Tag Archives: #exoplanet

The Second-coldest Imaged Exoplanet Found To Date (Planetary Science)

Astronomers have discovered thousands of exoplanets – planets beyond our solar system – but few have been directly imaged, because they are extremely difficult to see with existing telescopes. A University of Hawaiʻi Institute for Astronomy (IfA) graduate student has beaten the odds and discovered a directly imaged exoplanet, and it’s the closest one to Earth ever found, at a distance of only 35 light years. 

Using the COol Companions ON Ultrawide orbiTS (COCONUTS) survey, IfA graduate student Zhoujian Zhang and a team of astronomers, Michael Liu and Zach Claytor (IfA), William Best (University of Texas at Austin), Trent Dupuy (University of Edinburgh) and Robert Siverd (Gemini Observatory/National Optical-Infrared Astronomy Research Laboratory) identified a planet about six times the mass of Jupiter. The team’s research, published in The Astrophysical Journal Letters, led to the discovery of the low-temperature gas-giant planet orbiting a low-mass red dwarf star, about 6,000 times farther than the Earth orbits the Sun. They dubbed the new planetary system COCONUTS-2, and the new planet COCONUTS-2b.  

During the pandemic, Zhang met with fellow researchers on Zoom to discuss the discovery © University of Hawaiʻi at Manoa

“With a massive planet on a super-wide-separation orbit, and with a very cool central star, COCONUTS-2 represents a very different planetary system than our own solar system,” Zhang explained. The COCONUTS survey has been the focus of his recently-completed PhD thesis, aiming to find wide-separation companions around stars of all different types close to Earth.

Trapped heat helps detect planet 

COCONUTS-2b is the second-coldest imaged exoplanet found to date, with a temperature of just 320 degrees Fahrenheit, which is slightly cooler than most ovens use to bake cookies. The planet can be directly imaged thanks to emitted light produced by residual heat trapped since the planet’s formation. Still, the energy output of the planet is more than a million times weaker than the Sun’s, so the planet can only be detected using lower-energy infrared light. 

“Directly detecting and studying the light from gas-giant planets around other stars is ordinarily very difficult, since the planets we find usually have small-separation orbits and thus are buried in the glare of their host star’s light,” said Liu, Zhang’s thesis advisor. “With its huge orbital separation, COCONUTS-2b will be a great laboratory for studying the atmosphere and composition of a young gas-giant planet.” 

The planet was first detected in 2011 by the Wide-field Infrared Survey Explorer satellite, but it was believed to be a free-floating object, not orbiting a star. Zhang and his collaborators discovered that it is in fact gravitationally bound to a low-mass star, COCONUTS-2A, which is about one-third the mass of the Sun, and about 10 times younger. 

Darkness prevails 

Due to its wide-separation orbit and cool host star, COCONUTS-2b’s skies would look dramatically different to an observer there compared to the skies on Earth. Nighttime and daytime would look basically the same, with the host star appearing as a bright red star in the dark sky.  

Zhang’s discovery has fueled his desire to continue to explore exoplanets, brown dwarfs, and stars. The aspiring astronomer graduated from IfA this summer and will begin his postdoctoral research in fall 2021, with IfA alumnus Brendan Bowler, an astronomy professor at the University of Texas at Austin.  

Featured image: Illustration of gas-giant planet COCONUTS-2b. Credit: B. Bays (SOEST/UH)


Reference: Zhoujian Zhang et al, The Second Discovery from the COol Companions ON Ultrawide orbiTS (COCONUTS) Program: A Cold Wide-Orbit Exoplanet around a Young Field M Dwarf at 10.9 pc, arXiv:2107.02805v1 [astro-ph.EP] arxiv.org/abs/2107.02805


Provided by University of Hawaii at Manoa

First Measurement Of Isotopes in Atmosphere Of Exoplanet (Planetary Science)

An international team of astronomers have become the first in the world to detect isotopes in the atmosphere of an exoplanet. It concerns different forms of carbon in the gaseous giant planet TYC 8998-760-1 b at a distance of 300 light years in the constellation Musca (Fly). The weak signal was measured with ESO’s Very Large Telescope in Chile and seems to indicate that the planet is relatively rich in carbon-13. The astronomers speculate that this is because the planet formed at a great distance from its parent star. The research will be published in the scientific journal Nature on Thursday.

Isotopes are different forms of the same atom, but with varying number of neutrons in the nucleus. For example, carbon with six protons typically has six neutrons (carbon-12), but occasionally seven (carbon-13) or eight (carbon-14). This does not change much the chemical properties of carbon, but isotopes are formed in different ways and often react slightly differently to the prevailing conditions. Isotopes are therefore used in a wide range of research fields: from detecting cardiovascular disease or cancer to studying climate change and determining the age of fossils and rocks.

Quite special

The astronomers were able to distinguish carbon-13 from carbon-12 because it absorbs radiation at slightly different colours. “It is really quite special that we can measure this in an exoplanet atmosphere, at such a large distance,” says Leiden PhD student Yapeng Zhang, first author of the article.

The astronomers had expected to detect about one in 70 carbon atoms to be carbon-13, but for this planet it seems to be twice as much. The idea is that the higher carbon-13 is somehow related to the formation of the exoplanet.

Co-author Paul Mollière, from the Max Planck Institute for Astronomy in Heidelberg, Germany, explains:  “The planet is more than one hundred and fifty times further away from its parent star than our Earth is from our Sun. At such a great distance, ices have possibly formed with more carbon-13, causing the higher fraction of this isotope in the planet’s atmosphere today.”

Story continues below animation of isotopes on exoplanets.

‘My exoplanet’

The planet itself, TYC 8998-760-1 b, was discovered only two years ago by Leiden PhD student Alexander Bohn, co-author of the article. “It’ s awesome that this discovery has been made close to ‘my’ planet. It will probably be the first of many.”

Ignas Snellen, professor in Leiden and for many years the driving force behind this subject, is above all proud. “The expectation is that in the future isotopes will further help to understand exactly how, where and when planets form. This is just the beginning.”

Featured image: Cartoon about the discovery of carbon-13 in the atmosphere of an exoplanet. In reality, the astronomers were sitting behind their desks analysing the spectra of the exoplanet TYC-8998 b made by ESO’s Very Large Telescope in Chile. (c) Daniëlle Futselaar (Artsource)


Scientific paper
The 13CO-rich atmosphere of a young accreting super-Jupiter. By: Yapeng Zhang, Ignas A.G. Snellen, Alexander J. Bohn, Paul Mollière, Christian Ginski, H. Jens Hoeijmakers, Matthew A. Kenworthy, Eric E. Mamajek, Tiffany Meshkat, Maddalena Reggiani, Frans Snik. Nature, 15 July 2021. [original | preprint (pdf)]


Provided by NOVA

Haziness Of Exoplanet Atmospheres Depends On Properties of Aerosol Particles (Planetary Science)

A laboratory study of haze particles produced under different conditions helps explain why some exoplanets may be obscured by hazy atmospheres

Many exoplanets have opaque atmospheres, obscured by clouds or hazes that make it hard for astronomers to characterize their chemical compositions. A new study shows that haze particles produced under different conditions have a wide range of properties that can determine how clear or hazy a planet’s atmosphere is likely to be.

Photochemical reactions in the atmospheres of temperate exoplanets lead to the formation of small organic haze particles. Large amounts of these photochemical hazes form in Earth’s atmosphere every day, yet our planet has relatively clear skies. The reason has to do with how easily haze particles are removed from the atmosphere by deposition processes.

“It’s not just haze production but also haze removal that determines how clear the atmosphere is,” said Xinting Yu, a postdoctoral fellow at UC Santa Cruz and lead author of the study, published July 12 in Nature Astronomy.

Yu and her colleagues measured the properties of haze particles produced in the laboratory under conditions representative of exoplanet atmospheres, including a range of gas compositions, temperatures, and energy sources. Coauthor Xi Zhang, assistant professor of Earth and planetary sciences at UC Santa Cruz, said laboratory experiments like this are essential for understanding haze formation and its impact on observations.

“We can’t bring haze samples back from exoplanets, so we have to try to mimic the atmospheric conditions in the laboratory,” he said.

According to Yu, haze removal depends on a critical material property of the particles called surface energy. “Surface energy describes how cohesive or ‘sticky’ the material is,” she said.

Researchers measured the refractive indices at visible wavelengths (n) for haze samples created under a range of conditions. © Yu et al., Nature Astronomy, 2021

Sticky haze particles readily bond with each other when they collide, growing into larger particles that fall out of the atmosphere onto the surface of the planet (a process called dry deposition). They also make good condensation nuclei for cloud droplets and are easily removed by wet deposition. Hazes produced on Earth typically have high surface energy and are therefore ‘sticky’ and efficiently removed from the atmosphere.

Yu’s laboratory experiments show that the hazes produced in exoplanet atmospheres are highly diverse, with properties that depend on the conditions in which they are produced.

“Some of them are similar to the Earth haze, have high surface energy, and are easy to remove, leading to clear skies,” she said. “But some of them have very low surface energy, like a non-stick pan; they do not bond with other particles very well and remain as small particles hanging in the atmosphere for a long time.”

The study found that a critical factor is the temperature at which the haze particles are created. Hazes produced at around 400 Kelvin (260°F) tended to have the lowest surface energies, leading to less efficient removal and hazier atmospheres. This finding actually corresponds with observed trends, Yu said, noting that exoplanets at temperatures of 400 to 500 K tend to be the haziest.

Cooler planets located in the habitable zones of their host stars are more likely to have clear atmospheres, she said. “We may not have to worry about habitable exoplanets being too hazy for future observations, as hazes tend to have higher surface energies at lower temperatures,” Yu said. “So it is easy to remove these hazes, leaving relatively clear atmospheres.”

Astronomers are looking forward to having a powerful tool for characterizing exoplanet atmospheres with the upcoming James Webb Space Telescope (JWST). When an exoplanet transits across the face of its star, its atmosphere filters the light from the star, giving astronomers with a sensitive enough telescope (like JWST) an opportunity to identify the chemical components of the atmosphere using transmission spectroscopy.

A hazy atmosphere would interfere with transmission spectroscopy, but the hazes themselves may still yield valuable information, according to Zhang.

“Hazes are not featureless,” he said. “With better telescopes, we may be able to characterize the composition of exoplanet hazes and understand their chemistry. But the observations will be very hard to explain without data from laboratory experiments. This study has revealed the huge diversity of haze particles, and understanding their optical properties will be a high priority for future studies.”

In addition to Yu and Zhang, the coauthors of the paper include UCSC undergraduate Austin Dymont, astronomy professor Jonathan Fortney, and graduate student Diana Powell at UC Santa Cruz, as well as scientists at Johns Hopkins University, Cornell University, University of Texas at Austin, and University of Grenoble Alpes, France. This work was supported by NASA and the Heising-Simons Foundation.

Featured image: Xinting Yu, a 51 Pegasi b Postdoctoral Fellow at UCSC, measured the properties of haze particles produced in the laboratory under conditions representative of exoplanet atmospheres. Photo courtesy of Heising-Simons Foundation


Reference: Yu, X., He, C., Zhang, X. et al. Haze evolution in temperate exoplanet atmospheres through surface energy measurements. Nat Astron (2021). https://doi.org/10.1038/s41550-021-01375-3


Provided by University of California Santa Cruz

Scientists Discover New Exoplanet With An Atmosphere Ripe For Study (Planetary Science)

An international group of collaborators, including scientists from NASA’s Jet Propulsion Laboratory and The University of New Mexico, have discovered a new, temperate sub-Neptune sized exoplanet with a 24-day orbital period orbiting a nearby M dwarf star. The recent discovery offers exciting research opportunities thanks to the planet’s substantial atmosphere, small star, and how fast the system is moving away from the Earth.

The research, titled TOI-1231 b: A Temperate, Neptune-Sized Planet Transiting the Nearby M3 Dwarf NLTT 24399, will be published in a future issue of The Astronomical Journal. The exoplanet, TOI-1231 b, was detected using photometric data from the Transiting Exoplanet Survey Satellite (TESS) and followed up with observations using the Planet Finder Spectrograph (PFS) on the Magellan Clay telescope at Las Campanas Observatory in Chile. The PFS is a sophisticated instrument that detects exoplanets through their gravitational influence on their host stars. As the planets orbit their hosts, the measured stellar velocities vary periodically, revealing the planetary presence and information about their mass and orbit.

The observing strategy adopted by NASA’s TESS, which divides each hemisphere into 13 sectors that are surveyed for roughly 28 days, is producing the most comprehensive all-sky search for transiting planets. This approach has already proven its capability to detect both large and small planets around stars ranging from sun-like down to low-mass M dwarf stars. M dwarf stars, also known as a red dwarf, are the most common type of star in the Milky Way making up some 70 percent of all stars in the galaxy.

M dwarfs are smaller and possess a fraction of the sun’s mass and have low luminosity. Because an M dwarf is smaller, when a planet of a given size transits the star, the amount of light that is blocked out by the planet is larger, making the transit more easily detectable. Imagine an Earth-like planet passing in front of a star the size of the sun, it’s going to block out a tiny bit of light; but if it’s passing in front of a star that’s a lot smaller, the proportion of light that’s blocked out will be larger. In a sense, this creates a larger shadow on the surface of the star, making planets around M dwarfs more easily detectable and easier to study.

Although it enables the detection of exoplanets across the sky, TESS’s survey strategy also produces significant observational biases based on orbital period. Exoplanets must transit their host stars at least twice within TESS ‘s observing span to be detected with the correct period by the Science Processing Operations Center (SPOC) pipeline and the Quick Look Pipeline (QLP), which search the 2-minute and 30-minute cadence TESS data, respectively. Because 74 percent of TESS’ total sky coverage is only observed for 28 days, the majority of TESS exoplanets detected have periods less than 14 days. TOI-1231b’s 24-day period, therefore, makes its discovery even more valuable.

NASA JPL scientist Jennifer Burt, the lead author of the paper, along with her collaborators including Diana Dragomir, an assistant professor in UNM’s Department of Physics and Astronomy, measured both the radius and mass of the planet.

“Working with a group of excellent astronomers spread across the globe, we were able to assemble the data necessary to characterize the host star and measure both the radius and mass of the planet,” said Burt. “Those values in turn allowed us to calculate the planet’s bulk density and hypothesize about what the planet is made out of. TOI-1231 b is pretty similar in size and density to Neptune, so we think it has a similarly large, gaseous atmosphere.”

“Another advantage of exoplanets orbiting M dwarf hosts is that we can measure their masses easier because the ratio of the planet mass to the stellar mass is also larger. When the star is smaller and less massive, it makes detection methods work better because the planet suddenly plays a bigger role as it stands out more easily in relation to the star,” explained Dragomir. “Like the shadow cast on the star. The smaller the star, the less massive the star, the more the effect of the planet can be detected.

“Even though TOI 1231b is eight times closer to its star than the Earth is to the Sun, its temperature is similar to that of Earth, thanks to its cooler and less bright host star,” says Dragomir. “However, the planet itself is actually larger than earth and a little bit smaller than Neptune – we could call it a sub-Neptune.”

Burt and Dragomir, who actually initiated this research while they were Fellows at MIT’s Kavli Institute, worked with scientists specializing in observing and characterizing the atmospheres of small planets to figure out which current and future space-based missions might be able to peer into TOI-1231 b’s outer layers to inform researchers exactly what kinds of gases are swirling around the planet. With a temperature around 330 Kelvin or 140 degrees Fahrenheit, TOI-1231b is one of the coolest, small exoplanets accessible for atmospheric studies discovered thus far.

Past research suggests planets this cool may have clouds high in their atmospheres, which makes it hard to determine what types of gases surround them. But new observations of another small, cool planet called K2-18 b broke this trend and showed evidence of water in its atmosphere, surprising many astronomers.

“TOI-1231 b is one of the only other planets we know of in a similar size and temperature range, so future observations of this new planet will let us determine just how common (or rare) it is for water clouds to form around these temperate worlds,” said Burt.

Additionally, with its host star’s high Near-Infrared (NIR) brightness, it makes an exciting target for future missions with the Hubble Space Telescope (HST) and the James Webb Space Telescope (JWST). The first set of these observations, led by one of the paper’s co-authors, should take place later this month using the Hubble Space Telescope.

“The low density of TOI 1231b indicates that it is surrounded by a substantial atmosphere rather than being a rocky planet. But the composition and extent of this atmosphere are unknown!” said Dragomir. “TOI1231b could have a large hydrogen or hydrogen-helium atmosphere, or a denser water vapor atmosphere. Each of these would point to a different origin, allowing astronomers to understand whether and how planets form differently around M dwarfs when compared to the planets around our Sun, for example. Our upcoming HST observations will begin to answer these questions, and JWST promises an even more thorough look into the planet’s atmosphere.”

Another way to study the planet’s atmosphere is to investigate whether gas is being blown away, by looking for evidence of atoms like hydrogen and helium surrounding the planet as it transits across the face of its host star. Generally, hydrogen atoms are almost impossible to detect because their presence is masked by interstellar gas. But this planet-star system offers a unique opportunity to apply this method because of how fast it’s moving away from the Earth.

“One of the most intriguing results of the last two decades of exoplanet science is that, thus far, none of the new planetary systems we’ve discovered look anything like our own solar system,” said Burt. “They’re full of planets between the size of Earth and Neptune on orbits much shorter than Mercury’s, so we don’t have any local examples to compare them to. This new planet we’ve discovered is still weird – but it’s one step closer to being somewhat like our neighborhood planets. Compared to most transiting planets detected thus far, which often have scorching temperatures in the many hundreds or thousands of degrees, TOI-1231 b is positively frigid.”

In closing, Dragomir reflects that “this planet joins the ranks of just two or three other nearby small exoplanets that will be scrutinized with every chance we get and using a wide range of telescopes, for years to come so keep an eye out for new TOI1231b developments!”

** This article is in press at The Astronomical Journal. A pre-print version can be found here: https://arxiv.org/abs/2105.08077.

Featured image: An artist’s impression shows an exoplanet orbiting the Sun-like star. © ESO/M. Kornmesser


Provided by University of New Mexico

Evidence For Substance At Liquid-gas Boundary on Exoplanet WASP-31b (Planetary Science)

One of the properties that make a planet suitable for life is the presence of a weather system. Exoplanets are too far away to directly observe this, but astronomers can search for substances in the atmosphere that make a weather system possible. Researchers from SRON Netherlands Institute for Space Research and the University of Groningen have now found evidence on exoplanet WASP-31b for chromium hydride, which at the corresponding temperature and pressure is on the boundary between liquid and gas. Publication in Astronomy & Astrophysics on February 3rd.

While space probes scan the planets and moons around our Sun for extraterrestrial life, there are hundreds of billions of other stars in our galaxy, most of which probably also surrounded by planets. These so-called exoplanets are too far away to travel to, but we can study them with our telescopes. Although the spatial resolution is usually insufficient to make a picture of an exoplanet, astronomers can still get a lot of information from the fingerprints the atmosphere leaves behind in the light rays of the host star.

From those fingerprints—so-called transmission spectra—astronomers deduce which substances are in the atmosphere of an exoplanet. Those could one day give an indication of extraterrestrial life. Or they can show that there is a condition for life, such as a weather system. For the time being, however, this type of research is limited to giant planets close to their stars, so-called hot Jupiters. These planets are too hot to expect life, but they can already teach us a lot about how possible weather systems work. A research team from SRON Netherlands Institute for Space Research and the University of Groningen has now found evidence for a substance at the boundary between liquid and gas. On Earth this is reminiscent of clouds and rain.

First author Marrick Braam and his colleagues found evidence in Hubble data for chromium hydride (CrH) in the atmosphere of exoplanet WASP-31b. This is a hot Jupiter with a temperature of about 1,200 °C in the twilight zone between day and night—the place where starlight travels through the atmosphere towards Earth. And that happens to be around the temperature at which chromium hydride transitions from liquid to gas at the corresponding pressure in the outer layers of the planet, similar to the conditions for water on Earth. ‘Chromium hydride could play a role in a possible weather system on this planet, with clouds and rain,’ says Braam.

It is the first time that chromium hydride is found on a hot Jupiter and therefore at the right pressure and temperature. Braam: ‘We should add that we only found chromium hydride using the Hubble space telescope. We did not see it in the data from the ground telescope VLT. There are logical explanations for this, but we therefore use the term evidence instead of proof.’

When Hubble’s successor—the James Webb Space Telescope (JWST)—is launched later this year, the team plans to use it for further investigation. ‘Hot Jupiters, including WASP-31b, always have the same side facing their host star,’ says co-author and SRON Exoplanets program leader Michiel Min. ‘We therefore expect a day side with chromium hydride in gaseous form and a night side with liquid chromium hydride. According to theoretical models, the large temperature difference creates strong winds. We want to confirm that with observations.’

Floris van der Tak (SRON/UG), also co-author: ‘With JWST we will be looking for chromium hydride on ten planets with different temperatures, to better understand how the weather systems on those planets depend on the temperature.’

Featured image credit: ESA/ATG medialab, CC BY-SA 3.0 IGO


Reference: Marrick Braam, Floris F. S. van der Tak, Katy L. Chubb, and Michiel Min, ‘Evidence for chromium hydride in the atmosphere of hot Jupiter WASP-31b’, Astronomy & Astrophysics, 2021. https://www.aanda.org/articles/aa/full_html/2021/02/aa39509-20/aa39509-20.html


Provided by SRON

Terrestrial Exoplanet-exomoon Coupled Magnetospheres Work Together To Protect Their Early Atmospheres (Planetary Science)

Habitable terrestrial planets are those occupying orbits around a star that can maintain surface liquid water under a supporting atmosphere. The study of the evolution of planetary atmospheres has been an important research topic for the last several years, in an effort to determine the most important factors in creating a habitable environment for an exoplanet. A planet found in the habitable zone (HZ) around a star does not necessarily mean that the planet is habitable since stellar activity cannot be neglected. For example, it is well known that stellar emissions in the X-ray and extreme ultraviolet (EUV) leads to enhanced ionization and inflation of planetary ionospheres and atmospheres leading to atmospheric loss. Magnetically active stars produce very intense stellar flares that are often, but not always, accompanied by a coronal mass ejection (CME) and stellar winds leading to more atmospheric loss as observed at Mars. When the Martian dynamo shut down ~4.1 Ga and Mars lost its global magnetosphere, the intense solar wind and radiation ravished its atmosphere resulting in ocean evaporation and atmospheric loss transforming Mars from an early warmer and wetter world to a cold and dry planet with an average surface pressure of only 6 mbar. Simply stated, preservation of an atmosphere is one of the chief ingredients for surface habitability.

Artist impression of an exoplanet and an exomoon ©stockimage

It has been shown in previous studies that young stars, particularly one solar mass and smaller, produce extremes such as stellar flares and CMEs that lead to planetary atmospheric loss and that these extremes are closely related to the stellar rotation rate in addition to mass. Johnstone and colleagues found that almost all solar mass stars have converged to slow rotators by 500 Myr after formation producing slower stellar wind. Compared with solar-type stars, it takes a longer time for M-type stars (with lower masses) to slow down. M-type stars also keep magnetically active for a longer period of time. Therefore, the ravaging of planetary atmospheres in the young solar system due to extreme solar radiation and particle fluxes is believed to be a significant factor for our understanding of how an exoplanet will develop and maintain an atmosphere, which is a critical element of a habitable environment. Meanwhile, recent studies show that planetary magnetic fields may protect planets from atmospheric losses, indicating planetary magnetic fields play an important role in planetary habitability.

Another factor that should be considered with respect to the habitability of a terrestrial exoplanet, unrecognized until this paper, concerns the magnetic characteristics of an associated exomoon. With the existence of exoplanets well established, one of the next frontiers is the discovery of exomoons. Today there are a number of exomoon candidates waiting to be confirmed. Since it is without a doubt that they must exist around some exoplanets, it is important to examine what role, if any, they would have in creating an environment that contributes to the habitability of their host planet.

Although speculated for several decades, only recently have scientists determined that our Moon had an extensive magnetosphere for several hundred million years soon after it was formed. Recently, Green and colleagues investigated the expected magnetic topology of the early Earth-Moon magnetospheres and found that they would couple in such a way as to protect the atmosphere of both the Earth and Moon. Assuming similar formation processes for terrestrial planets and their moons, how would these two magnetospheres interact, and what protection would such a combined magnetosphere afford to the atmospheres of early exoplanets and their moons orbiting young stars is the subject of this current research done by James Green and colleagues.

In their paper, they modeled two dipole fields simulating the main field of the exoplanet and the exomoon when the exomoon was at several locations ranging from 4 to 18 Rp from the exoplanet in a stellar wind environment. They take the Earth-Moon dipole strengths presented in their previous paper as their starting conditions illustrating a basic magnetic topology that would evolve over time.

Their results demonstrated that terrestrial exoplanet-exomoon coupled magnetospheres work together to protect the early atmospheres of both the exoplanet and the exomoon. When exomoon magnetospheres are within the exoplanet’s magnetospheric cavity, the exomoon magnetosphere acts like a protective magnetic bubble providing an additional magnetopause confronting the stellar winds when the moon is on the dayside. In addition, magnetic reconnection would create a critical pathway for the atmosphere exchange between the early exoplanet and exomoon. When the exomoon’s magnetosphere is outside of the exoplanet’s magnetosphere it then becomes the first line of defense against strong stellar winds, reducing exoplanet’s atmospheric loss to space.

They have also given a brief discussion on how this type of exomoon would modify radio emissions from magnetized exoplanets.

“Based on the solar system, one of the most well-known wave phenomena for planets with magnetospheres is the release of escaping radio emissions generated by the cyclotron maser instability (CMI) mechanism that derives their named based on the frequency range of the observed emission. It is well known that this type of emission is closely related to the local gyrofrequency above a planet’s aurora and therefore provides important clues to the presence of a planet’s magnetic field and its strength. For example, the Earth’s intense auroral-related radio emission is called Auroral Kilometric Radiation (AKR) and the Jovian Decametric (DAM) emissions are CMI related emissions from Jupiter’s auroral zone.”, said James Green lead author of the study.

Some previous studies believed that the intense auroral related radio emission is the best indicator of planetary magnetospheres. The CMI generated radio emissions produce intense radiation perpendicular to the local magnetic field but the resulting emission cone can be filled-in by refraction or hollow. For instance, the emission cone of AKR has been observed to be relatively well filled in higher frequencies and may be hollow at the lower frequencies, while the Jovian DAM emissions produce hollow emission cones as illustrated in Figure 1A and Figure 1B, respectively. The Earth’s AKR emission cone points tailward with partially overlapping northern and southern hemisphere cones (only the northern hemisphere cone at one frequency is shown in Figure 1A) and is not dependent on Earth’s rotation or the location of the Moon.

Figure 1: A schematic view of radio emission cones, at one emission frequency, modeled after AKR at Earth (panel A) and Io-control DAM at Jupiter (panel B).

One aspect of the Jovian DAM emission is that it is strongly coupled with the moon Io which has a thin atmosphere allowing an ionospheric current to connect field-lines from Jupiter creating a constant current and therefore a constant aurora and resulting CMI related radio emissions. These Io controlled DAM emissions produce, hollow emission cones that move with Io around the planet which has an orbital period of about 42 hours. Io is in an elliptical orbit and is so close to the planet Jupiter that the energy from the very strong tidal forces is dissipated through volcanic activity on the moon that are so strong that a torus of escaping material is left in its wake that stretches around Jupiter. Alfven waves are set up in the Io torus that produce magnetospheric currents stretching all the way to the Jovian auroral regions that also trigger additional CMI emissions that produce a set of nested hollow emission cones. Near equatorial spacecraft, such as the Voyager 1 and 2 missions, observed the Io-DAM emissions as a series of arc-like structures in frequency-time spectrograms. The shape of the nested emission cones, in frequency-time spectrograms, are strongly controlled by the higher moments of the Jovian magnetic fields since the strongly right-hand polarized DAM radiation propagating from the source over these intense magnetic islands suffer significant refraction. In addition, Jupiter’s moon Ganymede also produces aurora, not only in the upper atmosphere of Jupiter, but also in the very tenuous Ganymede atmosphere since that moon is the only one in the solar system that has been observed to currently generate its own magnetosphere. The rather small Ganymede magnetosphere is anti aligned with Jupiter which is similar to the lower left panel in Figure 2. These connected field lines also facilitate the exchange of atmospheric constituents.

Figure 2: Simulation results for the exoplanet- exomoon coupled magnetospheres with the dipoles aligned (top panels) and dipoles antialigned (bottom panels) with three exomoon locations. This figure illustrates the evolution of the magnetic topology of an exoplanet-exomoon over a period of time delineated by the exomoon moving away from the exoplanet (expected to be a time period of 100’s of millions of years). © James Green et al.

In the case of both an exoplanet and exomoon with magnetospheres, Green and colleagues now observed a new situation in which the exomoon would be controlling the location of a potential CMI emission cone and producing either a hollow or filled in emission pattern. From a distant radio observer, periodicities in an observed CMI emission cone pattern along with the radio emission frequency not only could point to the existence and strength of an exoplanet’s magnetosphere but also the existence of an exomoon. The extent of the emission cone, ranging from completely hollow to completely filled-in provides additional information about the extent of the exoplanet’s ionosphere. In this manner, the detection and analysis of CMI generated radio emissions may provide additional information as to the habitability of the exoplanet.

“In order to understand the long-term evolution of exoplanetary atmospheres and their suitability for creating a habitable environment that may host life, we must understand not only the stellar environment, but also whether these planets and their associated moons have magnetic fields.”, said James Green.

Researchers concluded that, “future detection of exoplanet-exomoon magnetic fields from the detection of CMI radio emissions will provide a wealth of new information that will draw our attention to these systems having a greater chance of habitability.”

References: James Green, Scott Boardsen, Chuanfei Dong, “Magnetospheres of Terrestrial Exoplanets and Exomoons: Implications for Habitability and Detection”, ArXiv, pp. 1-13, 2020. https://arxiv.org/abs/2012.11694v1

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Cornell Postdoc Detects Possible Exoplanet Radio Emission (Planetary Science)

By monitoring the cosmos with a radio telescope array, an international team of scientists has detected radio bursts emanating from the constellation Boötes – that could be the first radio emission collected from a planet beyond our solar system.

In this artistic rendering of the Tau Boötes b system, the lines representing the invisible magnetic field are shown protecting the hot Jupiter planet from solar wind. © Jack Madden/Cornell University

The team, led by Cornell postdoctoral researcher Jake D. Turner, Philippe Zarka of the Observatoire de Paris – Paris Sciences et Lettres University and Jean-Mathias Griessmeier of the Université d’Orléans will publish their findings in the forthcoming research section of Astronomy & Astrophysics, on Dec. 16.

“We present one of the first hints of detecting an exoplanet in the radio realm,” Turner said. “The signal is from the Tau Boötes system, which contains a binary star and an exoplanet. We make the case for an emission by the planet itself. From the strength and polarization of the radio signal and the planet’s magnetic field, it is compatible with theoretical predictions.”

Among the co-authors is Turner’s postdoctoral advisor Ray Jayawardhana, the Harold Tanner Dean of the College of Arts and Sciences, and a professor of astronomy.

“If confirmed through follow-up observations,” Jayawardhana said, “this radio detection opens up a new window on exoplanets, giving us a novel way to examine alien worlds that are tens of light-years away.”

Using the Low Frequency Array (LOFAR), a radio telescope in the Netherlands, Turner and his colleagues uncovered emission bursts from a star-system hosting a so-called hot Jupiter, a gaseous giant planet that is very close to its own sun. The group also observed other potential exoplanetary radio-emission candidates in the 55 Cancri (in the constellation Cancer) and Upsilon Andromedae systems. Only the Tau Boötes exoplanet system – about 51 light-years away – exhibited a significant radio signature, a unique potential window on the planet’s magnetic field.

Observing an exoplanet’s magnetic field helps astronomers decipher a planet’s interior and atmospheric properties, as well as the physics of star-planet interactions, said Turner, a member of Cornell’s Carl Sagan Institute.

Earth’s magnetic field protects it from solar wind dangers, keeping the planet habitable. “The magnetic field of Earth-like exoplanets may contribute to their possible habitability,” Turner said, “by shielding their own atmospheres from solar wind and cosmic rays, and protecting the planet from atmospheric loss.”

Two years ago, Turner and his colleagues examined the radio emission signature of Jupiter and scaled those emissions to mimic the possible signatures from a distant Jupiter-like exoplanet. Those results became the template for searching radio emission from exoplanets 40 to 100 light-years away.

After poring over nearly 100-hours of radio observations, the researchers were able to find the expected hot Jupiter signature in Tau Boötes. “We learned from our own Jupiter what this kind of detection looks like. We went searching for it and we found it,” Turner said.

The signature, though, is weak. “There remains some uncertainty that the detected radio signal is from the planet. The need for follow-up observations is critical,” he said.

Turner and his team have already begun a campaign using multiple radio telescopes to follow up on the signal from Tau Boötes.

In addition to Turner, Jayawardhana, Griessmeier and Zarka, the co-authors are Laurent Lamy and Baptiste Cecconi of the Observatoire de Paris, France; Joseph Lazio from NASA’s Jet Propulsion Laboratory; J. Emilio Enriquez and Imke de Pater from the University of California, Berkeley; Julien N. Girard from Rhodes University, Grahamstown, South Africa; and Jonathan D. Nichols from the University of Leicester, United Kingdom.

Turner, who laid the groundwork for this research while earning his doctorate at the University of Virginia, received funding from the National Science Foundation.

References: J.D. Turner, P. Zarka, J.-M. Griessmeier, J. Lazio, B. Cecconi, J.-E. Enriquez, J.N. Girard, R. Jayawardhana, L. Lamy, J.D. Nichols, and I. Pater, “The search for radio emission from the exoplanetary systems 55 Cancri, upsilon Andromedae, and tau Boötis using LOFAR beam-formed observations”, A&A, 2020. https://www.aanda.org/component/article?access=doi&doi=10.1051/0004-6361/201937201

Provided by Cornell University

Mi­ra­cu­lous View Of a Nas­cent Planet (Planetary Science)

Mid-infrared ima­ging ob­ser­va­tions of PDS 70 b, an exo­planet that is still in the pro­cess of form­a­tion, provide unique in­sight into its at­mo­spheric prop­er­ties and the mech­an­isms by which plan­ets emerge from a cir­cum­s­tel­lar disk of gas and dust.

Image of the PDS 70 planetary system and circumstellar disk, containing the nascent planet PDS 70 b (from ref. [2])

Al­most 25 years ago to the day, on 23 Novem­ber 1995, the very first dis­cov­ery of a planet out­side our solar sys­tem was re­por­ted [1]. That break­through triggered a de­vel­op­ment that is of­ten dubbed the ‘exo­planet re­volu­tion’ — and one that earned the Swiss as­tro­nomers Michel Mayor and Didier Queloz the 2019 No­bel Prize in Phys­ics (shared with the Canadian-​American cos­mo­lo­gist James Peebles). Today, hun­dreds of new exo­plan­ets are dis­covered each year, and the rate of dis­cov­ery is still ac­cel­er­at­ing. At the same time, the level of de­tail with which the at­mo­spheres and or­bits of exo­plan­ets can be char­ac­ter­ized is ever in­creas­ing.

A case in point is a newly pub­lished study [2] by a team of in­ter­na­tional as­tro­nomers led by Dr. To­mas Stolker, a former ETH fel­low in the group of Pro­fessor Sascha Quanz at the In­sti­tute for Particle Phys­ics and As­tro­phys­ics and now a fel­low at Leiden Uni­ver­sity in the Neth­er­lands. The team has car­ried out a unique sur­vey at mid-​infrared wavelengths and ob­tained fresh in­sight into the pre­vi­ously dis­covered exo­planet PDS 70 b. In­triguingly, this gi­ant planet is still in the pro­cess of form­a­tion as it sweeps up gas and dust in its or­bit around a young star. The new ob­ser­va­tions help to gain a deeper un­der­stand­ing on the at­mo­spheres of such nas­cent plan­ets and the phys­ical pro­cesses that drive their form­a­tion.

The sur­vey that Stolker leads is known as MIR­ACLES (Mid-​InfraRed At­mo­spheric Char­ac­ter­iz­a­tion of Long-​period Exo­plan­ets and Sub­stel­lar com­pan­ions) and was de­signed to sys­tem­at­ic­ally char­ac­ter­ize the at­mo­spheres of dir­ectly im­aged exo­plan­ets and brown dwarfs at wavelengths of 4–5 µm [3]. The team — which brings to­gether sci­ent­ists from Switzer­land, Ger­many, the Neth­er­lands and the US — uses the Very Large Tele­scope (VLT) at the Paranal Ob­ser­vat­ory in Chile com­bined with high-​contrast ima­ging tech­niques to dir­ectly de­tect the light com­ing from re­l­at­ively young plan­ets which are still glow­ing at in­frared wavelengths. By ob­serving at 4–5 µm, the sur­vey provides an il­lu­min­at­ing win­dow into the phys­ical and chem­ical prop­er­ties of these gi­ant exo­plan­ets.

For the study now re­por­ted, the re­search­ers poin­ted the VLT to PDS 70, which is a young star in the con­stel­la­tion Cen­taurus at some 370 lightyears away. What makes this star stand out is that in its or­bit two plan­ets had been pre­vi­ously dis­covered, both of which are still grow­ing within the natal en­vir­on­ment of the cir­cum­s­tel­lar disk (see the fig­ure). One of the plan­ets, PDS 70 b, was now char­ac­ter­ized more closely as part of the MIR­ACLES sur­vey. The new data, to­gether with the re-​examination of archival ima­ging data, en­abled Stolker et al. to place im­port­ant con­straints on the mass, ra­dius and lu­min­os­ity of PDS 70 b. The ana­lysis also showed that the planet is em­bed­ded in an ex­ten­ded dusty en­vir­on­ment, which is re­plen­ished by gas and dust from the cir­cum­s­tel­lar disk.

Res­ults like these un­der­line that the exo­planet re­volu­tion is alive and well. In fact, a new im­ager and spec­tro­graph — ERIS, and to which ETH sci­ent­ists have made ma­jor con­tri­bu­tions — will enter into op­er­a­tion at the VLT in the near fu­ture. This in­stru­ment will make it pos­sible to de­tect and char­ac­ter­ize even fainter and closer-in exo­plan­ets.

References

  1. Mayor M, Queloz D: A Jupiter-mass companion to a solar-type star. Nature 378, 355 (1995). https://doi.org/10.1038/378355a0 Free-to-read version
  2. Stolker T, Marleau G-D, Cugno G, Mollière P, Quanz SP, Todorov KO, Kühn J: MIRACLES: atmospheric characterization of directly imaged planets and substellar companions at 4–5 μm – II. Constraints on the mass and radius of the enshrouded planet PDS 70 b. Astronomy & Astrophysics, 643, A13 (2020). https://doi.org/10.1051/0004-6361/202038878
  3. Stolker T, Quanz SP, Todorov KO, Kühn J, Mollière P, Meyer MR, Currie T, Daemgen S, Lavie B: MIRACLES: atmospheric characterization of directly imaged planets and substellar companions at 4–5 μm – I. Photometric analysis of β Pic b, HIP 65426 b, PZ Tel B, and HD 206893 B. Astronomy & Astrophysics, 635, A182 (2020). https://doi.org/10.1051/0004-6361/201937159
  4. T. Stolker et al., “MIRACLES: atmospheric characterization of directly imaged planets and substellar companions at 4–5 μm – II. Constraints on the mass and radius of the enshrouded planet PDS 70 b”, Published in Astronomy & Astrophysics, 2020, 644, A13. https://www.aanda.org/articles/aa/full_html/2020/12/aa38878-20/aa38878-20.html

Provided by ETH Zurich

NYUAD Study Finds Stellar Flares Can Lead To The Diminishment Of a Planet’s Habitability (Planetary Science)

Fast facts:

* Planetary habitability, defined by a planet’s ability to sustain liquid water on its surface, is one of the most important concepts in exoplanet science.
* Exoplanets (planets that orbit stars outside of our solar system) are subject to space weather in the form of stellar flares, emissions of radiation from stars.
* These emissions consist of extreme ultraviolet (XUV) photons and charged particles and can alter the upper atmosphere of the exoplanet. Current methods to determine a planet’s ability to support life do not take stellar activity into consideration.

Abu Dhabi, UAE, November 9, 2020: In a new study researchers, led by Research Scientist Dimitra Atri of the Center for Space Science at NYU Abu Dhabi (NYUAD), identified which stars were most likely to host habitable exoplanets based on the calculated erosion rates of the planetary atmospheres.

An artist’s conception of HD 209458 b, an exoplanet whose atmosphere is being torn off at more than 35,000 km/hour by the radiation of its close-by parent star. This hot Jupiter was the first alien world discovered via the transit method, and the first planet to have its atmosphere studied. ©NASA/European Space Agency/Alfred Vidal-Madjar (Institut d’Astrophysique de Paris, CNRS).

In the paper titled Stellar flares versus luminosity: XUV-induced atmospheric escape and planetary habitability, published in the journal Monthly Notices of Royal Astronomical Society: Letters, Atri and graduate student Shane Carberry Mogan present the process of analyzing flare emission data from NASA’s TESS (Transiting Exoplanet Survey Satellite) observatory.

It was found that more frequent, lower energy flares had a greater impact on an exoplanet’s atmosphere than less frequent, higher energy flares. The researchers also determined how different types of stars extreme ultraviolet radiation (XUV) through stellar flares, and how nearby planets are affected.

The ability to sustain an atmosphere is one of the most important requirements for a habitable planet. This research provides new insights into the habitability of exoplanets, as the effects of stellar activity were not well understood. This study also highlights the need for better numerical modeling of atmospheric escape – how planets release atmospheric gases into space – as it can lead to the erosion of atmosphere and the diminishment of the planet’s habitability.

“Given the close proximity of exoplanets to host stars, it is vital to understand how space weather events tied to those stars can affect the habitability of the exoplanet,” said Atri. “The next research step would be to expand our data set to analyze stellar flares from a larger variety of stars to see the long-term effects of stellar activity, and to identify more potentially habitable exoplanets.”

Provided by NYU Abu Dhabi