Superflares, extreme radiation bursts from stars, have been suspected of causing lasting damage to the atmospheres and thus habitability of exoplanets. A newly published study found evidence that they only pose a limited danger to planetary systems, since the radiation bursts do not explode in the direction of the exoplanets.
Using optical observations from the Transiting Exoplanet Survey Satellite (TESS), astronomers at the Leibniz Institute for Astrophysics Potsdam (AIP), in collaboration with scientists in the US and Spain, studied large superflares on young, small stars. These class of stars, also called “red dwarfs,” have a lower temperature and mass than our own Sun.
Many exoplanets have been found around these types of stars. The question is whether these exoplanets are habitable, since red dwarfs are more active than our Sun, and flare much more frequently and intensely. Flares are magnetic explosions in the atmospheres of stars that expel intense electromagnetic radiation into space. Large flares are associated with the emission of energetic particles that can hit exoplanets orbiting the flaring star, and alter or even evaporate the planetary atmospheres.
Ekaterina Ilin, PhD student at AIP, and the team developed a method to locate where on the stars’ surface flares are launched from. “We discovered that extremely large flares are launched from near the poles of red dwarf stars, rather than from their equator, as is typically the case on the Sun,” said Ilin. “Exoplanets that orbit in the same plane as the equator of the star, like the planets in our own solar system, could therefore be largely protected from such superflares, as these are directed upwards or downwards out of the exoplanet system. This could improve the prospects for the habitability of exoplanets around small host stars, which would otherwise be much more endangered by the energetic radiation and particles associated with flares compared to planets in the solar system.”
The detection of these flares is further evidence that strong and dynamic concentrations of stellar magnetic fields, which can manifest themselves as dark spots and flares, form close to the rotational poles of fast-rotating stars. The existence of such “polar spots” has long been suspected from indirect reconstruction techniques like (Zeeman) Doppler Imaging of stellar surfaces, but has not been detected directly so far. The team achieved this by analysing white-light flares on fast-rotating M dwarf stars that last long enough to have their brightness modulated by being rotated in and out of view on the stellar surface. The authors were able to directly infer the latitude of the flaring region from the shape of the light curve, and also showed that the detection method was not biased towards particular latitudes. “I’m particularly excited that we were finally able to substantiate the existence of polar spots for these fast rotating stars. In the future, this will help us to understand their magnetic field structure better,” adds Katja Poppenhäger, head of the section Stellar Physics and Exoplanets at AIP.
The scientists searched the entire archive of observations obtained by TESS for stars that exhibit large flares by processing the light curves of over 3000 red dwarf stars, totalling over 400 years of cumulative observing time. Among these stars, they found four which were suited for the new method. Their results show that all four flares occurred above ∼55 deg latitude, that is, much closer to the pole than solar flares and spots, which usually occur below 30 deg. This result, even with only four flares, is significant: if flares were spread equally across the stellar surface, the chances of finding four flares in a row at such high latitudes would be about 1:1000. This has implications for models of the magnetic fields of stars and for the habitability of exoplanets that orbit them.
Featured image: Small stars flare actively and expel particles that can alter and evaporate the atmospheres of planets that orbit them. New findings suggest that large superflares prefer to occur at high latitudes, sparing planets that orbit around the stellar equator. Credit: AIP/ J. Fohlmeister
Ekaterina Ilin, Katja Poppenhaeger, Sarah J Schmidt, Silva P Järvinen, Elisabeth R Newton, Julián D Alvarado-Gómez, J Sebastian Pineda, James R A Davenport, Mahmoudreza Oshagh, Ilya Ilyin, Giant white-light flares on fully convective stars occur at high latitudes, Monthly Notices of the Royal Astronomical Society, 2021.
Using the Atacama Large Millimetre/submillimeter Array (ALMA), in which the European Southern Observatory (ESO) is a partner, astronomers have unambiguously detected the presence of a disc around a planet outside our Solar System for the first time. The observations will shed new light on how moons and planets form in young stellar systems.“Our work presents a clear detection of a disc in which satellites could be forming,” says Myriam Benisty, a researcher at the University of Grenoble, France, and at the University of Chile, who led the new research published today in The Astrophysical Journal Letters. “Our ALMA observations were obtained at such exquisite resolution that we could clearly identify that the disc is associated with the planet and we are able to constrain its size for the first time,” she adds.
The disc in question, called a circumplanetary disc, surrounds the exoplanet PDS 70c, one of two giant, Jupiter-like planets orbiting a star nearly 400 light-years away. Astronomers had found hints of a “moon-forming” disc around this exoplanet before but, since they could not clearly tell the disc apart from its surrounding environment, they could not confirm its detection — until now.
In addition, with the help of ALMA, Benisty and her team found that the disc has about the same diameter as the distance from our Sun to the Earth and enough mass to form up to three satellites the size of the Moon.
But the results are not only key to finding out how moons arise. “These new observations are also extremely important to prove theories of planet formation that could not be tested until now,” says Jaehan Bae, a researcher from the Earth and Planets Laboratory of the Carnegie Institution for Science, USA, and author on the study.
Planets form in dusty discs around young stars, carving out cavities as they gobble up material from this circumstellar disc to grow. In this process, a planet can acquire its own circumplanetary disc, which contributes to the growth of the planet by regulating the amount of material falling onto it. At the same time, the gas and dust in the circumplanetary disc can come together into progressively larger bodies through multiple collisions, ultimately leading to the birth of moons.
But astronomers do not yet fully understand the details of these processes. “In short, it is still unclear when, where, and how planets and moons form,” explains ESO Research Fellow Stefano Facchini, also involved in the research.
“More than 4000 exoplanets have been found until now, but all of them were detected in mature systems. PDS 70b and PDS 70c, which form a system reminiscent of the Jupiter-Saturn pair, are the only two exoplanets detected so far that are still in the process of being formed,” explains Miriam Keppler, researcher at the Max Planck Institute for Astronomy in Germany and one of the co-authors of the study .
“This system therefore offers us a unique opportunity to observe and study the processes of planet and satellite formation,” Facchini adds.
PDS 70b and PDS 70c, the two planets making up the system, were first discovered using ESO’s Very Large Telescope (VLT) in 2018 and 2019 respectively, and their unique nature means they have been observed with other telescopes and instruments many times since .
The latest high resolution ALMA observations have now allowed astronomers to gain further insights into the system. In addition to confirming the detection of the circumplanetary disc around PDS 70c and studying its size and mass, they found that PDS 70b does not show clear evidence of such a disc, indicating that it was starved of dust material from its birth environment by PDS 70c.
An even deeper understanding of the planetary system will be achieved with ESO’s Extremely Large Telescope (ELT), currently under construction on Cerro Armazones in the Chilean Atacama desert. “The ELT will be key for this research since, with its much higher resolution, we will be able to map the system in great detail,” says co-author Richard Teague, a researcher at the Center for Astrophysics | Harvard & Smithsonian, USA. In particular, by using the ELT’s Mid-infrared ELT Imager and Spectrograph (METIS), the team will be able to look at the gas motions surrounding PDS 70c to get a full 3D picture of the system.
 Despite the similarity with the Jupiter-Saturn pair, note that the disc around PDS 70c is about 500 times larger than Saturn’s rings.
 PDS 70b was discovered using the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument, while PDS 70c was found using the VLT’s Multi Unit Spectroscopic Explorer (MUSE). The two-planet system has been investigated using the X-shooter instrument too, also installed on ESO’s VLT.
The team is composed of Myriam Benisty (Unidad Mixta Internacional Franco-Chilena de Astronomía, CNRS, Departamento de Astronomía, Universidad de Chile, Santiago de Chile, Chile and Université Grenoble Alpes, CNRS, Grenoble, France [UGA]), Jaehan Bae (Earth and Planets Laboratory, Carnegie Institution for Science, Washington DC, USA), Stefano Facchini (European Southern Observatory, Garching bei München, Germany), Miriam Keppler (Max Planck Institute for Astronomy, Heidelberg, Germany [MPIA]), Richard Teague (Center for Astrophysics | Harvard & Smithsonian, Cambridge, MA, USA [CfA]), Andrea Isella (Department of Physics and Astronomy, Rice University, Houston, TX, USA), Nicolas T. Kurtovic (MPIA), Laura M. Perez (Departamento de Astronomía, Universidad de Chile, Santiago de Chile, Chile [UCHILE]), Anibal Sierra (UCHILE), Sean M. Andrews (CfA), John Carpenter (Joint ALMA Observatory, Santiago de Chile, Chile), Ian Czekala (Department of Astronomy and Astrophysics, Pennsylvania State University, PA, USA, Center for Exoplanets and Habitable Worlds, Davey Laboratory, Pennsylvania State University, PA, USA, Center for Astrostatistics, Davey Laboratory, Pennsylvania State University, PA, USA and Institute for Computational & Data Sciences, Pennsylvania State University, PA, USA), Carsten Dominik (Anton Pannekoek Institute for Astronomy, University of Amsterdam, The Netherlands), Thomas Henning (MPIA), Francois Menard (UGA), Paola Pinilla (MPIA and Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, UK) and Alice Zurlo (Núcleo de Astronomía, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Santiago de Chile, Chile and Escuela de Ingeniería Industrial, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Santiago de Chile, Chile).
While studying two exoplanets in a bright, nearby star system, the CHEOPS satellite identified the third known planet in the system, which unexpectedly crossed the star’s surface. This transit reveals exciting details about an “unprecedented planet”, as emphasized by the scientific team headed by the Universities of Geneva and Bern and members of the National Research Center PlanetS.
So-called photo bombs – when an object or person unexpectedly falls into the camera’s field of view while taking a photo – happen every day. Sometimes it’s an acquaintance, other times a stranger, or maybe a bird. However, it is seldom an entire planet. But that is exactly what happened when the CHEOPS space telescope, managed by Switzerland, recorded images of a planetary system 50 light years away.
A planet like no other
The planetary system is located in the constellation Lupus (Latin for wolf), around a star called Nu2 Lupi, which is visible to the naked eye (but not from Switzerland). In 2019, Swiss astronomers announced the discovery of three exoplanets around this bright, sun-like star. The three exoplanets have masses between those of Earth and Neptune (17 times that of Earth) and take 12, 28 and 107 days to orbit their parent star. “What makes these exoplanets really stand out is that we can see them pass by right in front of their star; one speaks of a ‘transit’ ”, says Yann Alibert, professor of astrophysics at the University of Bern and co-author of the study, which has just been published in the journal Nature Astronomywas published. “We already knew that about the two inner planets, which prompted us to focus CHEOPS on the system in the first place. The third planet, however, is quite a long way from the star, and nobody expected its transit, ”adds Alibert. The further away the planet is from its star, the less likely it will be a transit.
It is the first time that an exoplanet with an orbital period of over 100 days – which is a distance from the star somewhere between that of Mercury and Venus from the Sun – has been discovered that crosses a star that is bright enough to to be visible to the naked eye.
“Due to its relatively long orbital period, the amount of stellar radiation reaching the planet is mild compared to many other exoplanets discovered. The less radiation a planet receives, the less it changes over time. Therefore, a planet with a long period could have retained more information about its formation, ”says David Ehrenreich, professor at the University of Geneva and missionary scientist from CHEOPS, who was involved in the study. But the few such exoplanets that astronomers had found so far orbited weakly shining stars. In other words, little of its light reaches the earth, making it difficult to study. Not so this time: «Since its bright host star is very close to us, it is easier to examine.
Further findings from other telescopes
The high-precision measurements from CHEOPS show that the third planet, now called Lupi d, is around 2.5 times as large as the earth and almost 9 times as heavy. By combining these measurements with archive data from other observatories and numerical models developed by the University of Bern, Laetitia Delrez, visiting researcher at the University of Geneva and lead author of the study, was able to precisely characterize the density and composition of the planet and its neighbors. “The innermost planet is mainly rocky, while the two outer ones appear to be enveloped in envelopes of hydrogen and helium gases, among which they contain large amounts of water,” explains Delrez. This is far more water than the earth has: a quarter of the mass of any planet is water, compared to less than 0.1% in the case of the earth. However, this water is not liquid, but is in the form of high pressure ice or high temperature steam, which makes the planets uninhabitable. But these insights could only be the beginning.
“Now that we have discovered that all three planets are transiting and have precisely measured their properties, the next step is to study them with larger and more powerful instruments than CHEOPS. Like the Hubble Space Telescope or its successor, the James Webb Space Telescope. They could reveal further details, such as the composition of the atmosphere, ”says Ehrenreich. Given its overall properties and orbit, planet d will become the poster child for exoplanets with an atmosphere of mild temperature around a sun-like star.
The collection of 19 open access papers in the SPIE Journal of Astronomical Telescopes, Instruments, and Systems, highlights recent research on the starlight-suppression technology that will be used in upcoming space missions
Section topics range from starshade programs and missions, to various aspects of related technologies, including formation flying, deployment, high-contrast imaging, and performance modeling. Together, the 19 open access articles provide an extensive overview and current status of this exponentially growing field.
“The starshade is a technology that has seen rapid development and wide interest at many institutions,” the editors write in their introduction. “Many of the advances in this field are spread over many journals and meetings. As a result they are difficult to collect in a single location in order to get a good view of the state of starshades…. As interest in developing starshade-based missions grows, we hope that this special section will serve as a tutorial, providing enough of a background for potential investigators who are not familiar with starshades to have a current overview of the field in one location.”
The special section guest editors – Jonathan W. Arenberg, of Northrop Grumman, Anthony D. Harness, of Princeton University, and Rebecca M. Jensen-Clem, of the University of California, Santa Cruz – are all members of the NASA-chartered Starshade Science and Industry Partnership’s Technology and Science Working Group.
Featured image: Example of an Inner Disk Subsystem, a key element of furled starshade architecture, deployed (left) and stowed, from “Demonstration of deployment repeatability of key subsystems of a furled starshade architecture.” Arya et al, one of the papers in the JATIS special section focusing on starshade technology. Credit: Jet Propulsion Laboratory.
New survey reveals that the presence of gaps in planet-forming disks is more common to higher mass stars and to the development of large, gaseous exoplanets
Using data for more than 500 young stars observed with the Atacama Large Millimeter/Submillimeter Array (ALMA), scientists have uncovered a direct link between protoplanetary disk structures–the planet-forming disks that surround stars–and planet demographics. The survey proves that higher mass stars are more likely to be surrounded by disks with “gaps” in them and that these gaps directly correlate to the high occurrence of observed giant exoplanets around such stars. These results provide scientists with a window back through time, allowing them to predict what exoplanetary systems looked like through each stage of their formation.
“We found a strong correlation between gaps in protoplanetary disks and stellar mass, which can be linked to the presence of large, gaseous exoplanets,” said Nienke van der Marel, a Banting fellow in the Department of Physics and Astronomy at the University of Victoria in British Columbia, and the primary author on the research. “Higher mass stars have relatively more disks with gaps than lower mass stars, consistent with the already known correlations in exoplanets, where higher mass stars more often host gas-giant exoplanets. These correlations directly tell us that gaps in planet-forming disks are most likely caused by giant planets of Neptune mass and above.”
Gaps in protoplanetary disks have long been considered as overall evidence of planet formation. However, there has been some skepticism due to the observed orbital distance between exoplanets and their stars. “One of the primary reasons that scientists have been skeptical about the link between gaps and planets before is that exoplanets at wide orbits of tens of astronomical units are rare. However, exoplanets at smaller orbits, between one and ten astronomical units, are much more common,” said Gijs Mulders, assistant professor of astronomy at Universidad Adolfo Ibáñez in Santiago, Chile, and co-author on the research. “We believe that planets that clear the gaps will migrate inwards later on.”
The new study is the first to show that the number of gapped disks in these regions matches the number of giant exoplanets in a star system. “Previous studies indicated that there were many more gapped disks than detected giant exoplanets,” said Mulders. “Our study shows that there are enough exoplanets to explain the observed frequency of the gapped disks at different stellar masses.”
The correlation also applies to star systems with low-mass stars, where scientists are more likely to find massive rocky exoplanets, also known as Super-Earths. Van der Marel, who will become an assistant professor at Leiden University in the Netherlands beginning September 2021 said, “Lower mass stars have more rocky Super-Earths–between an Earth mass and a Neptune mass. Disks without gaps, which are more compact, lead to the formation of Super-Earths.”
This link between stellar mass and planetary demographics could help scientists identify which stars to target in the search for rocky planets throughout the Milky Way. “This new understanding of stellar mass dependencies will help to guide the search for small, rocky planets like Earth in the solar neighborhood,” said Mulders, who is also a part of the NASA-funded Alien Earths team. “We can use the stellar mass to connect the planet-forming disks around young stars to exoplanets around mature stars. When an exoplanet is detected, the planet-forming material is usually gone. So the stellar mass is a ‘tag’ that tells us what the planet-forming environment might have looked like for these exoplanets.”
And what it all comes down to is dust. “An important element of planet formation is the influence of dust evolution,” said van der Marel. “Without giant planets, dust will always drift inwards, creating the optimal conditions for the formation of smaller, rocky planets close to the star.”
The current research was conducted using data for more than 500 objects observed in prior studies using ALMA’s high-resolution Band 6 and Band 7 antennas. At present, ALMA is the only telescope that can image the distribution of millimeter-dust at high enough angular resolution to resolve the dust disks and reveal its substructure, or lack thereof. “Over the past five years, ALMA has produced many snapshot surveys of nearby star-forming regions resulting in hundreds of measurements of disk dust mass, size, and morphology,” said van der Marel. “The large number of observed disk properties has allowed us to make a statistical comparison of protoplanetary disks to the thousands of discovered exoplanets. This is the first time that a stellar mass dependency of gapped disks and compact disks has been successfully demonstrated using the ALMA telescope.”
“Our new findings link the beautiful gap structures in disks observed with ALMA directly to the properties of the thousands of exoplanets detected by the NASA Kepler mission and other exoplanet surveys,” said Mulders. “Exoplanets and their formation help us place the origins of the Earth and the Solar System in the context of what we see happening around other stars.”
Rice team enhances models that will detect magnetospheres in distant solar systems
We can’t detect them yet, but radio signals from distant solar systems could provide valuable information about the characteristics of their planets.
A paper by Rice University scientists describes a way to better determine which exoplanets are most likely to produce detectable signals based on magnetosphere activity on exoplanets’ previously discounted nightsides.
The study by Rice alumnus Anthony Sciola, who earned his Ph.D. this spring and was mentored by co-author and space plasma physicist Frank Toffoletto, shows that while radio emissions from the daysides of exoplanets appear to max out during high solar activity, those that emerge from the nightside are likely to add significantly to the signal.
Planets that orbit within a star’s Goldilocks zone, where conditions may otherwise give rise to life, could be deemed uninhabitable without evidence of a strong enough magnetosphere. Magnetic field strength data would also help to model planetary interiors and understand how planets form, Sciola said.
Earth’s magnetosphere isn’t exactly a sphere; it’s a comet-shaped set of field lines that compress against the planet’s day side and tail off into space on the night side, leaving eddies in their wake, especially during solar events like coronal mass ejections. The magnetosphere around every planet emits what we interpret as radio waves, and the closer to the sun a planet orbits, the stronger the emissions.
Astrophysicists have a pretty good understanding of our own system’s planetary magnetospheres based on the Radiometric Bode’s Law, an analytical tool used to establish a linear relationship between the solar wind and radio emissions from the planets in its path. In recent years, researchers have attempted to apply the law to exoplanetary systems with limited success.
“The community has used these rule-of-thumb empirical models based on what we know about the solar system, but it’s kind of averaged and smoothed out,” Toffoletto said. “A dynamic model that includes all this spiky behavior could imply the signal is actually much larger than these old models suggest. Anthony is taking this and pushing it to its limits to understand how signals from exoplanets could be detected.”
Sciola said the current analytic model relies primarily on emissions expected to emerge from an exoplanet’s polar region, what we see on Earth as an aurora. The new study appends a numerical model to those that estimate polar region emissions to provide a more complete picture of emissions around an entire exoplanet.
“We’re adding in features that only show up in lower regions during really high solar activity,” he said.
It turns out, he said, that nightside emissions don’t necessarily come from one large spot, like auroras around the north pole, but from various parts of the magnetosphere. In the presence of strong solar activity, the sum of these nightside spots could raise the planet’s total emissions by at least an order of magnitude.
“They’re very small-scale and occur sporadically, but when you sum them all up, they can have a great effect,” said Sciola, who is continuing the work at Johns Hopkins University’s Applied Physics Laboratory. “You need a numerical model to resolve those events. For this study, Sciola used the Multiscale Atmosphere Geospace Environment (MAGE) developed by the Center for Geospace Storms (CGS) based at the Applied Physics Laboratory in collaboration which the Rice space plasma physics group.
“We’re essentially confirming the analytic model for more extreme exoplanet simulations, but adding extra detail,” he said. “The takeaway is that we’re bringing further attention to the current model’s limiting factors but saying that under certain situations, you can get more emissions than that limiting factor suggests.”
He noted the new model works best on exoplanetary systems. “You need to be really far away to see the effect,” he said. It’s hard to tell what’s going on at the global scale on Earth; it’s like trying to watch a movie by sitting right next to the screen. You’re only getting a little patch of it.”
Also, radio signals from an Earth-like exoplanet may never be detectable from Earth’s surface, Sciola said. “Earth’s ionosphere blocks them,” he said. “That means we can’t even see Earth’s own radio emission from the ground, even though it’s so close.”
Detection of signals from exoplanets will require either a complex of satellites or an installation on the far side of the moon. “That would be a nice, quiet place to make an array that won’t be limited by Earth’s ionosphere and atmosphere,” Sciola said.
He said the observer’s position in relation to the exoplanet is also important. “The emission is ‘beamed,’” Sciola said. “It’s like a lighthouse: You can see the light if you are in line with the beam, but not if you are directly above the lighthouse. So having a better understanding of the expected angle of the signal will help observers determine if they are in line to observe it for a particular exoplanet.”
Co-authors of the paper are Rice graduate student Alison Farrish and David Alexander, a professor of physics and astronomy and director of the Rice Space Institute, and computational physicist Kareem Sorathia and physicist Viacheslav Merkin at the Johns Hopkins Applied Physics Laboratory.
The National Science Foundation and NASA supported the research.
Featured image: Rice University scientists have enhanced models that could detect magnetosphere activity on exoplanets. The models add data from nightside activity that could increase signals by at least an order of magnitude. In this illustration, the planet’s star is at top left, and the rainbow patches are the radio emission intensities, most coming from the nightside. The white lines are magnetic field lines. Illustration by Anthony Sciola
An international collaboration of astronomers led by a researcher from the Astrobiology Center and Queen’s University Belfast has detected a new chemical signature in the atmosphere of an extrasolar planet – i.e., a planet that orbits a star other than our Sun. The hydroxyl radical (OH) was found on the dayside of the exoplanet WASP-33b. This planet is a so-called ‘ultra-hot Jupiter’, a gas-giant planet orbiting its host star much closer than Mercury orbits the Sun (Figure 1) and therefore reaching atmospheric temperatures of more than 2500 degrees C (hot enough to melt most metals). The lead researcher based at the Astrobiology Center and Queen’s University Belfast, Dr. Stevanus Nugroho, says, “This is the first direct evidence of OH in the atmosphere of a planet beyond the Solar System. It shows not only that astronomers can detect this molecule in exoplanet atmospheres, but also that they can begin to understand the detailed chemistry of this planetary population.”
In the Earth’s atmosphere, OH is mainly produced by the reaction of water vapor with atomic oxygen. It is a so-called ‘atmospheric detergent’ and plays a crucial role in the Earth’s atmosphere to purge pollutant gasses that are dangerous to life (e.g. methane, carbon monoxide). In a much hotter and bigger planet like WASP-33b (Figure 2, where astronomers have previously detected signs of iron and titanium oxide gas) OH plays a key role in determining the chemistry of the atmosphere through interactions with water vapor and carbon monoxide. Most of the OH in the atmosphere of WASP-33b is thought to have been produced by the destruction of water vapor due to the extremely high temperature. “We see only a tentative and weak signal from water vapor in our data, which would support the idea that water is being destroyed to form hydroxyl in this extreme environment.” explains Dr. Ernst de Mooij from Queen’s University Belfast, a co-author on this study.zoom
To make this discovery, the team used the InfraRed Doppler (IRD) instrument at the 8.2-meter diameter Subaru Telescope located in the summit area of Maunakea in Hawai`i (about 4,200 m above sea level). This new instrument can detect atoms and molecules through their ‘spectral fingerprints,’ unique sets of dark absorption features superimposed on the rainbow of colors (or spectrum) that is emitted by stars and planets. As the planet orbits its host star, its velocity relative to the Earth changes with time. Just like the siren of an ambulance or the roar of a racing car’s engine seems to changes pitch while speeding past us, the frequencies of light (i.e. color) of these spectral fingerprints change with the velocity of the planet. This allows us to separate the planet’s signal from its bright host star, which normally overwhelms such observations, despite modern telescopes being nowhere near powerful enough to take direct images of such ‘hot Jupiter’ exoplanets.
“The science of extrasolar planets is relatively new, and a key goal of modern astronomy is to explore these planets’ atmospheres in detail and eventually to search for ‘Earth-like’ exoplanets – planets similar to our own. Every new atmospheric species discovered further improves our understanding of exoplanets and the techniques required to study their atmospheres, and takes us closer to this goal” says Dr. Neale Gibson, assistant professor at Trinity College Dublin and co-author of this work. By taking advantage of the unique capabilities of IRD, the astronomers were able to detect the tiny signal from hydroxyl in the planet’s atmosphere. “IRD is the best instrument to study the atmosphere of an exoplanet in the infrared,” adds Prof. Motohide Tamura, one of the principal investigators of IRD, Director of the Astrobiology Center, and co-author of this work.
“These techniques for atmospheric characterization of exoplanets are still only applicable to very hot planets, but we would like to further develop instruments and techniques that enable us to apply these methods to cooler planets, and ultimately, to a second Earth,” says Dr. Hajime Kawahara, assistant professor at the University of Tokyo and co-author of this work.
Prof. Chris Watson (QUB) from Queen’s University Belfast, a co-author on this study, continues, “While WASP-33b may be a giant planet, these observations are the testbed for the next-generation facilities like the Thirty Meter Telescope and the European Extremely Large Telescope in searching for biosignatures on smaller and potentially rocky worlds, which might provide hints to one of the oldest questions of humankind, ‘Are we alone?'”
Featured image: Comparison of our Solar System (top) and the WASP-33 planetary system (bottom). The distances of planets in the Solar System are not to scale. WASP-33b is much closer to its host star than Mercury is to the Sun; it has a high temperature of 2500 degrees Celsius due to extreme radiation from its host star. One side of WASP-33b is constantly facing toward its host star, similar to how the same side of the Moon always faces the Earth. (Credit: WP, CC BY-SA 3.0, Wikimedia Commons (top), Astrobiology Center (bottom))
Before planets around other stars were first discovered in the 1990s, these far-flung exotic worlds lived only in the imagination of science fiction writers.
But even their creative minds could not have conceived of the variety of worlds astronomers have uncovered. Many of these worlds, called exoplanets, are vastly different from our solar system’s family of planets. They range from star-hugging “hot Jupiters” to oversized rocky planets dubbed “super Earths.” Our universe apparently is stranger than fiction.
Seeing these distant worlds isn’t easy because they get lost in the glare of their host stars. Trying to detect them is like straining to see a firefly hovering next to a lighthouse’s brilliant beacon.
That’s why astronomers have identified most of the more than 4,000 exoplanets found so far using indirect techniques, such as through a star’s slight wobble or its unexpected dimming as a planet passes in front of it, blocking some of the starlight.
These techniques work best, however, for planets orbiting close to their stars, where astronomers can detect changes over weeks or even days as the planet completes its racetrack orbit. But finding only star-skimming planets doesn’t provide astronomers with a comprehensive picture of all the possible worlds in star systems.
Another technique researchers use in the hunt for exoplanets, which are planets orbiting other stars, is one that focuses on planets that are farther away from a star’s blinding glare. Scientists have uncovered young exoplanets that are so hot they glow in infrared light using specialized imaging techniques that block out the glare from the star. In this way, some exoplanets can be directly seen and studied.
NASA’s upcoming James Webb Space Telescope will help astronomers probe farther into this bold new frontier. Webb, like some ground-based telescopes, is equipped with special optical systems called coronagraphs, which use masks designed to block out as much starlight as possible to study faint exoplanets and to uncover new worlds.
Two targets early in Webb’s mission are the planetary systems 51 Eridani and HR 8799. Out of the few dozen directly imaged planets, astronomers plan to use Webb to analyze in detail the systems that are closest to Earth and have planets at the widest separations from their stars. This means that they appear far enough away from a star’s glare to be directly observed. The HR 8799 system resides 133 light-years and 51 Eridani 96 light-years from Earth.
Webb’s Planetary Targets
Two observing programs early in Webb’s mission combine the spectroscopic capabilities of the Near Infrared Spectrograph (NIRSpec) and the imaging of the Near Infrared Camera (NIRCam) and Mid-Infrared Instrument (MIRI) to study the four giant planets in the HR 8799 system. In a third program, researchers will use NIRCam to analyze the giant planet in 51 Eridani.
The four giant planets in the HR 8799 system are each roughly 10 Jupiter masses. They orbit more than 14 billion miles from a star that is slightly more massive than the Sun. The giant planet in 51 Eridani is twice the mass of Jupiter and orbits about 11 billion miles from a Sun-like star. Both planetary systems have orbits oriented face-on toward Earth. This orientation gives astronomers a unique opportunity to get a bird’s-eye view down on top of the systems, like looking at the concentric rings on an archery target.
Many exoplanets found in the outer orbits of their stars are vastly different from our solar system planets. Most of the exoplanets discovered in this outer region, including those in HR 8799, are between 5 and 10 Jupiter masses, making them the most massive planets ever found to date.
These outer exoplanets are relatively young, from tens of millions to hundreds of millions of years old—much younger than our solar system’s 4.5 billion years. So they’re still glowing with heat from their formation. The images of these exoplanets are essentially baby pictures, revealing planets in their youth.
Webb will probe into the mid-infrared, a wavelength range astronomers have rarely used before to image distant worlds. This infrared “window” is difficult to observe from the ground because of thermal emission from and absorption in Earth’s atmosphere.
“Webb’s strong point is the uninhibited light coming through space in the mid-infrared range,” said Klaus Hodapp of the University of Hawaii in Hilo, lead investigator of the NIRSpec observations of the HR 8799 system. “Earth’s atmosphere is pretty difficult to work through. The major absorption molecules in our own atmosphere prevent us from seeing interesting features in planets.”
The mid-infrared “is the region where Webb really will make seminal contributions to understanding what are the particular molecules, what are the properties of the atmosphere that we hope to find which we don’t really get just from the shorter, near-infrared wavelengths,” said Charles Beichman of NASA’s Jet Propulsion Laboratory in Pasadena, California, lead investigator of the NIRCam and MIRI observations of the HR 8799 system. “We’ll build on what the ground-based observatories have done, but the goal is to expand on that in a way that would be impossible without Webb.”
How Do Planets Form?
One of the researchers’ main goals in both systems is to use Webb to help determine how the exoplanets formed. Were they created through a buildup of material in the disk surrounding the star, enriched in heavy elements such as carbon, just as Jupiter probably did? Or, did they form from the collapse of a hydrogen cloud, like a star, and become smaller under the relentless pull of gravity?
Atmospheric makeup can provide clues to a planet’s birth. “One of the things we’d like to understand is the ratio of the elements that have gone into the formation of these planets,” Beichman said. “In particular, carbon versus oxygen tells you quite a lot about where the gas that formed the planet comes from. Did it come from a disk that accreted a lot of the heavier elements or did it come from the interstellar medium? So it’s what we call the carbon-to-oxygen ratio that is quite indicative of formation mechanisms.”
To answer these questions, the researchers will use Webb to probe deeper into the exoplanets’ atmospheres. NIRCam, for example, will measure the atmospheric fingerprints of elements like methane. It also will look at cloud features and the temperatures of these planets. “We already have a lot of information at these near-infrared wavelengths from ground-based facilities,” said Marshall Perrin of the Space Telescope Science Institute in Baltimore, Maryland, lead investigator of NIRCam observations of 51 Eridani b. “But the data from Webb will be much more precise, much more sensitive. We’ll have a more complete set of wavelengths, including filling in gaps where you can’t get those wavelengths from the ground.”
Video: This video shows four Jupiter-sized exoplanets orbiting billions of miles away from their star in the nearby HR 8799 system. The planetary system is oriented face-on toward Earth, giving astronomers a unique bird’s-eye view of the planets’ motion. The exoplanets are orbiting so far away from their star that they take anywhere from decades to centuries to complete an orbit. The video consists of seven images of the system taken over a seven-year period with the W.M. Keck Observatory on Mauna Kea, Hawaii. Keck’s coronagraph blocks out most of the starlight so that the much fainter and smaller exoplanets can be seen.Credits: Jason Wang (Caltech) and Christian Marois (NRC Herzberg)
The astronomers will also use Webb and its superb sensitivity to hunt for less-massive planets far from their star. “From ground-based observations, we know that these massive planets are relatively rare,” Perrin said. “But we also know that for the inner parts of systems, lower-mass planets are dramatically more common than larger-mass planets. So the question is, does it also hold true for these further separations out?” Beichman added, “Webb’s operation in the cold environment of space allows a search for fainter, smaller planets, impossible to detect from the ground.”
Another goal is understanding how the myriad planetary systems discovered so far were created.
“I think what we are finding is that there is a huge diversity in solar systems,” Perrin said. “You have systems where you have these hot Jupiter planets in very close orbits. You have systems where you don’t. You have systems where you have a 10-Jupiter-mass planet and ones in which you have nothing more massive than several Earths. We ultimately want to understand how the diversity of planetary system formation depends on the environment of the star, the mass of the star, all sorts of other things and eventually through these population-level studies, we hope to place our own solar system in context.”
Video: This video shows a Jupiter-sized exoplanet orbiting far away—roughly 11 billion miles—from a nearby, Sun-like star, 51 Eridani. The planetary system is oriented face-on toward Earth, giving astronomers a unique bird’s-eye view of the planet’s motion. The video consists of five images taken over four years with the Gemini South Telescope’s Gemini Planet Imager, in Chile. Gemini’s coronagraph blocks out most of the starlight so that the much fainter and smaller exoplanet can be seen.Credits: Jason Wang (Caltech)/Gemini Planet Imager Exoplanet Survey
The NIRSpec spectroscopic observations of HR 8799 and the NIRCam observations of 51 Eridani are part of the Guaranteed Time Observations programs that will be conducted shortly after Webb’s launch later this year. The NIRCam and MIRI observations of HR 8799 is a collaboration of two instrument teams and is also part of the Guaranteed Time Observations program.
The James Webb Space Telescope will be the world’s premier space science observatory when it launches in 2021. Webb will solve mysteries in our solar system, look beyond to distant worlds around other stars, and probe the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and the Canadian Space Agency.
Featured image: Left: This is an image of the star HR 8799 taken by Hubble’s Near Infrared Camera and Multi-Object Spectrometer (NICMOS) in 1998. A mask within the camera (coronagraph) blocks most of the light from the star. Astronomers also used software to digitally subtract more starlight. Nevertheless, scattered light from HR 8799 dominates the image, obscuring four faint planets later discovered from ground-based observations. Right: A re-analysis of NICMOS data in 2011 uncovered three of the exoplanets, which were not seen in the 1998 images. Webb will probe the planets’ atmospheres at infrared wavelengths astronomers have rarely used to image distant worlds. Credits: NASA, ESA, and R. Soummer (STScI)
“Even finding and studying a small number of them could give us a great deal of information about dark matter that we don’t know now.”
Smirnov co-authored the paper with Rebecca Leane, a postdoctoral researcher at the SLAC National Accelerator Laboratory at Stanford University. It was published today (April 22, 2021) in the journal Physical Review Letters.
Smirnov said that when the gravity of exoplanets captures dark matter, the dark matter travels to the planetary core where it “annihilates” and releases its energy as heat. The more dark matter that is captured, the more it should heat up the exoplanet.
This heating could be measured by NASA’s James Webb Space Telescope, an infrared telescope scheduled to launch in October that will be able to measure the temperature of distant exoplanets.
“If exoplanets have this anomalous heating associated with dark matter, we should be able to pick it up,” Smirnov said.
Exoplanets may be particularly useful in detecting light dark matter, Smirnov said, which is dark matter with a lower mass. Researchers have not yet probed light dark matter by direct detection or other experiments.
Scientists believe that dark matter density increases toward the center of our Milky Way galaxy. If that is true, researchers should find that the closer planets are to the galactic center, the more their temperatures should rise.
“If we would find something like that, it would be amazing. Clearly, we would have found dark matter,” Smirnov said.
Smirnov and Leane propose one type of search that would involve looking close to Earth at gas giants – so called “Super Jupiters” – and brown dwarfs for evidence of heating caused by dark matter. One advantage of using planets like this as dark matter detectors is that they don’t have nuclear fusion, like stars do, so there is less “background heat” that would make it hard to find a dark matter signal.
In addition to this local search, the researchers suggest a search for distant rogue exoplanets that are no longer orbiting a star. The lack of radiation from a star would again cut down on interference that could obscure a signal from dark matter.
One of the best parts of using exoplanets as dark matter detectors is that it doesn’t require any new types of instrumentation such as telescopes, or searches that aren’t already being done, Smirnov said.
As of now, researchers have identified more than 4,300 confirmed exoplanets and an additional 5,695 candidates are currently under investigation. Gaia, a space observatory of the European Space Agency, is expected to identify tens of thousands more potential candidates in the next few years.
“With so many exoplanets being studied, we will have a tremendous opportunity to learn more than ever before about dark matter,” Smirnov said.