Category Archives: Planetary Science

Do you regulate the end? Don’t look for it on porous asteroids (Planetary Science)

If the surface of carbonaceous asteroids such as Bennu and Ryugu – destinations respectively of the Osiris-Rex and Hayabusa 2 space missions – is not covered by large quantities of material known as fine regolith, similar to the sand of our beaches, the cause is to be found in the high porosity of their rocks. These are the results obtained by an international team, which sees an important participation of researchers from the National Institute of Astrophysics

When the NASA mission Osiris-Rex reached the asteroid Bennu in 2018, scientists expected to find a surface covered with fine regolith : material formed by small grains or pebbles at most a few centimeters large, similar to the sand found on beaches of our planet. There were also similar expectations for the Hayabusa 2 mission of the Japanese space agency Jaxa, which reached the asteroid Ryugu in the same year . Once in the vicinity of these two small bodies, however, both probes revealed rocky terrain, surprisingly covered with boulders and with very little fine regolith.

A new study, led by the Italian researcher Saverio Cambioni , with the participation of Giovanni Poggiali and John R. Brucato of the National Institute of Astrophysics, has finally solved the mystery: it would be the very porous nature of the rocks of these asteroids to explain the absence of fine regolith on their surface.

“When the first images from Bennu arrived, we noticed some regions where the resolution wasn’t enough to distinguish between small rocks and fine regolith, so we started using our new AI method to recognize fine regolith from rocks. using the infrared emission recorded by the probe “, explains Saverio Cambioni, first author of the article published today in Nature , who conducted this research during his doctorate at the University of Arizona, in the United States, and is now a postdoctoral researcher -doc at the Massachusetts Institute of Technology .

Osiris-Rex collected high-resolution images – up to 3 millimeters per pixel – across Bennu’s surface to map it out and study it in detail: characterizing the asteroid is one of the mission’s main objectives. The absence of fine regolith on the surface posed not only a scientific question, but also a technical challenge: the probe was designed to collect a sample of the asteroid from a surface rich in fine regolith, not among boulders and rocks. Thus, the sampling area on Bennu had to be drastically reduced from a region of about 160 meters, the size of a parking lot for 100 cars, down to 8 meters, comparable to a parking lot for only 5 cars. The operation was successful on 20 October.

Detail of the Bennu surface. Credits: Nasa / University of Arizona / Csa / York University / Open University / Mda

“To distinguish the contribution of fine regolith from that of rock and to make a global study of the surface, it was necessary to put together a large database of regions observed both in the day and in the night,” says Giovanni Poggiali, co-author of the study and researcher. INAF in Florence.

The thermal emission released by the fine regolith, which depends on the size of its grains, is different from the emission produced by larger rocks, which instead depends on their porosity. For this the team simulated the emission produced by different mixtures of fine regolith and rocks of varying porosity, teaching an algorithm how to recognize the different soils and compare them with observations of the Bennu surface.

The analysis of the data showed that the fine regolith is not randomly distributed on Bennu but that, where the rocks are more porous – that is, on most of the asteroid – it is systematically found less. This suggests that the highly porous rocks of the celestial body produce less fine regolith because they are compacted – and not fragmented, as is the case with less porous rocks – as a result of meteoric impacts: the cavities inside the rocks would help to cushion the impact of the meteorites. resulting in less fragment production. Furthermore, the porous rocks would break down more slowly due to the diurnal cycle of heating and cooling of the asteroid, further inhibiting the formation of the fine regolith.

“With artificial intelligence, used for the first time in this type of research, we were able to go much deeper into the analysis of spectroscopic data by highlighting unique properties of the materials that make up primitive asteroids such as Bennu and Ryugu,” comments John Brucato, co-author of the study and INAF researcher in Florence.

Saverio Cambioni, researcher at the Massachusetts Institute of Technology.

The results of Bennu’s analysis are in agreement with findings from independent laboratory experiments. Also for the asteroid Ryugu, Cambioni and colleagues propose that the porosity of its rocks would explain the lack of fine regolith observed by Hayabusa 2. Similarly, the abundance of this material recorded in 2005 by the previous Jaxa mission, Hayabusa, on a another asteroid of a different type, Itokawa, would be due to a lower porosity of its rocks, determined by the researchers using observations from Earth. According to the team, fine regolith would be rare on carbonaceous asteroids such as Bennu and Ryugu, which are the most common type of asteroid and are thought to be formed from very porous rocks. The fine regolith would instead be abundant on S-type asteroids such as Itokawa,

“Asteroids are fossils of the formation of the Solar System, but recent studies – including ours – are showing how in reality some asteroids are very evolved,” Cambioni concludes. «Understanding the evolution processes of asteroids is important for understanding the evolution of the Solar System and our planet. To shed some light on this, we will need to visit more asteroids in the future to collect samples to be reported and analyzed on Earth. Our study will allow us to understand in advance the nature of the asteroid surfaces, and therefore to plan the missions accordingly ».

Featured image: The asteroid Bennu in a mosaic of images taken by the Nasa Osiris-Rex probe. Credits: NASA / Goddard / University of Arizona

To know more:

  • Read in  Nature  the article ” Fine-regolith production on asteroids controlled by rock porosity “, by S. Cambioni, M. Delbo, G. Poggiali, C. Avdellidou, AJ Ryan, JDP Deshapriya, E. Asphaug, R.-L . Ballouz, MA Barucci, CA Bennett, WF Bottke, JR Brucato, KN Burke, E. Cloutis, DN DellaGiustina, JP Emery, B. Rozitis, KJ Walsh and DS Lauretta

Provided by INAF

Late-time Small-body Disruptions Can Protect the Earth (Planetary Science)

If an asteroid is determined to be on an Earth-impacting trajectory, scientists typically want to stage a deflection, where the asteroid is gently nudged by a relatively small change in velocity, while keeping the bulk of the asteroid together.

A kinetic impactor or a standoff nuclear explosion can achieve a deflection. However, if the warning time is too short to stage a successful deflection, another option is to couple a lot of energy to the asteroid and break it up into many well-dispersed fragments. This approach is called disruption and it is often what people think of when they picture planetary defense. While scientists would prefer to have more warning time, they need to be prepared for any possible scenario, as many near-Earth asteroids remain undiscovered.

Now, new research takes a closer look into at how different asteroid orbits and different fragment velocity distributions affect the fate of the fragments, using initial conditions from a hydrodynamics calculation, where a 1-Megaton-yield device was deployed a few meters off the surface of a Bennu-shaped, 100-meter diameter asteroid (1/5 the scale of Bennu, a near-Earth asteroid discovered in 1999). See the video.

The work is featured in a paper published in Acta Astronautica with lead author Patrick King, a former Lawrence Livermore National Laboratory Graduate Scholar Program fellow who worked with LLNL’s Planetary Defense group on this research as part of his Ph.D. thesis. King currently works at the Johns Hopkins University Applied Physics Laboratory (JHUAPL) as a physicist in the Space Exploration Sector. Co-authors of the paper include Megan Bruck Syal, David Dearborn, Robert Managan, Michael Owen and Cody Raskin.

The results highlighted in the paper are reassuring: for all five asteroid orbits considered, carrying out the disruption just two months before the Earth impact date was able to reduce the fraction of impacting mass by factor of 1,000 or more (99.9 percent of the mass misses Earth). For a larger asteroid, the dispersal would be less robust, but even dispersal velocities reduced by an order of magnitude would result in 99 percent of the mass missing Earth, if disruption is staged at least six months ahead of the impact date.

“One of the challenges in assessing disruption is that you need to model all of the fragment orbits, which is generally far more complicated than modeling a simple deflection,” King said. “Nevertheless, we need to try to tackle these challenges if we want to assess disruption as a possible strategy.”

King said the principal finding of the work was that nuclear disruption is a very effective defense of last resort. “We focused on studying ‘late’ disruptions, meaning that the impacting body is broken apart shortly before it impacts,” he said. “When you have plenty of time — typically decade-long timescales — it is generally preferred that kinetic impactors are used to deflect the impacting body.”

Kinetic impactors have many advantages: for one, the technique is well-known and is being tested on real missions, such as the DART mission, and is capable of handling a wide range of possible threats if you have enough time. However, they do have some limitations, so it is important that if an actual emergency does arise that multiple options are available to deal with a threat, including some ways that can handle pretty short warning times.

Owen said this paper is critically important for understanding the consequences and requirements for disrupting a hazardous asteroid approaching Earth. Owen wrote the software, called Spheral, that was used to model the nuclear disruption of the original asteroid, following the detailed physics of shocking and breaking up the original rocky asteroid and capturing the properties of the resulting fragments. From there, the team used Spheral to follow the gravitational evolution of the fragment cloud, accounting for the effects of the fragments on one another as well as the gravitational influence of the sun and planets.

“If we spotted a hazardous object destined to strike the Earth too late to safely divert it, our best remaining option would be to break it up so thoroughly the resulting fragments would largely miss the Earth,” he said. “This is a complicated orbital question though — if you break up an asteroid into pieces, the resulting cloud of fragments will each pursue their own path around the sun, interacting with each other and the planets gravitationally. That cloud will tend to stretch out into a curved stream of fragments around the original path the asteroid was on. How quickly those pieces spread out (combined with how long until the cloud crosses Earth’s path) tells us how many will strike the Earth.”

Bruck Syal said the work addresses a major goal defined in the White House OSTP’s National Near-Earth Object (NEO) Preparedness Strategy and Action Plan: to improve NEO modeling, prediction and information integration.

“Our group continues to refine our modeling approaches for nuclear deflection and disruption, including ongoing improvements to X-ray energy deposition modeling, which sets the initial blowoff and shock conditions for a nuclear disruption problem,” she said. “This latest paper is an important step in demonstrating how our modern multiphysics tools can be used to simulate this problem over multiple relevant physics regimes and timescales.”

Featured image: The hydro simulation in Spheral that provided the basis for the analysis: 1 Megaton at a few meters standoff distance from a 100-meter diameter asteroid (with Bennu shape). Colors denote velocities. The legend is cm/us, which is equivalent to 10 km/s. © LLNL


Provided by LLNL

Spectrum Reveals Extreme Exoplanet is Even More Exotic (Planetary Science)

Considered an ultra-hot Jupiter – a place where iron gets vaporized, condenses on the night side and then falls from the sky like rain – the fiery, inferno-like WASP-76b exoplanet may be even more sizzling than scientists had realized.

An international team, led by researchers at Cornell, University of Toronto and Queen’s University Belfast, reports the discovery of ionized calcium on the planet – in high-resolution spectra obtained with Gemini North near the summit of Mauna Kea in Hawaii.

Hot Jupiters are so named for their high temperatures, due to proximity to their stars. WASP-76b, discovered in 2016, is a Jupiter-sized planet about 640 light-years from Earth, but so close to its F-type star, which is slightly hotter than the sun, that the giant planet completes one orbit every 1.8 Earth days.

The research results are the first of a multiyear, Cornell-led project, Exoplanets with Gemini Spectroscopy survey, or ExoGemS, that explores the diversity of planetary atmospheres.

“As we do remote sensing of dozens of exoplanets, spanning a range of masses and temperatures,” said co-author Ray Jayawardhana, the Harold Tanner Dean of the College of Arts and Sciences (A&S), and a professor of astronomy, “we will develop a more complete picture of the true diversity of alien worlds – from those hot enough to harbor iron rain to others with more moderate climates, from those heftier than Jupiter to others not much bigger than the Earth.

“It’s remarkable that with today’s telescopes and instruments, we can already learn so much about the atmospheres – their constituents, physical properties, presence of clouds and even large-scale wind patterns – of planets that are orbiting stars hundreds of light-years away,” Jayawardhana said.

The group spotted a rare trio of spectral lines in highly sensitive observations of the exoplanet WASP-76b’s atmosphere, published in the Astrophysical Journal Letters on Sept. 28 and presented on Oct. 5 at the annual meeting of the Division for Planetary Sciences of the American Astronomical Society.

“We’re seeing so much calcium; it’s a really strong feature,” said first author Emily Deibert, a University of Toronto doctoral student, whose adviser is Jayawardhana.

“This spectral signature of ionized calcium could indicate that the exoplanet has very strong upper atmosphere winds,” Deibert said. “Or the atmospheric temperature on the exoplanet is much higher than we thought.”

Since WASP-76b is tidally locked – in that one side of it always faces the star – it has a permanent night side that sports a relatively cool 2,400-degree Fahrenheit average temperature. Its day side, facing toward the star, has an average temperature at 4,400 degrees F.

Deibert and her colleagues examined the moderate temperature zone, on the planet’s limb between day and night. “The exoplanet moves fast on its orbit and that’s how we were able to separate its signal from starlight,” she said. “You can see that the calcium imprint on the spectra is moving quickly along with the planet.”

The ExoGemS survey – intended to study 30 or more planets – is led by Jake Turner, a Carl Sagan Fellow in NASA’s Hubble Fellowship program, who is in Cornell’s Department of Astronomy (A&S) and is also advised by Jayawardhana.

Astronomers continue to delve deeper to understand exoplanets – considered just a dream two decades ago. “Our work, and that of other researchers, is paving the way for exploring the atmospheres of terrestrial worlds beyond our solar system,” Turner said.

Other authors on the paper, “Detection of Ionized Calcium in the Atmosphere of the Ultra-Hot Jupiter WASP-76b,” include Ernst J. W. de Mooij of the Queen’s University Belfast; Luca Fossati of the Austrian Academy of Sciences; Callie E. Hood and Jonathan J. Fortney, both from University of California, Santa Cruz; Romain Allart of the University of Montreal; and David K. Sing of Johns Hopkins University. Cornellians included researchers Andrew Ridden-Harper and Laura Flagg, both in Jayawardhana’s group, and Ryan MacDonald. Portions of this research were funded by NASA.

Gemini North is part of the international Gemini Observatory, a program of National Science Foundation’s NOIRLab.

The image used with this story is provided as a courtesy of the European Southern Observatory.

Featured image: The fiery exoplanet WASP-76b – a so-called hot Jupiter, where it rains iron – may be hotter than previously thought. © ESO/M. Kornmesser


Provided by Cornell University

New Study Finds Evidence of the Origin of Metal-Rich Near-Earth Asteroids (Planetary Science)

Little is known about the population of metal-rich Near-Earth Asteroids (NEAs), their number, origin, and relationship with meteorites found on Earth. A new paper by Planetary Science Institute Associate Research Scientist Juan Sanchez explains how near-infrared spectroscopic data of two NEAs reveals new information about the composition and physical properties of these bodies. 

“We find that both NEAs are composed of mostly metal and a small fraction of silicate minerals, similar to mesosiderites, a rare type of stony-iron meteorites found on Earth,” said Sanchez, lead author of the paper “Physical Characterization of Metal-rich Near-Earth Asteroids 6178 (1986 DA) and 2016 ED85” that appears in Planetary Science Journal. PSI Laboratory Technician Neil Pearson is also an author.

“Analysis of their orbits allows us to trace their origin to a region in the outer asteroid belt where the largest metal-rich asteroids reside,” Sanchez said. The asteroid belt is located between the orbits of Mars and Jupiter. 

“According to some studies, there are more than 60 parent bodies represented among iron meteorites found on Earth; however, those parent bodies have not been identified so far. There are also stony-iron meteorites and metal-rich carbonaceous chondrites whose origin is unknown,” Sanchez said. “Because NEAs represent a direct link between meteorites found on Earth and their parent bodies throughout the Solar System, the identification of metal-rich NEAs get us closer to determining the specific origin of the meteorites that derive from them.” 

The larger NEA, (1986 DA), is shown to be primarily metal by using radar data from an earlier study. Metal has a much higher radar reflectivity than rocky bodies composed of silicate minerals. The team’s new near-infrared spectra of 1986 DA confirmed that the asteroid surface is a mixture of about 85% metal and 15% pyroxene, a rock-forming silicate mineral found in igneous and metamorphic rocks. 

 For the other NEA, 2016 ED85, there is no radar data available, but Sanchez finds that its near-infrared spectrum is almost identical to the spectrum of 1986 DA and other metal-rich asteroids, suggesting that this object has a similar composition. 

The paper’s findings are based on observations from the NASA Infrared Telescope Facility on the island of Hawaii. The work was funded by the NASA Near-Earth Object Observations Program, which also funds the NASA Infrared Telescope Facility.

asteroid 1986 DA
An artist’s impression of a close flyby of the metal-rich near-Earth asteroid 1986 DA. Astronomers using the NASA Infrared Telescope Facility have confirmed that the asteroid is made of 85% metal. Credit: Addy Graham/University of Arizona.

Featured image: Photograph of stony-iron meteorite called mesosiderite showing iron-nickel metal mixed with silicate rocky material. Two metal-rich near-Earth asteroids observed by Planetary Science Institute astronomer Juan Sanchez are thought to be made of this rare class of meteorite. Credit: University of Arizona.


Provided by PSI

‘Planet Confusion’ Could Slow Earth-like Exoplanet Exploration (Planetary Science)

When it comes to directly imaging Earth-like exoplanets orbiting faraway stars, seeing isn’t always believing.

A new Cornell study finds that next-generation telescopes used to see exoplanets could confuse Earth-like planets with other types of planets in the same solar system.

With today’s telescopes, dim distant planets are hard to see against the glare of their host stars, but next-generation tools such as the Nancy Grace Roman Space Telescope, currently under development by NASA, will be better at imaging Earth-like planets, which orbit stars at just the right distance to offer prime conditions for life.

“Once we have the capability of imaging Earth-like planets, we’re actually going to have to worry about confusing them with completely different types of planets,” said Dmitry Savransky, associate professor in the Sibley School of Mechanical and Aerospace Engineering (College of Engineering) and the Department of Astronomy (College of Arts and Sciences).

“The future telescopes that will enable these observations will be so huge, expensive, and difficult to build and launch that we can’t afford to waste a single second of time on them,” Savransky said, “which is why it is so important to think through all of these potential issues ahead of time.”

By using Earth’s own solar system as a model of an unexplored star system, Savransky and Dean Keithly, doctoral student in the field of mechanical and aerospace engineering, calculated that even with direct-imaging techniques and the increased capabilities of future, high-powered telescopes, exoplanets as different as Uranus and Earth could be mistaken for one another.

The research was published Sept. 23 in Astrophysical Journal Letters, and details how measurements estimating planet-star separation and brightness can cause “planet confusion.” The modeling finds that when two planets share the same separation and magnitude along their orbits, one planet can be confused for the other.

“I’m asking the question, ‘Is it possible that Jupiter could have the same separation and brightness as Earth? Can we possibly confuse these two things that we have just detected?’ And the answer is yes,” Keithly said. “A habitable Earth-like exoplanet around a star in a different solar system could be confused with many other types of planets.”

Keithly and Savransky – both members of Cornell’s Carl Sagan Institute – identified 21 cases within their solar system model in which an individual planet had the same apparent planet-star separation and brightness as another planet. Using this data, it was calculated that an Earth-like planet could be misidentified with a Mercury-like planet in 36% of randomly generated solar systems; with a Mars-like planet in about 43% of randomly generated solar systems; and with a Venus-like planet in more than 72% of randomly-generated solar systems.

In contrast, confusion between Earth-like planets and larger gas-giant planets similar to Neptune, Saturn and Uranus was less likely, and could occur in 1-4% of randomly generated solar systems.

Confusing planets for one another can be an expensive and time-consuming problem for researchers. Extensive planning and funds go into each use of a high-powered telescope, so the false identification of a habitable exoplanet wastes valuable telescope time. With this problem identified, researchers can design more efficient exoplanet direct-imaging mission surveys. The researchers warn that further improvements to instrument contrast and inner-working angles could exacerbate the problem, and advise that future exoplanet direct-imaging missions make multiple observations to more accurately differentiate between planets.

The research was funded by NASA through the Science Investigation Team of the Nancy Grace Roman Space Telescope.

Featured image: Artistic rendering of the Nancy Grace Roman Space Telescope, currently under development by NASA, which will be used in the search for distant planets beyond our solar system. © Credit:NASA/Provided


Reference: Dean Robert Keithly, Dmitry Savransky. The Solar System as an Exosystem: Planet Confusion. The Astrophysical Journal Letters, 2021; 919 (1): L11 DOI: 10.3847/2041-8213/ac20cf


Provided by Cornell University

UNLV Astronomers May Have Discovered First Planet to Orbit 3 Stars (Planetary Science)

Potential discovery of a circumtriple
planet has implications for bolstering
our understanding of planet formation.

In a distant star system — a mere 1,300 light years away from Earth — UNLV researchers and colleagues may have identified the first known planet to orbit three stars.

Unlike our solar system, which consists of a solitary star, it is believed that half of all star systems, like GW Ori where astronomers observed the novel phenomenon, consist of two or more stars that are gravitationally bound to each other. 

But no planet orbiting three stars – a circumptriple orbit – has ever been discovered. Perhaps until now.

Takeaways

Using observations from the powerful Atacama Large Millimeter/submillimeter Array (ALMA) telescope, UNLV astronomers analyzed the three observed dust rings around the three stars, which are critical to forming planets.

But they found a substantial, yet puzzling, gap in the circumtriple disc. 

The research team investigated different origins, including the possibility that the gap was created by gravitational torque from the three stars. But after constructing a comprehensive model of GW Ori, they found that the more likely, and fascinating, explanation for the space in the disc is the presence of one or more massive planets, Jupiter-like in nature. Gas giants, according to Jeremy Smallwood, lead author and a recent Ph.D. graduate in astronomy from UNLV, are usually the first planets to form within a star system. Terrestrial planets like Earth and Mars follow.

The planet itself cannot be seen, but the finding – highlighted in a September study in the Monthly Notices of the Royal Astronomical Society – suggests that this is the first circumtriple planet ever discovered. Further observations from the ALMA telescope are expected in the coming months, which could provide direct evidence of the phenomenon.

“It’s really exciting because it makes the theory of planet formation really robust,” Smallwood said. “It could mean that planet formation is much more active than we thought, which is pretty cool.”

Featured image: An image of GW Orionis, a triple star system with a mysterious gap in its surrounding dust rings. UNLV astronomers hypothesize the presence of a massive planet in the gap, which would be the first planet ever discovered to orbit three stars. The left image, provided by the Atacama Large Millimeter/submillimeter Array (ALMA) telescope, shows the disc’s ringed structure, with the innermost ring separated from the rest of the disc. The observations in the right image show the shadow of the innermost ring on the rest of the disc. UNLV astronomers used observations from ALMA to construct a comprehensive model of the star system. Credit: ALMA (ESO/NAOJ/NRAO), ESO/Exeter/Kraus et al.


Reference: Jeremy L Smallwood, Rebecca Nealon, Cheng Chen, Rebecca G Martin, Jiaqing Bi, Ruobing Dong, Christophe Pinte, GW Ori: circumtriple rings and planets, Monthly Notices of the Royal Astronomical Society, Volume 508, Issue 1, November 2021, Pages 392–407, https://doi.org/10.1093/mnras/stab2624


Provided by University of Nevada, Las Vegas

Stellar Winds and Evaporating Exoplanet Atmospheres (Planetary Science)

Most stars including the Sun generate magnetic activity that drives a fast-moving, ionized wind and also produces X-ray and ultraviolet emission (often referred to as XUV radiation). XUV radiation from a star can be absorbed in the upper atmosphere of an orbiting planet, where it is capable of heating the gas enough for it to escape from the planet’s atmosphere. M-dwarf stars, the most common type of star by far, are smaller and cooler than the Sun, and they can have very active magnetic fields. Their cool surface temperatures result in their habitable zones (HZ) being close to the star (the HZ is the range of distances within which an orbiting planet’s surface water can remain liquid). Any rocky exoplanets that orbit an M-dwarf in its HZ, because they are close to the star, are especially vulnerable to the effects of photoevaporation which can result in partial or even total removal of the atmosphere. Some theorists argue that planets with substantial hydrogen or helium envelopes might actually become more habitable if photoevaporation removes enough of the gas blanket.

The effects of XUV radiation on exoplanet atmospheres have been studied for almost twenty years, but the effects of the stellar wind on exoplanet atmospheres are only poorly understood. CfA astronomers Laura Harbach, Sofia Moschou, Jeremy Drake, Julian Alvarado-Gomez, and Federico Frascetti and their colleagues have completed simulations modeling the effects of a stellar wind on an exoplanet with a hydrogen-rich atmosphere orbiting close to an M-dwarf star. As an example, they use the exoplanet configuration in TRAPPIST-1, a cool M-dwarf star with a system of seven planets, six of which are close enough to the star to be in its HZ.

The simulations show that, depending on the details, the stellar wind can generate outflows from a planet’s atmosphere. The team finds that both the star’s and the planet’s magnetic fields play significant roles in defining many of the details of the outflow, which could be observed and studied via atomic hydrogen lines in the ultraviolet. The complex simulation results indicate that planets around M-dwarf host stars are likely to display a diverse range of atmospheric properties, and some of the physical conditions can vary over short timescales making observational interpretations of sequential exoplanet transits more complex. The simulation results highlight the need to use 3-D simulations that include magnetic effects in order to interpret observational results for planets around M-dwarf stars.

Featured image: An illustration of the TRAPPIST-1 system of seven planets around an M-dwarf star.  The star has both strong UV and X-ray emission as well as an ionized wind that can evaporate the atmosphere of a planet orbting nearby. Astronomers have completed simulations using the TRAPPIST-1 system parameters that reveal the complex possible consequences of a stellar wind on a planet’s atmosphere. © NASA/ CalTech-JPL


Reference: “Stellar Winds Drive Strong Variations in Exoplanet Evaporative Outflow Patterns and Transit Absorption Signatures,” Laura M. Harbach, Sofia P. Moschou, Cecilia Garraffo, Jeremy J. Drake, Julián D. Alvarado-Gómez, Ofer Cohen, and Federico Fraschetti, The Astrophysical Journal 913, 130, 2021.


Provided by CFA Harvard

SwRI Scientists Confirm Decrease in Pluto’s Atmospheric Density (Planetary Science)

When Pluto passed in front of a star on the night of August 15, 2018, a Southwest Research Institute-led team of astronomers had deployed telescopes at numerous sites in the U.S. and Mexico to observe Pluto’s atmosphere as it was briefly backlit by the well-placed star. Scientists used this occultation event to measure the overall abundance of Pluto’s tenuous atmosphere and found compelling evidence that it is beginning to disappear, refreezing back onto its surface as it moves farther away from the Sun.

The occultation took about two minutes, during which time the star faded from view as Pluto’s atmosphere and solid body passed in front of it. The rate at which the star disappeared and reappeared determined the density profile of Pluto’s atmosphere.

“Scientists have used occultations to monitor changes in Pluto’s atmosphere since 1988,” said Dr. Eliot Young, a senior program manager in SwRI’s Space Science and Engineering Division. “The New Horizons mission obtained an excellent density profile from its 2015 flyby, consistent with Pluto’s bulk atmosphere doubling every decade, but our 2018 observations do not show that trend continuing from 2015.”

Several telescopes deployed near the middle of the shadow’s path observed a phenomenon called a “central flash,” caused by Pluto’s atmosphere refracting light into a region at the very center of the shadow. When measuring an occultation around an object with an atmosphere, the light dims as it passes through the atmosphere and then gradually returns. This produces a moderate slope on either end of the U-shaped light curve. In 2018, refraction by Pluto’s atmosphere created a central flash near the center of its shadow, turning it into a W-shaped curve.

During the August 15, 2018, Pluto occultation event, several telescopes deployed near the middle of the shadow’s path observed a phenomenon called a “central flash,” caused by Pluto’s atmosphere refracting light into a region at the very center of the shadow. This central flash indicates that the occultation data are very robust, bolstering SwRI’s findings that confirm that Pluto’s atmosphere is freezing out onto its surface as it moves farther away from the Sun. © NASA/SwRi

“The central flash seen in 2018 was by far the strongest that anyone has ever seen in a Pluto occultation,” Young said. “The central flash gives us very accurate knowledge of Pluto’s shadow path on the Earth.”

Like Earth, Pluto’s atmosphere is predominantly nitrogen. Unlike Earth, Pluto’s atmosphere is supported by the vapor pressure of its surface ices, which means that small changes in surface ice temperatures would result in large changes in the bulk density of its atmosphere. Pluto takes 248 Earth years to complete one full orbit around the Sun, and its distance varies from its closest point, about 30 astronomical units from the Sun (1 AU is the distance from the Earth to the Sun), to 50 AU from the Sun.

For the past quarter century, Pluto has been receiving less and less sunlight as it moves farther away from the Sun, but, until 2018, its surface pressure and atmospheric density continued to increase. Scientists attributed this to a phenomenon known as thermal inertia.

“An analogy to this is the way the Sun heats up sand on a beach,” said SwRI Staff Scientist Dr. Leslie Young, who specializes in modeling the interaction between the surfaces and atmospheres of icy bodies in the outer solar system. “Sunlight is most intense at high noon, but the sand then continues soaking up the heat over course of the afternoon, so it is hottest in late afternoon. The continued persistence of Pluto’s atmosphere suggests that nitrogen ice reservoirs on Pluto’s surface were kept warm by stored heat under the surface. The new data suggests they are starting to cool.”

The largest known nitrogen reservoir is Sputnik Planitia, a bright glacier that makes up the western lobe of the heart-shaped Tombaugh Regio. The data will help atmospheric modelers improve their understanding of Pluto’s subsurface layers, particularly regarding compositions that are compatible with the observed limits on heat transfer.

Eliot Young will discuss these results at a press conference Monday, October 4, at the 53rd American Astronomical Society Division for Planetary Sciences Annual Meeting.

Featured image: When Pluto passed in front of a star on the night of August 15, 2018, a SwRI-led team of astronomers measured the abundance of Pluto’s atmosphere, shown here in New Horizons 2015 flyby data, as it was briefly backlit by the well-placed star. These data indicate that the surface pressure on Pluto is decreasing and that its nitrogen atmosphere is condensing, forming ice on its surface as the object moves away from the Sun. Courtesy of NASA/JHU-APL/SwRI 


Provided by Southwest Research Institute

Is New Finding an Asteroid or a Comet? It’s Both (Planetary Science)

The newest known example of a rare type of object in the Solar System – a comet hidden among the main-belt asteroids – has been found and studied, according to a new paper by Planetary Science Institute Senior Scientist Henry Hsieh. 

Discovered to be active on July 7, 2021, by the Asteroid Terrestrial-Impact Last Alert System (ATLAS) survey, asteroid (248370) 2005 QN137 is just the eighth main-belt asteroid, out of more than half a million known main-belt asteroids, confirmed to not only be active, but to have been active on more than one occasion. “This behavior strongly indicates that its activity is due to the sublimation of icy material,” said Hsieh, lead author of the paper “Physical Characterization of Main-Belt Comet (248370) 2005 QN173” that he presented at a press conference today at the 53rd annual meeting of the American Astronomical Society’s Division for Planetary Sciences. “As such, it is considered a so-called main-belt comet, and is one of just about 20 objects that have currently been confirmed or are suspected to be main-belt comets, including some that have only been observed to be active once so far. 

“248370 can be thought of as both an asteroid and a comet, or more specifically, a main-belt asteroid that has just recently been recognized to also be a comet. It fits the physical definitions of a comet, in that it is likely icy and is ejecting dust into space, even though it also has the orbit of an asteroid,” Hsieh said. “This duality and blurring of the boundary between what were previously thought to be two completely separate types of objects – asteroids and comets – is a key part of what makes these objects so interesting.” 

Hsieh found that size of the nucleus, the solid object at the “head” of the comet that is surrounded by a dust cloud, is 3.2 kilometers (2 miles) across, the length of the tail in July 2021 was more than 720,000 kilometers (450,000 miles) long, or three times the distance from the Earth to the Moon, and the tail at that time was just 1,400 kilometers (900 miles) wide.  These dimensions mean that if the length of the tail was scaled to the length of a football field, the tail would be just 7 inches wide and the nucleus would be half a millimeter across. 

“This extremely narrow tail tells us that dust particles are barely floating off of the nucleus at extremely slow speeds and that the flow of gas escaping from the comet that normally lifts dust off into space from a comet is extremely weak. Such slow speeds would normally make it difficult for dust to escape from the gravity of the nucleus itself, so this suggests that something else might be helping the dust to escape. For example, the nucleus might be spinning fast enough that it’s helping to fling dust off into space that has been partially lifted by escaping gas.  Further observations will be needed to confirm the rotation speed of the nucleus though,” Hsieh said. 

“Cometary activity is generally thought to be caused by sublimation – the transformation from ice to gas – of icy material in a Solar System object, which means that most comets are found to come from the cold outer Solar System, beyond the orbit of Neptune, and spend most of their time there, with their highly elongated orbits only bringing them close to the Sun and the Earth for short periods at a time,” Hsieh said. “During those times when they are close enough to the Sun, they heat up and release gas and dust as a result of ice sublimation, producing the fuzzy appearance and often spectacular tails associated with comets.” 

By contrast, main-belt asteroids, which orbit between the orbits of Mars and Jupiter, are thought to have been in the warm inner Solar System where we see them today (inside the orbit of Jupiter) for the last 4.6 billion years. Any ice in these objects was expected to be long gone from being so close to the Sun for so long, meaning that cometary activity was not expected to be possible from any of these objects. However, a few rare objects that challenge this expectation called main-belt comets, first discovered as a new class of comets by Hsieh and David Jewitt in 2006, have been found over the last several years. These objects are interesting because a substantial part of Earth’s water is thought to have been delivered via impacts by asteroids from the main asteroid belt when the Earth was being formed. Given that the activity observed for these objects means they are likely to still contain ice, they offer a potential way to test that hypothesis and learn more about the origin of life on Earth by learning more about the abundance, distribution, and physical properties of icy objects in the inner Solar System. 

Hsieh’s work was funded by a grant to PSI from NASA’s Solar System Observations program (Grant 80NSSC19K0869).  This work also made use of observations carried out under the Las Cumbres Observatory Outbursting Objects Key Project (LOOK) and the Faulkes Telescope Project’s Comet Chasers program, and from Lowell Observatory’s Lowell Discovery Telescope and Palomar Observatory’s Hale Telescope. 

orbits
Diagram showing the orbit of (248370) 2005 QN173 along with the main asteroid belt and the orbits of Jupiter, Saturn, Uranus, and Neptune, and Comet Halley for comparison, showing how “traditional” comets like Comet Halley spend much more time far from the Sun in the cold outer Solar System than (248370) 2005 QN173, whose orbit keeps it much closer to the Sun and therefore in a much warmer environment all the time. Credit: Henry H. Hsieh (PSI).

Featured image: Composite image of (248370) 2005 QN173 taken with Palomar Observatory’s Hale Telescope in California on July 12, 2021. The head, or nucleus, of the comet is in the upper left corner, with the tail stretching down and to the right, getting progressively fainter farther from the nucleus. Stars in the field of view appear as short dotted lines due to the apparent motion of Solar System objects against background stars and the process of adding together multiple images to increase the visibility of the tail. Credit: Henry H. Hsieh (PSI), Jana Pittichová (NASA/JPL-Caltech).


Provided by Planetary Science Institute