Tag Archives: #neptune

Astronomers Discover Trio Of Planets Orbiting An M-Dwarf Star (Planetary Science)

A team of international astronomers reported on the discovery of one super-Earth- (TOI-1749b) and two sub-Neptune-sized planets (TOI1749c and TOI-1749d) transiting an early M dwarf TOI-1749 at a distance of 100 pc. Their study recently appeared in Arxiv.

Astronomers first identified these systems as planetary candidates using data from the TESS photometric survey. Later, they have followed up this system from the ground by means of multiband transit photometry, adaptive-optics imaging, and low-resolution spectroscopy, from which they have validated the planetary nature of the candidates.

They found that TOI-1749 b, c, and d have orbital periods of 2.39, 4.49, and 9.05 days, and radii of 1.4, 2.1, and 2.5 R, respectively. (More derived parameters are given in Table 1 below)

Table 1: Derived parameters from the results of photodynamical analysis. © Fukui et al.

In addition, using photodynamical models they have been able to place 95% confidence level upper limits on the masses of TOI-1749b, TOI-1749c, and TOI-1749d of 57, 14, and 15 M, respectively.

Figure 1: Period and radius diagram for planets around M dwarfs. The gray dots are known planets around M dwarfs taken from the NASA Exoplanet Archive. The magenta squares and black dots are for planets in the TOI-1749 and TOI-270 systems, respectively. The orange dashed line indicates the location of the proposed radius valley for M dwarf planets by Van Eylen et al. (2021). © Fukui et al.

The periods, sizes, and tentative masses of these planets are in line with a scenario in which all three planets initially had a hydrogen envelope on top of a rocky core, and only the envelope of the innermost planet has been stripped away by photoevaporation and/or core-powered mass loss mechanisms.

“The compositions of the innermost planet and the outer two planets can be explained by a bare rocky core and a rocky core + thin hydrogen envelope respectively.”

In addition, they have confirmed that the system is dynamically stable for at least 109 orbits of the innermost planet.

Figure 2. Period ratios of planetary trios including a near 2:1 commensurability pair. P1, P2, and P3 denote the orbital period of the inner planet in a near 2:1 commensurability pair, that of the outer planet in the same pair, and that of another planet inside (left panels) or outside (right panels) of the near 2:1 commensurability pair, respectively. Top and bottom panels show planetary trios around M dwarfs (Teff ≤ 4000K) and FGK dwarfs (Teff > 4000K), respectively. The gray dashed, dash-dotted, and dotted lines indicate the locations of the exact 2:1, 2:3, and 3:5 commensurabilities, respectively. This figure shows that additional planets to planetary pairs near 2:1 commensurability around M (FGK) dwarfs tend to have an inside (outside) orbit. Note that the TOI-178 system shows an opposite property; the host star however has an effective temperature that is close to the boundary (Teff = 4316K) © Fukui et al.

The outer planetary pair has a period ratio very close to the 2:1 commensurability (2.015), sharing the orbital architecture with the other M dwarf systems TOI-270 and TOI-175. This characteristic architecture might be a consequence of common planetary formation and migration processes in these systems.

“Further follow-up observations of this system would be worth pursuing to characterize the system in more detail.”

— concluded authors of the study

Featured image credit: Getty Images


Reference: A. Fukui, J. Korth, J. H. Livingston, J. D. Twicken, M. R. Zapatero Osorio, J. M. Jenkins, M. Mori, F. Murgas, M. Ogihara, N. Narita, E. Pallé, K. G. Stassun, G. Nowak, D. R. Ciardi, L. Alvarez-Hernandez, V. J. S. Béjar, N. Casasayas-Barris, N. Crouzet, J. P. de Leon, E. Esparza-Borges, D. Hidalgo Soto, K. Isogai, K. Kawauchi, P. Klagyivik, T. Kodama, S. Kurita, N. Kusakabe, R. Luque, A. Madrigal-Aguado, P. Montanes Rodriguez, G. Morello, T. Nishiumi, J. Orell-Miquel, M. Oshagh, H. Parviainen, M. Sánchez-Benavente, M. Stangret, N. Watanabe, G. Chen, M. Tamura, P. Bosch-Cabot, M. Bowen, K. Eastridge, L. Freour, E. Gonzales, P. Guerra, Y. Jundiyeh, T. K. Kim, L. V. Kroer, A. M. Levine, E. H. Morgan, M. Reefe, R. Tronsgaard, C. K. Wedderkopp, J. Wittrock, K. A. Collins, K. Hesse, D. W. Latham, G. R. Ricker, S. Seager, R. Vanderspek, J. Winn, E. Bachelet, M. Bowman, C. McCully, M. Daily, D. Harbeck, N. H. Volgenau, “TOI-1749: an M dwarf with a Trio of Planets including a Near-Resonant Pair”, Arxiv, pp. 1-30, 2021. https://arxiv.org/abs/2107.05430


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NYU Abu Dhabi Researchers Discover Orbital Patterns of Trans-Neptunian Objects (Planetary Science)

Data collected can be used to provide new insights into the evolution of the Kuiper Belt, and the larger solar system

Trans-Neptunian Objects (TNOs), small objects that orbit the sun beyond Neptune, are fossils from the early days of the solar system which can tell us a lot about its formation and evolution.

A new study led by Mohamad Ali-Dib, a research scientist at the NYU Abu Dhabi Center for Astro, Particle, and Planetary Physics, reports the significant discovery that two groups of TNOs with different surface colors also have very different orbital patterns. This new information can be compared to models of the solar system to provide fresh insights into its early chemistry. Additionally, this discovery paves the way for further understanding of the formation of the Kuiper Belt itself, an area beyond Neptune comprised of icy objects, that is also the source of some comets.

In the paper, The rarity of very red TNOs in the scattered disk, published in The Astronomical Journal, the researchers explain how they studied the chemical composition of TNOs in order to understand the dynamical history of the Kuiper Belt. TNOs are either deemed “Less Red” (often referred to as Gray), or “Very Red” (often referred to as Red) based on their surface colors. By re-analyzing a 2019 data set, the researchers discovered that gray and red TNOs have vastly different orbital patterns. Through additional calculations, the researchers determined that the two groups of TNOs formed in different locations, and this led to the dichotomy in both their orbits and colors.

Many models of the solar system have been designed to show how the Kuiper Belt has evolved, but these models only study the origins of its orbital structure or colors, not both simultaneously.

“With more data, our team’s work could be applied to more detailed solar system models and has the potential to reveal new insights about the solar system and how it has changed over the course of time,” said Ali-Dib.


Reference: Mohamad Ali-Dib, Michaël Marsset, Wing-Cheung Wong, and Rola Dbouk, “The Rarity of Very Red Trans-Neptunian Objects in the Scattered Disk”, Astronomical Journal, 162(1), 2021. Link to paper


Provided by NYU Abu Dhabi

Deep Water On Neptune and Uranus May Be Magnesium-rich (Planetary Science)

While scientists have amassed considerable knowledge of the rocky planets in our solar system, like Earth and Mars, much less is known about the icy water-rich planets, Neptune and Uranus.

In a new study recently published in Nature Astronomy, a team of scientists re-created the temperature and pressure of the interiors of Neptune and Uranus in the lab, and in so doing have gained a greater understanding of the chemistry of these planets’ deep water layers. Their findings also provide clues to the composition of oceans on water-rich exoplanets outside our solar system. 

Neptune and Uranus are conventionally thought to have distinct separate layers, consisting of an atmosphere, ice or fluid, a rocky mantle and a metallic core. For this study, the research team was particularly interested in possible reaction between water and rock in the deep interiors.

“Through this study, we were seeking to extend our knowledge of the deep interior of ice giants and determine what water-rock interactions at extreme conditions might exist,” said lead author Taehyun Kim, of Yonsei University in South Korea.

“Ice giants and some exoplanets have very deep water layers, unlike terrestrial planets. We proposed the possibility of an atomic-scale mixing of two of the planet-building materials (water and rock) in the interiors of ice giants.”

To mimic the conditions of the deep water layers on Neptune and Uranus in the lab, the team first immersed typical rock-forming minerals, olivine and ferropericlase, in water and compressed the sample in a diamond-anvil to very high pressures. Then, to monitor the reaction between the minerals and water, they took X-ray measurements while a laser heated the sample to a high temperature.

The resulting chemical reaction led to high concentrations of magnesium in the water. Based on these findings, the team concluded that oceans on water-rich planets may not have the same chemical properties as the Earth’s ocean and high pressure would make those oceans rich in magnesium.

“We found that magnesium becomes much more soluble in water at high pressures. In fact, magnesium may become as soluble in the water layers of Uranus and Neptune as salt is in Earth’s ocean,” said study co-author Sang-Heon Dan Shim of Arizona State University’s School of Earth and Space Exploration.    

A diamond-anvil (top right) and laser were used in the lab on a sample of olivine to reach the pressure-temperature conditions expected at the top of the water layer beneath the hydrogen atmosphere of Uranus (left). In this experiment, the magnesium in olivine dissolved in the water. Credit: Shim/ASU

These characteristics may also help solve the mystery of why Uranus’ atmosphere is much colder than Neptune’s, even though they are both water-rich planets. If much more magnesium exists in the Uranus’ water layer below the atmosphere, it could block heat from escaping from the interior to the atmosphere.

“This magnesium-rich water may act like a thermal blanket for the interior of the planet,” said Shim.

Beyond our solar system, these high-pressure and high-temperature experiments may also help scientists gain a greater understanding of sub-Neptune exoplanets, which are planets outside of our solar system with a smaller radius or a smaller mass than Neptune.

Sub-Neptune planets are the most common type of exoplanets that we know of so far, and scientists studying these planets hypothesize that many of them may have a thick water-rich layer with a rocky interior. This new study suggests that the deep oceans of these exoplanets would be much different from Earth’s ocean and may be magnesium-rich.

An electron microscopy image of the olivine sample shows a large empty dome structure where magnesium under high-pressure water precipitated as magnesium oxide. Credit: Kim et al.

“If an early dynamic process enabled a rock–water reaction in these exoplanets, the topmost water layer may be rich in magnesium, possibly affecting the thermal history of the planet,” said Shim.

For next steps, the team hopes to continue their high-pressure/high-temperature experiments under diverse conditions to learn more about the composition of planets.

“This experiment provided us with a plan for further exploration of the unknown phenomena in ice giants,” said Kim.

Additional authors of this study include Stella Chariton and Vitali Prakapenka of the University of Chicago; Anna Pakhomova and Hanns-Peter Liermann of the Deutsches Elektronen Synchrotron, Germany; Zhenxian Liu of the University of Illinois, Chicago; Sergio Speziale of the German Research Center for Geosciences, Germany; and Yongjae Lee of Yonsei University, South Korea.

Featured image: Uranus, taken by the NASA spacecraft Voyager 2. This ice giant, which is the seventh planet from the sun in our solar system, is nearly four times larger than Earth, and most of its mass is a dense fluid above a rocky core. Image by NASA/JPL


Reference: Kim, T., Chariton, S., Prakapenka, V. et al. Atomic-scale mixing between MgO and H2O in the deep interiors of water-rich planets. Nat Astron (2021). https://doi.org/10.1038/s41550-021-01368-2


Provided by Arizona State University

Two Strange Planets (Planetary Science)

Uranus and Neptune both have a completely skewed magnetic field, perhaps due to the planets’ special inner structures. But new experiments by ETH Zurich researchers now show that the mystery remains unsolved.

The two large gas planets Uranus and Neptune have strange magnetic fields. These are each strongly tilted relative to the planet’s rotation axes and are significantly offset from the physical centre of the planet. The reason for this has been a longstanding mystery in planetary sciences. Various theories assume that a unique inner structure of these planets could be responsible for this bizarre phenomenon. According to these theories, the skewed magnetic field is caused by circulations in a convective layer, which consists of an electrically conductive fluid. This convective layer in turn surrounds a stably layered, non-convective layer in which there is no circulation of the material due to its high viscosity and thus no contribution to the magnetic field.

The magnetic fields of Earth, Uranus and Neptune differ markedly. (Image: ETH Zurich / T. Kimura)

Extraordinary states

Computer simulations show that water and ammonia, the main components of Uranus and Neptune, enter an unusual state at very high pressures and temperatures: a “superionic state”, which has the properties of both a solid and a liquid. In this state, the hydrogen ions become mobile within the lattice structure formed by oxygen or nitrogen.

This is what the inner structure of the two gas planets could look like, according to previous theories. (Image: ETH Zurich / T. Kimura)

Recent experimental studies confirm that superionic water can exist at the depth where, according to theory, the stably layered region is located. It could therefore be that the stratified layer is formed by superionic components. However, it is unclear whether the components are actually able to suppress convection, since the physical properties of the superionic state are not known.

High pressure in the smallest space

Tomoaki Kimura and Motohiko Murakami from the Department of Earth Sciences at ETH Zurich are now one step closer to finding the answer. The two researchers have conducted high-​pressure and high-​temperature experiments with ammonia in their laboratory. The aim of the experiments was to determine the elasticity of the superionic material. Elasticity is one of the most important physical properties that influences thermal convection in the planetary mantle. It is remarkable that the elasticity of the materials in their solid and liquid states is completely different.

For their investigations, the researchers used a high-pressure apparatus called a diamond anvil cell. In this apparatus, the ammonia is placed in a small container with a diameter of about 100 micrometres, which is then clamped between two diamond tips that compress the sample. This makes it possible to subject materials to extremely high pressures, such as those found inside Uranus and Neptune.

Schematic representation of the diamond anvil cell. The chemical structure can be determined with the Raman spectrum, and the elasticity of the sample material with Brillouin scattering. (Image: ETH Zurich / T. Kimura)

The sample is then heated to over 2,000 degrees Celsius with an infrared laser. At the same time, a green laser beam illuminates the sample. By measuring the wave spectrum of the scattered green laser light, the researchers can determine the elasticity of the material and the chemical bonding in the ammonia. The shifts in the wave spectrum at different pressures and temperatures can be used to determine the elasticity of ammonia at different depths.

A new phase discovered

In their measurements, Kimura and Murakami have discovered a new superionic ammonia phase (γ phase) that exhibits an elasticity similar to that of the liquid phase. This new phase may be stable in the deep interior of Uranus and Neptune and therefore occur there. However, the superionic ammonia behaves like a liquid and thus it would not be viscous enough to contribute to the formation of the non-​convective layer.

The question of what properties the superionic water has inside Uranus and Neptune is all the more urgent in light of the new results. For even now, the mystery of why the two planets have such an irregular magnetic field still remains unsolved.

Featured image: Neptune and Uranus are the outermost two planets of our solar system and two gas giants. (Image: NASA)


Reference

Kimura T, Murakami M.: Fluid-like elastic response of superionic NH3 in Uranus and Neptune. PNAS April 6, 2021 118 (14) e2021810118; doi: 10.1073/pnas.2021810118 https://www.pnas.org/content/118/14/e2021810118


Provided by ETH Zurich

Planetary Scientists Revealed Triton’s Haze Formation Mechanism (Planetary Science)

Summary:

⦿ Ohno and colleagues presented the first microphysical model of haze formation on Triton.

⦿ Their results support the idea that Triton hazes are predominantly composed of hydrocarbon ices.

⦿ They simulated the formation of sphere and aggregate hazes with and without condensation of the C2H4 ice.

⦿ They found that haze particles can grow into fractal aggregates even in the Triton’s tenuous atmosphere.

⦿ UV occultation observations of Voyager 2 at short wavelength < 0.15 µm may slightly favor the icy aggregates on Triton.

⦿ They also answered why the haze formation on Triton is different from that of Pluto, even though they had similar atmosphere.


Organic aerosols (haze hereafter) produced via photochemistry of hydrocarbons are of great interest to study atmospheric properties and surface environment. The opacity of haze has crucial impacts on the radiative energy balance of atmospheres on Titan, Jupiter, and Pluto. It has been suggested that the haze veiled Archean Earth and played an important role to maintain warm climates. In the atmospheric chemistry context, hazes act as loss sites of gaseous species via condensation and heterogeneous reactions. Recent studies also have suggested that the presence of haze greatly impacts observations of exoplanetary atmospheres.

In the outer Solar System, observations of Titan, Pluto, and Triton provide important insights on organic haze formation in reduced (N2-CH4-CO) atmospheres. The presence of haze on Titan was discovered by ground-based observations and images from Voyager 1. Polarimetric and photometric observations of Titan’s haze particles by Pioneer 11 and Voyager 1 are consistent with fractal aggregates—nonspherical particles constituted by numerous spherical monomers. Cassini observations found that the Titan hazes extend from the ground to ionosphere above 1100 km.

Pluto’s hazes were discovered by the stellar occulations and recently have been investigated in detail by the New Horizons spacecraft. The UV extinction coefficients of hazes are nearly proportional to the atmospheric (N2) density from 26 to 100 km above the ground. The haze has a blue color that is consistent with Rayleigh scattering from particles with radii of ∼0.01 µm, whereas the strong forward scattering is consistent with particles with radii of ∼0.5 µm. This observational characteristic also indicates the aggregate nature of haze particles similar to the Titan haze.

Triton has also been found to possess near-surface haze layer in its thin atmosphere. The Voyager 2 imaging observations found that the optically thin hazes extend to an altitude of ∼30 km. Using the high phase angle images, Pollack et al. estimated the particle size of ∼0.1 µm, haze scattering optical depth of ∼0.003, and the particle production rate of ∼4.6 × 10¯15 g cm¯21. From disk-averaged photometry for a wavelength of λ = 0.414–0.561 µm, Hillier et al. reported that Triton hazes have single scattering albedo of nearly unity and cause strong forward scattering with an asymmetry factor of ∼0.6, although their results are highly influenced by discrete clouds near the ground. They also suggested that the scattering optical depth is nearly proportional to λ¯2. From the disk-resolved photometry at the similar wavelength range, Rages & Pollack later constrained the particle size of ∼0.17 µm and scattering optical depths of 0.001–0.01 that is higher at shorter wavelength. Solar occultation observations at UV wavelengths (λ = 0.14–0.165 µm) constrained the extinction optical depth to ∼0.024, significantly higher than the scattering optical depth at visible wavelength. In sum, hazes on Titan, Pluto, and Triton all exhibit the wavelength-dependent opacity and the strong forward scattering.

Microphysical models have been used to investigate the haze formation processes and constrain fundamental parameters, such as the haze production rate and charge to radius ratio. The models inferred the production rate of 0.5– 3 × 10¯14 g cm¯21 for Titan and 1.2 × 10¯14 g cm¯21 for Pluto, respectively. The microphysical models can also give insight on the degree of particle charge, which is associated with ionization processes in atmospheres. Previous studies suggested the charge-to-radius ratio of qe∼15 e µm¯1 for Titan and qe∼30 e µm¯1 for Pluto to explain the degree of forward scattering of haze particles.

In contrast to Titan and Pluto, Triton haze has not been thoroughly studied by a detailed microphysical model. Now, Ohno and colleagues presented the first microphysical model of haze formation on Triton.

Our model solves the evolution of both size and porosity distributions of haze particles in a self-consistent manner. We simulated the formation of sphere and aggregate hazes with and without condensation of the C2H4 ice.

— told Ohno, first author of the study

Figure 1. (Top) Vertical optical depth of the best models of ice balls (green line) and ice aggregates (black line). The gray, blue, and orange symbols denote the optical depth reported by Herbert & Sandel (1991), Krasnopolsky et al. (1992), and Rages & Pollack (1992), respectively. The extinction optical depth is identical to the scattering one since they have assumed the imaginary refractive index of zero for C2H4 ice. (Bottom) The visible I/F for all scattering angles in other best models. The gray shaded region denotes the I/F constrained in Rages & Pollack (1992). © Ohno et al.

Their model simulates the evolution of both size and porosity distributions of haze particles in a self-consistent manner. They have compared the model results with the observed UV extinction coefficient and visible scattered light intensity from Voyager 2 and showed that ice-free hazes, often assumed for Titan and Pluto hazes, cannot explain the Trion observations. Their results support the idea that Triton hazes are predominantly composed of hydrocarbon ices, which has been inferred from the Triton’s cold environment but not assessed in detail. They have proposed two possible models of haze formation with ice condensation, namely ice ball and ice aggregate scenarios, that can successfully explain the existing observations of Triton hazes. Their findings are summarized as follows.

⦿ Haze particles can grow into fractal aggregates even in the Triton’s tenuous atmosphere. The aggregates can grow into the mass-equivalent sphere radius of 0.2–1 µm, while the spheres can grow into only small sizes of 0.03–0.06 µm. Due to collisional growth, the fractal dimension of fractal aggregates is Df = 1.8–2.2, varying with the particle mass and altitude. The mass-dominating aggregates have the fractal dimension of Df ≈ 1.9. The obtained Df is in agreement with the outcome of cluster-cluster aggregation and similar to the fractal dimension of Titan hazes.

⦿ The ice-free hazes cannot simultaneously explain both UV and visible observations of Voyager 2, while including the condensation of C2H4 ices provides two better solutions. For ice aggregates, the required total haze mass flux is ∼2 × 10¯15 g cm¯21. For the icy sphere scenario, the column integrated C2H4 production rate is ∼8 × 10¯15 g cm¯21, and the ice-free mass flux of ∼6 × 10¯17 g cm¯21.

⦿ Future observations on the UV optical depth with greater wavelength coverage and scattering phase function with more phase angles would distinguish the ice balls and ice aggregates scenarios. The optical depth of ice aggregates increases with decreasing the wavelength at λ < 0.15 µm, while the optical depth of the ice balls is invariant at this wavelength range. The ice aggregates are slightly more consistent with the UV solar occultation observations of Voyager 2. The ice aggregates also cause forward scattering stronger than the ice balls do. These observational signatures would help to shed light on the nature of haze formation on Triton for future observations, such as NASA Ice Giants Mission and TRIDENT.

⦿ Lastly, although they did not focus on atmospheric thermal structure in this study, hazes may play an important role in controlling the temperature structure on Triton. In 2017, Zhang et al. suggested that radiative cooling by hazes is a key to explain the cold temperature on Pluto. It would be interesting to include the feedback of haze radiative effects on Triton’s temperature structure in future haze formation models. Since their results suggest that Triton hazes at the lower atmosphere are likely composed of hydrocarbon ices, the haze radiative feedback may have different effects as compared to that on Pluto, for which optical constants of Titan tholin is often assumed. On the other hand, since Triton hazes are likely ice-free in the hot upper atmosphere, they may act as coolants in the upper atmosphere, as suggested for Pluto.

Why are Triton and Pluto Hazes different?

Triton’s N2-CH4-CO atmosphere is similar to Pluto; nevertheless, Ohno and Colleagues results suggested that the properties of Triton hazes are quite different from those of Pluto hazes. According to their results, Triton hazes are predominantly composed of hydrocarbon ices, while Pluto hazes are suggested to be similar to Titan hazes, although there are no direct observational constraints on the Pluto haze compositions. They also note that a recent study of Lavvas et al. suggested that Pluto hazes may be composed of hydrocarbon ices, such as C4H2. Nonetheless, it would be important to understand the cause in terms of their composition and formation processes if Triton and Pluto hazes were indeed different.

One straightforward explanation of the difference is the temperature structure. Triton’s lower atmosphere is very cold (∼ 40–60 K), while the Pluto’s lower atmosphere is relatively hot (∼ 60–100 K). Thus, the ice condensation onto haze particles can be inhibited at Pluto’s lower hot atmosphere, explaining why Pluto hazes look different from Triton hazes. They note that the C2 hydrocarbons might condense or stick onto the haze particles in the cold upper atmosphere of Pluto.

CH4 abundance in the upper atmosphere might also cause the difference of Triton and Pluto haze properties. The Voyager 2 observation found that CH4 abundance steeply decreases with increasing altitudes on Triton, while the abundance rather increases with altitudes Pluto as revealed by the New Horizons spacecraft. The New Horizons observation also found that high order hydrocarbons, such as C2H2, C2H4, and C2H6, are abundantly present in the upper atmosphere of Pluto. Since photochemistry of hydrocarbons eventually yields photochemical haze as known from Titan, Pluto seems to be more favored to form Titan-like hazes than Triton does.

Water delivery by interplanetary dust particles (IDPs) might be an alternative factor that causes the different haze formation processes. In the outer solar system relevant to Triton and Pluto, IDPs are mostly coming from Edgeworth Kuiper belts and Oort Cloud comets. Based on dust dynamics simulations, Poppe & Horanyi suggested that meteoroidal water influx on Triton is two orders of magnitude higher than that on Pluto because of strong gravitational acceleration and focusing by Neptune’s gravity field. The estimated H2O mass flux is 1.8 × 10¯17 g cm¯21, which is about two orders of magnitude lower than the total haze mass flux suggested in this study. However, the deposited water can influence the photochemistry of hydrocarbons; for example, photolysis of H2O produces OH radicals that eventually form CO through reactions with carbon-based molecules. It would be interesting to study how the IDPs’ water delivery may affect hydrocarbon photochemistry and subsequent haze formation.


Reference: Kazumasa Ohno, Xi Zhang, Ryo Tazaki, Satoshi Okuzumi, “Haze Formation on Triton”, pp. 1-29, ArXiv, 2021. https://arxiv.org/abs/2012.11932


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Saturn’s Tilt Caused By its Moons (Planetary Science)

Two scientists from CNRS and Sorbonne University working at the Institute of Celestial Mechanics and Ephemeris Calculation (Paris Observatory – PSL/CNRS) have just shown that the influence of Saturn’s satellites can explain the tilt of the rotation axis of the gas giant. Their work, published on 18 January 2021 in the journal Nature Astronomy, also predicts that the tilt will increase even further over the next few billion years.

Artist’s impression of the migration of Titan and the tilt of Saturn. © Coline SAILLENFEST / IMCCE

Rather like David versus Goliath, it appears that Saturn’s tilt may in fact be caused by its moons. This is the conclusion of recent work carried out by scientists from the CNRS, Sorbonne University and the University of Pisa, which shows that the current tilt of Saturn’s rotation axis is caused by the migration of its satellites, and especially by that of its largest moon, Titan.

Recent observations have shown that Titan and the other moons are gradually moving away from Saturn much faster than astronomers had previously estimated. By incorporating this increased migration rate into their calculations, the researchers concluded that this process affects the inclination of Saturn’s rotation axis: as its satellites move further away, the planet tilts more and more.

The decisive event that tilted Saturn is thought to have occurred relatively recently. For over three billion years after its formation, Saturn’s rotation axis remained only slightly tilted. It was only roughly a billion years ago that the gradual motion of its satellites triggered a resonance phenomenon that continues today: Saturn’s axis interacted with the path of the planet Neptune and gradually tilted until it reached the inclination of 27° observed today.

A schematic animation showing Titan’s migration and Saturn entering into resonance. The frame is rotating, so the axis stops moving when resonance is achieved. © Melaine SAILLENFEST / IMCCE

These findings call into question previous scenarios. Astronomers were already in agreement about the existence of this resonance. However, they believed that it had occurred very early on, over four billion years ago, due to a change in Neptune’s orbit. Since that time, Saturn’s axis was thought to have been stable. In fact, Saturn’s axis is still tilting, and what we see today is merely a transitional stage in this shift. Over the next few billion years, the inclination of Saturn’s axis could more than double.

The research team had already reached similar conclusions about the planet Jupiter, which is expected to undergo comparable tilting due to the migration of its four main moons and to resonance with the orbit of Uranus: over the next five billion years, the inclination of Jupiter’s axis could increase from 3° to more than 30°.

Reference: Melaine Saillenfest, Giacomo Lari and Gwenaël Boué, “The large obliquity of Saturn explained by the fast migration of Titan”, Nature Astronomy, 18 January 2021.
https://www.nature.com/articles/s41550-020-01284-x

Provided by CNRS

UH Astronomers Find Evidence for Planets Shrinking Over Billions of Years (Planetary Science)

A team of astronomers led by University of Hawaiʻi Institute for Astronomy (IfA) graduate student Travis Berger has shown that an intriguing class of Neptune-sized planets shrinks over billions of years.

Credit: NASA/Ames Research Center/W. Stenzel/D. Rutter

From centuries of studying the planets within our solar system, astronomers have wondered how planets form and evolve to become the ones we observe them today. One of the most surprising findings of the past decade was the discovery of a new branch in the planetary “family tree”, separating slightly larger than Earth (super-Earths) from those somewhat smaller than Neptune (sub-Neptunes). However, it is unclear how these different-sized planets formed, as our observations are only a single snapshot out of a billions of years long lifetime for each individual planetary system.

Astronomers can’t watch planets evolve in real time, so they analyze populations of planets to infer how they form and evolve. Indeed, using observations from the NASA Kepler and the ESA Gaia missions, Berger and his team have uncovered another piece of the planet formation and evolution puzzle:  as planets are bombarded with intense light from their host stars, they gradually lose their atmospheres over billions of years.

“The loss of planet atmospheres on billion year timescales shows that these planets lose mass, even at old age,” explained Berger. “One of our main discoveries is that planet sizes shrink on longer timescales than previously thought.” 

NASA’s Kepler mission hunted for planets by staring at one patch of the sky near the constellation Cygnus for roughly four years, detecting small, regular brightness dips from hundreds of thousands of stars within our Milky Way Galaxy. The size of a dip corresponds to the relative size of the planet compared to its host star. Therefore, to determine the actual size of a planet, it is first necessary to measure the size of the star.

The ESA Gaia mission provided an essential ingredient to measuring the sizes of Kepler planet-hosting stars:  parallax. Human eyes use parallax to measure distances to objects, giving us depth perception. Similarly, astronomers use parallax for astronomical-scale depth perception to measure the distances to stars, which in turn assists in measuring the sizes of stars. Distance information is needed to distinguish between a closer small star and a distant, larger star. Combining size and stellar colors also allows astronomers to determine the relative ages of stars.

The UH team used the Gaia constraints on star sizes to revise estimates of planet sizes, and combined it with stellar color data to determine the ages of the planet-hosting stars. They then compared the effects of stellar age on over 2600 planets detected by Kepler. Some planets, especially those that receive over 150 times the light that Earth receives from the Sun, lose their atmospheres over a billion years, as they are inundated with heat and light from the host star.

“While astronomers have long predicted that planets should shrink in size as they age, we did not know whether this can occur over timescales of billions of years. We do now,” says Berger. “The fact that we see planet sizes change on billion-year timescales suggests that there is an evolutionary pathway, where highly-illuminated sub-Neptune-sized planets transition to becoming super-Earth-sized planets.”

In the future, similar work could be conducted on planets discovered by the NASA K2 and TESS Missions in order to resolve the timescale for atmosphere loss with finer precision. 

The study, titled “The Gaia-Kepler Stellar Properties Catalog. II. Planet Radius Demographics as a Function of Stellar Mass and Age,” was funded by NASA and the National Science Foundation, and is published in The Astronomical Journal.

Provided by University of Hawaiʻi at Mānoa

NGTS-14Ab: A Neptune-sized Transiting Planet in the Desert (Planetary Science)

Smith and colleagues presented the discovery of a new transiting planetary system within the Neptunian desert, NGTS-14A.

Fig. 1. NGTS deep stacked image (2′ × 2′) centred on NGTS-14A. The Gaia positions of NGTS-14A and NGTS-14B are indicated with a red ’+’ and a blue ’x’, respectively. The pink circle indicates the NGTS photometric aperture, which has a radius of 3 pixels (15”). Other Gaia sources are marked with orange triangles. North is up, and East to the left. © Smith et al.

The first generation of wide-field transit surveys, the most prolific of which were SuperWASP and HAT-Net, unveiled the rich diversity of hot Jupiters. These planets, although intrinsically rare, have been discovered in numbers large enough to enable statistical population analyses such as those investigating inflation. Transiting hot Jupiters also remain the best-studied individual planetary systems, with characterisation observations such as atmospheric transmission spectroscopy pioneered on these objects.

The Kepler and K2 missions, and the ongoing TESS (Transiting Exoplanet Survey Satellite) mission subsequently revealed a large population of small (less than 2 – 3 Earth radii), short-period planets. In between these two populations, however, lies a relatively unpopulated region of parameter space, often referred to as the sub-Jovian or Neptunian desert.

Fig. 2. NGTS-14Ab (light green point) within the Neptunian desert, the boundary of which is indicated with blue dashed lines. The black dashed lines indicate the region of interest. The parameters of other planets come from the NASA Exoplanet Archive, and the colour scale indicates the number of planets per grid element. The plot comprises 200 × 200 elements, uniformly spaced in log10 P and log10 R. ©Smith et al.

The Next Generation Transit Survey (NGTS) consists of twelve independent 0.2-m telescopes, each equipped with a red-sensitive 2k × 2k CCD covering eight square degrees. The survey is optimised for detecting short-period planets transiting K-dwarf stars, with the goal of detecting Neptune-sized planets. One of the main drivers for the extremely high-precision photometry achieved by NGTS is the telescope guiding, which uses Donuts to ensure sub-pixel pointing precision throughout the duration of an observing season. This has allowed the detection of significantly shallower transits than previously achieved from the ground, and the discovery of planets in the Neptunian desert, such as NGTS-4b.

Now, Smith and colleagues reported the discovery of the NGTS-14 system, which consists of an early K-type dwarf star, NGTS-14A, with a long period companion, NGTS-14B, which is likely to be a mid-M dwarf. The primary star is orbited by a transiting exoplanet,
NGTS-14Ab, which is slightly larger and more massive than Neptune.

Transits of NGTS-14Ab were discovered in photometry from the Next Generation Transit Survey (NGTS). Follow-up transit photometry was conducted from several ground-based facilities, as well as extracted from TESS full-frame images. They combine radial velocities from the HARPS spectrograph with the photometry in a global analysis to determine the system parameters (shown in table 1 below).

“We use the neural network model of planetary interiors to predict the internal structure of NGTS-14Ab (Fig. 3), using the planetary mass and radius (Table 1) as model inputs. Although internal structure is notoriously degenerate with mass and radius, this model suggests that NGTS-14Ab has a significant gaseous envelope.”, said Smith.

Table 1. Parameters from light curve and RV data analysis © Smith et al.

They found that NGTS-14Ab has a radius about 30 per cent larger than that of Neptune (i.e. it has radius, 0.444 times than that of the radius of the Jupiter), and is around 70 per cent more massive than Neptune (i.e. it has mass of 0.092 than that of the mass of the Jupiter). They also found that it transits the main-sequence K1 star, NGTS-14A, with a period of 3.54 days, just far enough to have maintained at least some of its primordial atmosphere.

Fig. 3. Possible internal structure for NGTS-14Ab, the result of the neural network model. Shown are the predictions of the relative interior layer thicknesses, using derived planetary mass and radius as inputs. The colored lines showed the combined Gaussian mixture prediction of the model, with the area under each curve normalised to unity. © Smith et al.

“NGTS-14Ab appears to just far enough from its star that it is able to maintain a significant atmosphere, resulting in a density similar to those of Uranus and Neptune”, said Smith.

They have also identified a possible long-period stellar mass companion to the system, NGTS-14B, and investigated the binarity of exoplanet host stars inside and outside the Neptunian desert using Gaia.

Reference: A. M. S. Smith, J. S. Acton, D. R. Anderson, D. J. Armstrong, D. Bayliss, C. Belardi, F. Bouchy, R. Brahm, J. T. Briegal, E. M. Bryant, M. R. Burleigh, J. Cabrera, A. Chaushev, B. F. Cooke, J. C. Costes, Sz. Csizmadia, Ph. Eigmüller, A. Erikson, S. Gill, E. Gillen, M. R. Goad, M. N. Günther, B. A. Henderson, A. Hogan, A. Jordán, M. Lendl, J. McCormac, M. Moyano, L. D. Nielsen, H. Rauer, L. Raynard, R. H. Tilbrook, O. Turner, S. Udry, J. I. Vines, C. A. Watson, R. G. West, P. J. Wheatley, “NGTS-14Ab: a Neptune-sized transiting planet in the desert”, ArXiv, pp. 1-11, 7 Jan 2021. https://arxiv.org/abs/2101.01470v1

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Researchers Puzzled At Storm’s U-turn and Sudden Appearance of New Dark Spot (Planetary Science)

Summary:

When NASA’s Voyager 2 spacecraft flew by Neptune in 1989 after a nearly 3-billion-mile odyssey, astronomers expected to get a close-up look at a blue-green planet that seemed as featureless as a marble. Instead, they were shocked and intrigued to see a dynamic and turbulent world of whirling storms, including a giant feature dubbed the Great Dark Spot, looming in Neptune’s far southern hemisphere.

The vortex was reminiscent of Jupiter’s legendary Great Red Spot, a monstrous storm that has been raging for hundreds of years. Had this Great Dark Spot been brewing for the same amount of time? Or, was it a more ephemeral tempest?

Scientists had to wait until 1994, when the Hubble Space Telescope and its crisp vision peered at distant Neptune. The mysterious spot had vanished! This game of planetary peek-a-boo continued when Hubble spotted another dark storm appearing in Neptune’s northern hemisphere in 1995. Over the past three decades, Hubble has continued to observe the planet, watching several more dark spots come and go.

Only Hubble can study these spots because it has the sharp vision to observe them in visible light. Hubble has shown that these storms live for a few years before vanishing or fading away.

Researchers thought the current giant storm in the northern hemisphere was heading to destruction when it mysteriously halted its southern journey and began drifting northward. At the same time as the spot’s stunning reversal, a new, slightly smaller dark feature appeared near its bigger cousin and later disappeared. These surprising events add to the mystery of this dynamic world.

Neptune dark spot © NASA, ESA, M.H. Wong (University of California, Berkeley), and A. Simon (NASA Goddard Space Flight Center)

Astronomers using NASA’s Hubble Space Telescope watched a mysterious dark vortex on Neptune abruptly steer away from a likely death on the giant blue planet.

The storm, which is wider than the Atlantic Ocean, was born in the planet’s northern hemisphere and discovered by Hubble in 2018. Observations a year later showed that it began drifting southward toward the equator, where such storms are expected to vanish from sight. To the surprise of observers, Hubble spotted the vortex change direction by August 2020, doubling back to the north. Though Hubble has tracked similar dark spots over the past 30 years, this unpredictable atmospheric behavior is something new to see.

Equally as puzzling, the storm was not alone. Hubble spotted another smaller dark spot in January this year that temporarily appeared near its larger cousin. It might possibly have been a piece of the giant vortex that broke off, drifted away, and then disappeared in subsequent observations.

“We are excited about these observations because this smaller dark fragment is potentially part of the dark spot’s disruption process,” said Michael H. Wong of the University of California at Berkeley. “This is a process that’s never been observed. We have seen some other dark spots fading away and they’re gone, but we’ve never seen anything disrupt, even though it’s predicted in computer simulations.”

The large storm, which is 4,600 miles across, is the fourth dark spot Hubble has observed on Neptune since 1993. Two other dark storms were discovered by the Voyager 2 spacecraft in 1989 as it flew by the distant planet, but they had disappeared before Hubble could observe them. Since then, only Hubble has had the sharpness and sensitivity in visible light to track these elusive features, which have sequentially appeared and then faded away over a duration of about two years each. Hubble uncovered this latest storm in September 2018.

Wicked Weather

Neptune’s dark vortices are high-pressure systems that can form at mid-latitudes and may then migrate toward the equator. They start out remaining stable due to Coriolis forces, which cause northern hemisphere storms to rotate clockwise, due to the planet’s rotation. (These storms are unlike hurricanes on Earth, which rotate counterclockwise because they are low-pressure systems.) However, as a storm drifts toward the equator, the Coriolis effect weakens and the storm disintegrates. In computer simulations by several different teams, these storms follow a more-or-less straight path to the equator, until there is no Coriolis effect to hold them together. Unlike the simulations, the latest giant storm didn’t migrate into the equatorial “kill zone.”

“It was really exciting to see this one act like it’s supposed to act and then all of a sudden it just stops and swings back,” Wong said. “That was surprising.”

Dark Spot Jr.

The Hubble observations also revealed that the dark vortex’s puzzling path reversal occurred at the same time that a new spot, informally deemed “dark spot jr.,” appeared. The newest spot was slightly smaller than its cousin, measuring about 3,900 miles across. It was near the side of the main dark spot that faces the equator—the location that some simulations show a disruption would occur.

However, the timing of the smaller spot’s emergence was unusual. “When I first saw the small spot, I thought the bigger one was being disrupted,” Wong said. “I didn’t think another vortex was forming because the small one is farther towards the equator. So it’s within this unstable region. But we can’t prove the two are related. It remains a complete mystery.

“It was also in January that the dark vortex stopped its motion and started moving northward again,” Wong added. “Maybe by shedding that fragment, that was enough to stop it from moving towards the equator.”

The researchers are continuing to analyze more data to determine whether remnants of dark spot jr. persisted through the rest of 2020.

Dark Storms Still Puzzling

It’s still a mystery how these storms form, but this latest giant dark vortex is the best studied so far. The storm’s dark appearance may be due to an elevated dark cloud layer and it could be telling astronomers about the storm’s vertical structure.

Another unusual feature of the dark spot is the absence of bright companion clouds around it, which were present in Hubble images taken when the vortex was discovered in 2018. Apparently, the clouds disappeared when the vortex halted its southward journey. The bright clouds form when the flow of air is perturbed and diverted upward over the vortex, causing gases to likely freeze into methane ice crystals. The lack of clouds could be revealing information on how spots evolve, say researchers.

Weather Eye on the Outer Planets

Hubble snapped many of the images of the dark spots as part of the Outer Planet Atmospheres Legacy (OPAL) program, a long-term Hubble project, led by Amy Simon of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, that annually captures global maps of our solar system’s outer planets when they are closest to Earth in their orbits.

OPAL’s key goals are to study long-term seasonal changes, as well as capture comparatively transitory events, such as the appearance of dark spots on Neptune or potentially Uranus. These dark storms may be so fleeting that in the past some of them may have appeared and faded during multi-year gaps in Hubble’s observations of Neptune. The OPAL program ensures that astronomers won’t miss another one.

“We wouldn’t know anything about these latest dark spots if it wasn’t for Hubble,” Simon said. “We can now follow the large storm for years and watch its complete life cycle. If we didn’t have Hubble, then we might think the Great Dark Spot seen by Voyager in 1989 is still there on Neptune, just like Jupiter’s Great Red Spot. And, we wouldn’t have known about the four other spots Hubble discovered.” Wong will present the team’s findings Dec. 15 at the fall meeting of the American Geophysical Union.

The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.

Provided by Hubble