Tag Archives: #exoplanets

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

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

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

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

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

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

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

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

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

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

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

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

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

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


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


Provided by Cornell University

Can We Directly Detect Exoplanets and Protoplanetary Disks? (Astronomy)

What would be your answer if I asked you, how can we directly detect exoplanets and protoplanetary disks? Or what kind of technology needed for it? I know many of you dont know the answer of this question, but today you will get it.

Guys, detecting exoplanets and protoplanetary disks directly with the help of current extremely large telescopes (ELT’s) is not at all possible. However, the next/new generation of ELTs provides the necessary resolution to probe close to a significant number of M-type stars.

Secondly, if we want to directly detect exoplanets and protoplanetary disks, we will need high accuracy wavefront sensing and control (WFS&C) technologies, especially for ground-based Extremely Large Telescopes (ELTs).

Figure 1: On-sky PSF in seeing limited mode (left), after the first stage of correction by AO188 (middle), after SCExAO correction (right). With SCExAO correction, the PSF is more stable and the speckle halo surrounding the core is fainter © Kyohoon Ahn et al.

One instrument that will help us target Earth-like planets in the habitable zone of M-type stars in the future is the Planetary System Imager (PSI), planned to be installed on the Thirty Meter Telescope (TMT). But, some major hardware upgrades like replacement of the current 188-actuator DM with the ALPAO 64×64 actuator DM or addition of beam switcher etc. and software upgrades like new advanced HCI technologies such as coherent differential imaging, predictive control, sensor fusion or real-time post-processing etc. are needed and this shall be achieved in next few years.

Figure 2: Hardware upgrades scheduled for the AO188: (a) NIR PyWFS, (b) new 64×64 ALPAO DM, (c) future configuration at the Nasmyth platform including a new beam switcher, allowing to split the light between the SCExAO and the IRCS, as well as other potential future instruments, and (d) a optomechanical design of the beam switcher. © Kyohoon Ahn et al.

Moreover, another high contrast imaging instrument like Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) will be able to directly image young Jupiter-mass planets closer to the habitable zone, down to 3 AU, where they should be more abundant. This will give us more insight on the planet population around the habitable zone. Finally, a few older Jupiter-size planets should be reached by looking at the reflected light for the first time.


For more:

Kyohoon Ahn et al., “SCExAO, a testbed for developing high-contrast imaging technologies for ELTs”, pp. 1-13, 2021. https://arxiv.org/abs/2109.13353


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Astronomers Discovered Massive Exoplanet Orbiting Its Host Star Once Every 1.1 Million Years (Planetary Science)

Astronomers with COol Companions ON Ultrawide orbiTS (COCONUTS) program, the COCONUTS-2 system, have identified a massive exoplanet, “COCONUTS-2b” in the COCONUTS-2A system, orbiting a low mass red dwarf star L 34-26. This planet has 6.3 times the mass of the Jupiter and orbits the parent star once every 1.1 million years at a distance of 6,471 AU (astronomical units). Their study recently appeared in Astrophysical Journal Letters.

L 34-26 also called COCONUTS-2A and TYC 9381-1809-1, is a M3-type dwarf star located 35 light-years away in the constellation of Chamaeleon. The star is about one-third the mass of the Sun and between 150 and 800 million years old.

Astronomers estimated that the planet has temperature of about 434 K. They also showed that, it is the nearest imaged exoplanet to Earth known to date.

In addition, astronomers suggested that, the skies of COCONUTS-2b would look dramatically different compared to the skies on the earth, due to its wide-separation orbit and cool host star.

While, the nighttime and daytime would look basically the same, with the host star appearing as a bright red star in the dark sky.

After TYC 9486-927-1b at a projected separation of 6,900 AU, this planet is the second widest and after WD 0806-661b with a temperature of 328 K this planet is the second coldest imaged exoplanet discovered so far.

COCONUTS-2b 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.

The results were published in the Astrophysical Journal Letters.

Featured image: This image shows COCONUTS-2b (left) and its host star, COCONUTS-2A (right). Image credit: Zhang et al., doi: 10.3847/2041-8213/ac1123.


Reference: Zhoujian Zhang et al. 2021. The Second Discovery from the COCONUTS Program: A Cold Wide-orbit Exoplanet around a Young Field M Dwarf at 10.9 pc. ApJL 916, L11; doi: 10.3847/2041-8213/ac1123


Note for editors of other websites: To reuse this article fully or partially kindly give credit either to our author S. Aman or provide a link of our article

Superflares: Less Harmful To Exoplanets Than Previously Thought (Planetary Science)

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


Further information

Original publication

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.

DOI: https://doi.org/10.1093/mnras/stab2159

https://arxiv.org/abs/2108.01917


Provided by AIP

Astronomers Make First Clear Detection Of A Moon-forming Disc Around An Exoplanet (Planetary Science)

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 [1].

“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 [2].

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.              

Notes

[1] Despite the similarity with the Jupiter-Saturn pair, note that the disc around PDS 70c is about 500 times larger than Saturn’s rings.

[2] 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.

More information

This research was presented in the paper “A Circumplanetary Disk Around PDS 70c” to appear in The Astrophysical Journal Letters.

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).


Provided by ESO

Unique Exoplanet Bursts into CHEOPS Study (Planetary Science)

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 Astronomy was 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.

This infographic reveals the details of the Nu2 Lupi planetary system. This bright, Sun-like star is located just under 50 light-years away from Earth in the constellation of Lupus (the Wolf), as shown to the left of the frame, and is known to host three planets (named ‘b’, ‘c’ and ‘d’, with the star deemed to be object ‘A’). The relative sizes, orbital periods, and possible compositions of these three planets are depicted to the centre and lower right of the frame, while planet d’s comparative position within our Solar System is shown to the upper right (as defined by the amount of incident light it receives from its star, Nu2 Lupi). © ESA

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.

Laetitia Delrez, Visiting researcher at the University of Geneva, now at the University of Liège, Belgium © ULiege/JLWertz

“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.

Featured image: Artist’s impression of the Nu2 Lupi planetary system © ESA


Publication information:

L. Delrez et al .: Transit detection of the long-period volatile-rich super-Earth ν 2 Lupi d with CHEOPS, Nature Astronomy. https://doi.org/10.1038/s41550-021-01381-5


Provided by University of Bern

Collection of Starshade Research to Help Advance Exoplanet Imaging By Space Telescopes (Astronomy)

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

The Journal of Astronomical Telescopes, Instruments, and Systems (JATIS) has published a special section on the latest science, engineering, research, and programmatic advances of starshades, the starlight-suppression technology integral to extra-solar and exoplanet detection.

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.”

NASA’s starshade technology development activity,” “Antireflection coatings on starshade optical edges for solar glint suppression,” “Exoplanet imaging performance envelopes for starshade-based missions,” and “Mapping the observable sky for a Remote Occulter working with ground-based telescopes” are just a few of the articles featured in this collection of open access papers.

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.

JATIS is one of 12 journals published by SPIE, the international society for optics and photonics, on its Digital Library platform.

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.


Provided by SPIE

Mind The Gap: Scientists Use Stellar Mass to Link Exoplanets to Planet-forming Disks (Planetary Science)

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.” 

Featured image: Protoplanetary disks are classified into three main categories: transition, ring, or extended. These false-color images from the Atacama Large Millimeter/submillimeter Array (ALMA) show these classifications in stark contrast. On left: the ring disk of RU Lup is characterized by narrow gaps thought to be carved by giant planets with masses ranging between a Neptune mass and a Jupiter mass. Middle: the transition disk of J1604.3-2130 is characterized by a large inner cavity thought to be carved by planets more massive than Jupiter, also known as Super-Jovian planets. On right: the compact disk of Sz104 is believed not to contain giant planets, as it lacks the telltale gaps and cavities associated with the presence of giant planets. © ALMA (ESO/NAOJ/NRAO), S. Dagnello (NRAO)


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“A stellar mass dependence of structured disks: A possible link with exoplanet demographics,” N. van der Marel and G. Mulders, ApJ, DOI: 10.3847/1538-3881/ac0255, preview [https://arxiv.org/pdf/2104.06838.pdf]


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Nightside Radio Could Help Reveal Exoplanet Details (Planetary Science)

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.

This interests the exoplanet community because the strength of a given planet’s magnetosphere indicates how well it would be protected from the solar wind that radiates from its star, the same way Earth’s magnetic field protects us.

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.

The study appears in The Astrophysical Journal.

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.”

Anthony Sciola, pictured at Kaldidalur (The Cold Valley) in Iceland, has developed a numerical model to enhance the analysis of radio signals from exoplanets. Though the instruments to obtain such data are not yet available, they could help determine what planets have protective magnetospheres. Photo courtesy of Anthony Sciola

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


Provided by Rice University