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An Infant System Laden with Gas Giants

We’ve never found a ‘hot Jupiter’ around a star as young as CI Tau. This well studied system, some 2 million years old, has drawn attention for its massive disk of dust and gas, one that extends hundreds of AU from the star. But radial velocity examination recently revealed CI Tau b, a hot Jupiter that in and of itself raises questions. Couple that to the likelihood of three other gas giant planets emerging in the disk with extreme differences in orbital radii and it’s clear that CI Tau challenges our ideas of how gas giants, especially hot Jupiters, emerge and evolve.

Can a hot Jupiter form in place, or is migration from a much more distant orbit the likely explanation? The latter seems likely, and in that case, what was the mechanism here around such a young star? Most hot Jupiter host stars have lost their protoplanetary disks, which means that astronomers have been working with theoretical formation models to produce the observed tight orbits. And because about 1 percent of main sequence solar type stars (CI Tau is a K4IV object) have hot Jupiters around them, the riddle of their formation demands resolution.

The new work on CI Tau comes via high resolution imaging at submillimeter wavelengths at the Atacama Large Millimeter/submillimeter Array (ALMA), where Cathie Clarke (Cambridge Institute of Astronomy) and colleagues have found three distinct gaps in the star’s protoplanetary disk at ∼ 13, 39 and 100 au. The team’s paper in Astrophysical Journal Letters reports on computer modeling showing that these gaps are likely caused by additional gas giant planets.

Image: From Figure 1 of the paper, showing the protoplanetary disk around CI Tau. Credit: Clarke et al.

Making the find even more intriguing is that while CI Tau b is in an orbit not dissimilar from Mercury’s, the farthest putative planet orbits at a distance three times that of Neptune. All are large objects — the two outer worlds are roughly the mass of Saturn, while the two inner planets weigh in at 1 Jupiter mass and 10 Jupiter masses for the hot Jupiter. We’re left with the question of how these other planets affected the hot Jupiter’s orbital position, and whether there is a mechanism at work here that could apply to hot Jupiters in other systems.

For that matter, how did the two Saturn-class planets emerge where they are?

“Planet formation models tend to focus on being able to make the types of planets that have been observed already, so new discoveries don’t necessarily fit the models,” said Clarke. “Saturn mass planets are supposed to form by first accumulating a solid core and then pulling in a layer of gas on top, but these processes are supposed to be very slow at large distances from the star. Most models will struggle to make planets of this mass at this distance.”

No doubt. Complicating the picture further is that we have only learned about these planet candidates because of their effects on the protoplanetary disk, so whether or not extreme orbital parameters like these are common in hot Jupiter systems remains an open question. After all, older systems like those we’ve found other hot Jupiters in have already lost their disks.

The Jovian-class worlds may turn out to be easier to explain. Despite CI Tau’s relative youth, the authors argue that its hot Jupiter would still have had time to make the migration into hot Jupiter range. From the paper:

The hot Jupiter… could have been formed by a variety of mechanisms; from the modeled masses in disc and planets and from the accretion on to the star the inferred timescale for its inward migration is ∼ 0.4 Myr (Dürmann & Kley 2015) so that there would have been plenty of time for it to have migrated from a range of outward lying locations. The roughly Jovian mass planet inferred at 14 au is also easy to account for in terms of existing planet formation models (i.e. core accretion models involving either planetesimal or pebble accretion.

But those two outer ‘Saturns’ will need further work. We have a number of planetary disks with well-defined substructure to look at (HL Tau, HD 163296 and HD 169142, among others), but none with a hot Jupiter. Is this orbital configuration one that can survive on billion-year timescales? The authors believe these planets may still end up at small radii, suggesting they could be eventually ejected from the system by gravitational interactions. It will take future imaging surveys to tell us whether systems like the emerging one at CI Tau can be long-lived.

The paper is Clarke et al., “High-resolution Millimeter Imaging of the CI Tau Protoplanetary Disk: A Massive Ensemble of Protoplanets from 0.1 to 100 au,” Astrophysical Journal Letters Vol. 866, No. 1 (4 October 2018). Abstract / preprint.

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{ 13 comments… add one }
  • Geoffrey Hillend October 17, 2018, 16:32

    I reject the idea of migration. It seems to me using the scientific intuition and principles of physics, we can come only to one conclusion. The idea that gas giants like Jupiter and Saturn have to have solid cores must become obsolete. I never liked the migration idea since there would have to just too many migrations of the exoplanet examples in our galaxy and large bodies like Jupiter and Saturn are not easily moved out of their orbits at least to a position much nearer to a G glass star.

    I now think there might not be a solid core in Jupiter and Saturn and that would explain how gas giants form close to their stars. They form early on like a double star system. If double star systems can form, then there can be enough gas for a gas giant to form near the star. It forms at the same time as the star, early in the accretion disk. I thought this might not be possible because the bigger star might with the bigger gravity might capture or hog all the gas and there could be no red giants. It seems to me I think counter intuitive to try and force the birth of gas giants into an old, obsolete idea. The rocky planets formed from collisions in the early bombardment period, but there were small bodies with low gravity. If we have enough of an accumulation of gas, a Jupiter or Saturn sized body will form early in the birth of an another star system and just keep capturing more gas when there was not a lot of light, heat and solar wind for it’s star.

    • Ron S. October 17, 2018, 19:00

      There can be more than one way to form these planets. It isn’t a competition where the winner takes all.

    • AlexT October 18, 2018, 4:24

      “there would have to just too many migrations of the exoplanet examples in our galaxy and large bodies like Jupiter and Saturn are not easily moved”
      I suppose that the Earth civilisation age is instant moment even if you compare it against young CI tau star 2 millions years age…
      I suppose we do not have enough possibility to see this process in real time. The fact that there is billion similar star dies not help much – we have still photo of different systems you cannot combine those photo from multiple sustems together to produce “live video”.

  • Wojciech J October 17, 2018, 17:31
    • Michael Fidler October 18, 2018, 2:46

      Could this be used to develop a ground based Kepler Schmidt telescope array. The main problem with adaptive optics is it limited field of view, but with this advanced Extreme-AO algorithms could the field of view be expanded? NVIDIA GPUs useing the algorithm now learns to optimize itself and they are no longer outsmarted by turbulence. So in conclusion would this work with wide field transit surveys and radial velocity spectrums to give the same results or better then space born telescopes?

  • Geoffrey Hillend October 17, 2018, 18:33

    Quote by me: “I thought this might not be possible because the bigger star might with the bigger gravity might capture or hog all the gas and there could be no red giants.” I meant I thought there could be no gas giants like Jupiter and Saturn near a star without migration, not red giants which is a typo.

  • Michael C. Fidler October 17, 2018, 19:02

    Spectroscopic binary stars make up about 50 percent or more of all binary stars and these close in systems did not need migration to form. The other characteristic that could be involved in the development of these planets may be the Lagrange point concentrating large amounts of material in the whirlpool that form at the Lagrange points. These could be the areas that other smaller Saturn or Neptune mass planets form and are ejected into different orbits around the developing protoplanetary disk. This could be how the large moon around the planet Kepler 1625b formed. As I have said before the earths moons large seas that now face earth may of resulted from impacts of asteroids that formed at the moons L4 Lagrange point. This would be a good model to see if many of the multi-planet system did not form in this way – after all there are 5 Lagrange points around every planet and moon that forms. So did Jupiter form Saturn, Uranus, Neptune, Venus and the Earth at it’s 5 Lagrange points when the protoplanetary disk formed around the Sun?

  • Laintal October 17, 2018, 20:01

    Another interesting post Paul

    I have just read the following articles along the same topic lines, they make for interesting reading

    Planets around Other Stars are like Peas in a Pod
    http://keckobservatory.org/planets-around-other-stars-are-like-peas-in-a-pod

    The California-Kepler Survey V. Peas in a Pod
    https://arxiv.org/abs/1706.06204

    Cheers Laintal

    • Paul Gilster October 17, 2018, 20:24

      Thank you, Laintal. I appreciate the links and will check them out.

  • Geoffrey Hillend October 17, 2018, 23:13

    I agree that it’s not about competition, but which model or theory will best fit observations in all circumstances, so it will become a general principle. The idea that gas giants might not need a solid, rocky core to form the way rocky, terrestrial planets form makes sense and it would explain why M class stars have Neptune and Jupiter sized bodies near to them without the need for a migration theory.

    • Ron S. October 18, 2018, 13:38

      “which model or theory will best fit observations in all circumstances”

      This doesn’t address my point. Giant planets could form in more than one way. There is no a priori reason for there to be only one, nor therefore to search for a a singular formation theory.

      • Rob Flores October 20, 2018, 1:03

        Unfortunately both, solid core gas giants and planetary migration
        have become indoctrinated simply because of the volumes of Post Doc research supporting this view(albeit mathematical/geophysical constructs, not direct evidence supporting such mechanisms/states),

        There is always a resistance when trying to discard something with
        alot of “graduate papers”. Nobody want their work invalidated and
        wont accept their view as obsolete until 99% evidence says it should be, and sometimes minds are never changed (see plate tectonics and the older generation of Geologists.)

  • Geoffrey Hillend October 18, 2018, 18:43

    It would be very hard for a gas giant to form near a star by having a rocky core first due to the solar wind or T tauri phase of a star blowing away all the gas. The hot Jupiter in CI Tau is there too early; It would not have time to have formed a rocky core or become large like a terrestrial planet These Jupiter don’t seem to depend on location. If these gas giants are there this early in the formation of the infant planetary star system, then they might have no rocky solid cores which is how stars form. All gas giants might form like stars.

    This is of course a theory, idea and an assumption, but it would explain hot gas giants. I agree that they might just as easily form with a rocky core first as some large proto planets might form early in an infant star system. It would interesting to know what is the best theory once we get more data on many infant proto planetary systems.

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