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How Common Are Giant Planets around Red Dwarfs?

A planet like GJ 3512 b is hard to explain. Here we have a gas giant that seems to be the result of gravitational instabilities inside the ring of gas and dust that circles its star. This Jupiter-like world is unusual because of the ratio between planet and star. The Sun, for example, is about 1050 times more massive than Jupiter. But for GJ 3512 b, that ratio is 270, a reflection of the fact that this gas giant orbits a red dwarf with about 12 percent of the Sun’s mass. How does a red dwarf produce a debris disk that allows such a massive planet to grow?

Image: Comparison of GJ 3512 to the Solar System and other nearby red-dwarf planetary systems. Planets around solar-mass stars can grow until they start accreting gas and become giant planets such as Jupiter, in a few millions of years. However, up to now astronomers suspected that, except for some rare exceptions, small stars such as Proxima, TRAPPIST-1, Teegarden’s star, and GJ 3512 were not able to form Jupiter mass planets. Credit: © Guillem Anglada-Escude – IEEC/Science Wave, using SpaceEngine.org (Creative Commons Attribution 4.0 International; CC BY 4.0).

I bring up GJ 3512 b (discovered in 2019) as an example of the kind of anomaly that highlights the gaps in our knowledge. While we often talk about red dwarf systems in these pages, the processes of planet formation around these stars remain murky. M-dwarfs may make up 80 percent of the stars in the Milky Way, but they host a scant 10 percent of the exoplanets we’ve found thus far. To tackle this gap, a team of scientists led by Nicolas Kurtovic (Max Planck Institute for Astronomy, Heidelberg) has released the results of its analysis of six very low mass stars n the Taurus star-forming region. These are stars with mass less than 20% that of the Sun.

The new work relies on data from the Atacama Large Millimeter/submillimeter Array (ALMA), taken at a wavelength of 0.87 millimeters in order to trace dust and gas in the disk around these stars at an angular resolution of 0.1 arcseconds. What’s intriguing here are the signs of ring-like structures in the dust that extend between 50 and 90 AU from the stars, which reminds us of similar, much larger disks around more massive stars. Half of the disks the Kurtovic team studied showed structures at these distances. The consensus is that such rings are markers for planets in the process of formation as they accumulate gas and dust.

Image: Artistic representation of a planet-forming disk of dust and gas around a very low-mass star (VLMS). The inner dust disk contains a ring structure that indicates the formation of a new planet. The dust disk resides inside a larger gas disk whose thickness increases towards the edge. Credit: MPIA graphics department.

In the case of the M-dwarfs under study, the gaps in the rings being cleared by these planets would require worlds about as massive as Saturn, an indication that the material for gas giant formation is available. But time is a problem: Inward-movng dust evaporates close to the star, and in the case of red dwarfs, the migration is twice as fast as for more massive stars. There is little time in such a scenario for the planetary embryos needed for core accretion to form.

This is, I think, the key point in the paper, which notes the problem and explains current thinking:

The core accretion scenario for planet formation assumes collisional growth from sub-µm-sized dust particles from the interstellar medium (ISM) to kilometer-sized bodies or planetesimals… The collisions of particles and their dynamics within the disk are regulated by the interaction with the surrounding gas. Different physical processes lead to collisions of particles and their potential growth, such as Brownian motion, turbulence, dust settling, and radial drift…

Much depends upon the star in question. The paper continues (italics mine):

All of these processes have a direct or indirect dependency on the properties of the hosting star, such as the temperature and mass. For instance, from theoretical calculations, settling and radial drift are expected to be more efficient in disks around VLMS [Very Low Mass Stars] and BDs [Brown Dwarfs], with BD disks being 15-20% flatter and with radial drift velocities being twice as high or even more in these disks compared to T-Tauri disks.

The authors estimate that the ringed structures they’ve found around three of the red dwarfs under study formed approximately 200,000 years before the dust would have migrated to the central star. So planet formation must be swift: The planetary embryos need to accumulate enough mass to create gaps in the disk, which effectively block the dust from further inward migration. Without these gaps, the likelihood of planet formation drops.

Image: Observational data and model of the dust disk around the VLMS MHO6. Left: Image of the dust disk. Middle: The disk model with a 20 au wide central hole, which is consistent with a Saturn-mass planet located at a distance of 7 au from the star, accreting disk material. Right: Radial profile of the model (blue) and after convolving it with the telescope’s angular resolution (red). The black symbols represent the data obtained from the measured brightness distribution. The grey bar corresponds to the angular resolution of the observations. Credit: Kurtovic et al./MPIA.

Remember, we only have six stars to work with here, and the other three stars under investigation are likewise problematic. They appear to show dust concentrations between 20 and 40 AU from their stars, but in all three cases lack structure that can be defined by ALMA. Better resolution would, the authors believe, tease out rings inside even these smaller disks.

But the paper acknowledges that disks around stars that are still lower in mass demand that inward migration be reduced through ‘gas pressure bumps’ that can trap dust efficiently. Here’s the process (once again, the italics are mine):

The presence of pressure bumps produces substructures, such as rings, gaps, spiral arms, and lopsided asymmetries, with a different amplitude, contrasts, and locations depending on the origin of the pressure variations… Currently, due to sensitivity limitations, most of our observational knowledge about substructures comes from bright (and probably massive) disks, such as the DSHARP sample… A less biased sample of ALMA observations of disks in the star-formation region of Taurus has demonstrated that at least 33% of disks host substructures at a resolution of 0.1”, and the disks that do not have any substructures are compact (dust disk radii lower that ∼50 au…). It remains an open question if compact disks are small because they lack pressure bumps or because current observations lack the resolution to detect rings and gaps in these disks.

Such pressure bumps are a workable explanation but one without the kind of observational evidence we’ll need as we continue to investigate planet formation around low-mass stars and brown dwarfs. We are a long way from having a fully developed model for planet formation in this environment, and the Kurtovic et al. paper drives the point home. On a matter as fundamental as how common planets around red dwarfs are, we are still in the early stages of data gathering.

The paper is Kurtovic, Pinilla, et al., “Structures of Disks around Very Low Mass Stars in the Taurus Star-Forming Region,” Astronomy & Astrophysics, 645, A139 (2021). Abstract / Preprint.

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Comments on this entry are closed.

  • Alex Tolley February 1, 2021, 14:46

    When theory is at odds with observations, then the theory must be changed.

    I would guess that theories on planetary formation will have to be modified. Explaining observations is the grist of science and theories will be bounded by these observations. It is exactly these unexpected observations that are interesting and informative. The more and better we can observe, the more likely unexpected observations will be made, and the more we must find ways to explain these observations in a cohesive way. We have come a long way from the observations of planetary motions that resulted in the discarding of epicycles. We still have a huge discrepancy between methods to define the Hubble constant. The universe may indeed be queerer than we can imagine, but hopefully, we can explain it all eventually.

    The richer and more complete our models, the better we can detect true, non-natural phenomena…

  • Geoffrey Hillend February 1, 2021, 17:13

    I wouldn’t consider the Jupiter sized world unusual if there are not rocky planets there with it, if the large gas giant swept away or deflected all the other material available for other inner rocky planets. Also the gas pressure bumps idea seems dubious when we consider that it is the angular momentum that the accretion disk borrows from the angular momentum of the Star’s rotation that keeps the dust and planets from migrating inwards? The cloud of gas and dust collapses into aa flat accretion disk when it forms a star.

  • Geoffrey Hillend February 1, 2021, 17:20

    Maybe the disk closer to the star does not have gaps because the gravity is stronger being closer to the star or there is more angular momentum? Consequently, the rings would be smaller and harder to detect.

  • Spaceman February 2, 2021, 8:47

    Interesting work! I thought microlensing surveys had already put strong constraints on the prevalence of giant planets around red dwarfs?

  • Michael Fidler February 2, 2021, 11:14

    Half the stars in the sky are binary or higher as we see in TYC 7037-89-1, so these M dwarfs may have been ejected from such systems. This would give the extra mass for large gas giants to form. The other situation might be the star forming cloud having a higher density and larger fraction of heaver elements to help build a Jovian planet in such systems.

  • Edwin February 2, 2021, 17:49

    Yes this sure is interesting and one subject I have a lot of research on.

    It seems to be rare for small red dwarfs to have Jovian planets around them but perhaps 10-20% of them do? This could be caused by a large disc and gravitational collapse also Binary stars leading to Jovian sized planets in orbit.

    The thing that really interests me is the Jupiter like planets orbiting at HZ distances with terrestrial moons. These places could be very promising places for life.

  • Michael Fidler February 2, 2021, 19:14

    Could M dwarf planets have diamond cores? We assume iron core planets are the norm but very low-mass star may form organic planets from the disk of dust and gas.

    New research on carbon cracks open secrets deep inside exoplanets.

    n international team of researchers, including researchers at the University of Rochester’s Laboratory for Laser Energetics (LLE), has successfully measured carbon at pressures reaching 2,000 GPa (five times the pressure in Earth’s core), nearly doubling the maximum pressure at which carbon’s crystal structure has ever been directly probed. Their results were published in the journal Nature.

    “This is the highest pressure any atomic structure has been measured, placing key constraints on the equation of state, material strength, melting, and chemical bonding of carbon,” says Gilbert (Rip) Collins, the Tracy Hyde Harris Professor of Mechanical Engineering and associate director of science, technology, and academics at the LLE. “In our studies of the many recently discovered and yet-to-be discovered massive, carbon-rich planets, we will have to consider the diamond structure of carbon at pressures well beyond its predicted stability range.”

    The researchers found that even when subjected to the intense conditions, solid carbon retains its diamond structure, far beyond its range of predicted stability. The findings indicate that the strength of the molecular bonds in diamond persists even under enormous pressure, resulting in large energy barriers that hinder carbon’s conversion to other possible structures.

    “The diamond phase of carbon appears to be the most stubborn structure ever explored,” says Ryan Rygg, an assistant professor of mechanical engineering and of physics and a senior scientist at the LLE. “This could have implications for carbon in the deep interiors of planets, where the precipitation of diamond is expected. Now we anticipate the diamond structure of carbon will persist over a much greater range of planetary conditions than we previously thought.”

    • Edwin February 3, 2021, 5:56

      Yes

      I have seen a few papers on this

    • Andrei February 8, 2021, 18:39

      Yes the pressure inside Earth is enough to make diamond, so a carbon planet could develop a diamond layer at some depth. Yet an entire core without any heavy element and instead diamond sound like a very rare thing – extremely rare but it’s a big universe, even very unlikely events might happen somewhere.

  • Geoffrey Hillend February 2, 2021, 20:01

    The generally accepted idea is that the M dwarfs accretion disk is smaller and dust could is smaller, but their may be exceptions like separate clouds of collapsing gas?

  • Geoffrey Hillend February 2, 2021, 20:04

    What I meant to say was gas giants exist around red dwarfs but they might be rarer.

  • Michael Fidler February 4, 2021, 9:22

    This should quickly even up the odds;

    In Search of New Super Earths: CRIRES+ on ESO’s Very Large Telescope Sees First Light.

    “CRIRES+, an instrument built by ESO in collaboration with a consortium of European institutes, will search the sky for super-Earths located within the habitable zones of nearby low-mass stars, the range of planetary orbits within which scientists believe a planet can support life. These types of planets are difficult to detect due to their relatively low masses. With CRIRES+, scientists will be better equipped to search for these super-Earths as they will be able to survey most stars in our stellar neighbourhood. Other science goals of CRIRES+ include studying the atmosphere of transiting exoplanets and the origin and evolution of stellar magnetic fields.”

    https://www.eso.org/public/announcements/ann21003/

  • Ronald February 11, 2021, 7:03

    A few very interesting papers out:
    On the origin of super-earths and mini-Neptunes:
    http://dx.doi.org/10.3847/1538-4357/abd6c7
    And on detecting (nearby) habitable planets using infrared;
    http://dx.doi.org/10.1038/s41467-021-21176-6

  • ljk February 18, 2021, 13:19

    Predicting how many worlds are in systems with multiple planets:

    http://astrobiology.com/2021/02/exoplanets-prediction-in-multiplanetary-systems.html

  • ljk March 11, 2021, 13:08

    http://astrobiology.com/2021/03/nemesis-exoplanet-transit-survey-of-nearby-m-dwarfs-in-tess-ffis-i.html

    NEMESIS: Exoplanet Transit Survey of Nearby M-Dwarfs in TESS FFIs I

    Source: astro-ph.EP

    Posted March 10, 2021 10:33 PM

    In this work, we present the analysis of 33,054 M-dwarf stars located within 100 parsecs in the Transiting Exoplanet Survey Satellite (TESS) Full Frame Images (FFIs) of the observed sectors 1 to 5.

    We present a new pipeline called NEMESIS which was developed to extract detrended photometry and perform transit searches of single sector data in TESS FFIs. As many M-dwarfs are faint and are not observed with a 2 minute cadence by TESS, FFI transit surveys can give an empirical validation of how many planets are missed by using the 30 minute cadence data.

    In this work, we detected 183 threshold crossing events and present 29 planet candidates for sectors 1 to 5, 24 of which are new detections. Our sample contains orbital periods ranging from 1.25 to 6.84 days and planetary radii from 1.26 to 5.31 Earth radii. With the addition of our new planet candidate detections along with previous detections observed in sectors 1 to 5, we calculate an integrated occurrence rate of 2.49 +/- 1.58 planets per star for the period range between [1,9] days and planet radius range between [0.5,11] Earth radii.

    We project an estimated yield of 122 +/- 11 transit detections of nearby M-dwarfs. 23 of our new candidates have Signal to Noise ratios > 7, Transmission Spectroscopy Metrics > 38 and Emission Spectroscopy Metrics > 10. We provide all of our data products for our planet candidates through the Filtergraph data visualization service located at this https URL.

    Dax L. Feliz, Peter Plavchan, Samantha N. Bianco, Mary Jimenez, Kevin I. Collins, Bryan Villarreal Alvarado, Keivan G. Stassun

    Comments: 19 pages, 16 figures and 3 tables. Accepted for publication in The Astronomical Journal

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)

    Cite as: arXiv:2103.05647 [astro-ph.EP] (or arXiv:2103.05647v1 [astro-ph.EP] for this version)

    Submission history

    From: Dax Feliz

    [v1] Tue, 9 Mar 2021 19:00:02 UTC (4,384 KB)

    https://arxiv.org/abs/2103.05647

    Astrobiology