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Do ‘Hot Jupiters’ Rule Out Terrestrial Planets?

Meetings like the Royal Astronomical Society’s gathering in Glasgow can be overwhelming, with all kinds of news to track via emails, news releases and Twitter. Yesterday we looked at the possible signature of rocky planets in the atmospheres of white dwarfs. But the unusual orbits of planets newly discovered by the WASP project (and follow-up studies of older hot Jupiters) get pride of place as perhaps the most notable announcement so far. WASP (Wide Angle Search for Planets) turned up nine new planets, bumping the exoplanet total to 454), but follow-up studies of these worlds, along with other ‘hot Jupiters’ from earlier surveys, showed that six out of the 27 examined orbited opposite to the rotation of their host star. Moreover, more than half of the planets studied are misaligned with their star’s rotation axis.

Image: Exoplanets, discovered by WASP together with ESO telescopes, that unexpectedly have been found to have retrograde orbits are shown here. In all cases the star is shown to scale, with its rotation axis pointing up and with realistic colours. The exoplanets are shown during the transit of their parent star, just before mid-transit. The last object at the lower right is for comparison and has a “normal” orbital direction. The size of each image is three solar diameters. Credit: ESO/A. C. Cameron.

If misaligned orbits sound like a minor point, consider this: We’d like to know whether the existence of a hot Jupiter in a tight orbit rules out the possibility of a terrestrial world in a wider orbit. And the answer seems to depend on how these planets form. They’re surely born in the outer reaches of their solar systems, with cores made of the rock and ice particles we’d expect to exist there. If the hot Jupiters then went through gravitational interactions with the dust and debris disk from which they formed, they could migrate in to their present, tight orbits and should show alignment with the parent star’s rotation axis. That’s a scenario that allows planets like the Earth to form later, a cheery thought for life but not one enhanced by the new findings.

For if we’re looking at numerous misaligned orbits including retrograde paths around the primary star, then the other migration scenario comes into play. Here, we forego interactions with the dust disk and talk about much slower gravitational processes operating due to the effect of more distant planets or close stars. As opposed to millions of years, we’re now talking about taking hundreds of millions to move a giant planet into an orbit that eventually undergoes tidal friction, causing its orbit to circularize but remain randomly tilted close to the star.

Exoplanet hunter Didier Queloz (Geneva Observatory), who was closely involved with this work, notes the result: “A dramatic side-effect of this process is that it would wipe out any other smaller Earth-like planet in these systems.” To learn more, of course, we’ll need fuller descriptions of exoplanetary systems, so we can learn how many hot Jupiters occur in systems with more distant massive companions. Andrew Cameron (University of St. Andrews), who presented the current study at the RAS meeting in Glasgow, points out that the new results present quite a challenge to conventional wisdom holding that planets should orbit in the same direction as their stars spin.

Preprints of the papers from the SuperWASP Consortium can be found here. An ESO news release is also available, as is a release from Las Cumbres Observatory. Las Cumbres used robotic telescopes located in Hawaii and Australia to provide brightness measurements that helped researchers determine the size of the planets. But follow-up radial velocity observations of these transiting worlds were performed by observatories around the world, from the Nordic Optical Telescope in the Canaries to the HARPS spectrograph on the 3.6-meter ESO telescope at La Silla. This animation showing how distant objects can perturb a gas giant’s orbit may be helpful.

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Comments on this entry are closed.

  • Duncan Ivry April 14, 2010, 10:02

    “… the new results present quite a challenge to conventional wisdom holding that planets should orbit in the same direction as their stars spin.”

    The rotating dusk and gas disk of a young stellar system is a turbulent medium, as far as I know. In a physical system of this kind it is not unusual, that some small parts rotate in the direction opposite to that of the whole system.

    Because of this it should be no surprise, that in some stellar systems (a) at least one planet, having been created out of a small, ring-like part of the disk, rotates in a direction opposite to that of the star, (b) the star, having been a small part of the disk, rotates in one direction and all or most planets in the other direction, (c) some planets rotate around themselfs opposed (or with a very tilted axis like Uranus).

  • Thomas April 14, 2010, 10:05

    You know the graphic showing the retrograde orbits has an imposter, right? The bottom right one, WASP-5 b, has a prograde orbit (and indeed the graphic shows this). The sixth retrograde planet is actually HAT-P-7 b.

  • Administrator April 14, 2010, 10:26

    Thomas, I think the caption already states this — that image is for comparison.

  • andy April 14, 2010, 17:07

    Well the “clobber the planetary system” scenarios would seem to fit with the apparent deficit of additional (giant) planets in hot Jupiter systems. The majority of multi-planet systems do not appear to contain hot Jupiters, so the exceptions (especially HAT-P-13, currently the only well-characterised multi-planet system containing a transiting hot Jupiter) should prove interesting.

  • Mike April 14, 2010, 17:24

    I was of the opinion that any stellar system with hot Jupiters,pro-grade or retrograde would not contain terrestial planets in the inner system or habitable zone. The migration of the Jovian sized planet inward would displace and likely eject any planets and also remove the remaining planet forming disk eliminating any more planet formation in the inner system.
    There may still be terrestial planets in the outer system but if there is not
    much left of the original disk what could cause any outer terrestial planets
    to migrate inward?
    So,pro-grade or retrograde,hot Jupiters are bad news for Earth-type worlds.

  • djlactin April 15, 2010, 1:40

    Duncan Ivry: The point is not that the planets ROTATE retrograde, but REVOLVE retrograde. Given that a collapsing cloud would have initially evolved around its centre (source of the star?), I’m skeptical that the centre of the cloud would have been rotating in an opposite direction to the overall cloud, turbulence notwithstanding.

    I also note that the 5 retrograde orbits in the illustration are all highly inclined. This orientation is inconsistent my [simplistic?] conception that the cloud initially collapsed into a disk, before spawning planets. Does the presence of planets in highly-inclined orbits imply gravitational interactions that flung the planet far from the local ecliptic? (To reorient the orbit of a Jupiter-sized planet in this way would require some very strong interactions!) Could some early impact have reoriented the primary?

    In my opinion, the best results are the ones that raise further questions….

  • Ronald April 15, 2010, 4:49

    I agree with mike (and andy) that it was already supposed from the early days of the ‘inward migration scenario’ of hot giant planets, that these would spoil the fun for any other planet, particularly small terrestrials in the inner system.
    Really relevant question now is what dominates and delimits the formation of these ‘bad Jupiters’ (as they were sometimes called from the beginning of their discovery). As far as we know now particularly metallicity (maybe specific elements within metallicity), and stellar mass.

  • kzb April 15, 2010, 7:43

    Let’s bear in mind that Hot Jupiters were not PREDICTED by the models. Yet they are quite common. This fact alone should tell us the modellers think they know more than they really do.

  • Eniac April 15, 2010, 9:09

    Could these be ex-interstellar objects that got captured in a near-collision with the star? That would make their orbits random, I would think. I know the cross-section is minuscule, but then again, the time available is LONG. Maybe gas giant migration is not common (good for Earths), which is why most hot Jupiters are from some other process (such as capture)?

    And let’s not forget that our sample is heavily biased. Hot Jupiters may loom large now, but once our detection methods improve to where we can reliably detect “regular” Jupiters and Earth-like planets, we may find they are quite rare.

  • Mike April 15, 2010, 11:42

    Eniac says,”our samples are heavily biased.”

    That is something that can’t be overstated,our ability to detect every type of exoplanet in all types of orbits is many years away yet. However Kepler may provide us with a pretty good population sampling.

  • Thomas April 15, 2010, 15:52

    @ Administrator
    D’oh! Sorry :(

    @ kzb,
    that holds true 15 years ago. However since then, the models have been updated to incorporate migration through planet-disk interaction to explain the observed population of hot Jupiters.

    This would make sense if the orbit of the planet is coplanar with the disk, and thus the rest of the system. Now we find the planets are awkwardly aligned.

  • andy April 15, 2010, 15:54

    Actually hot Jupiters seem to be quite rare, the results of the OGLE surveys suggest an occurrence of about 0.5%. Microlensing surveys suggest that the occurrence of long-period Jupiters is somewhere near the 10% level. The high numbers of hot Jupiters in the exoplanet catalogues is due to the fact that these are the easiest planets to find.

    As for the hot Jupiters not being predicted, bear in mind that there were also limitations based on computational power on the level of details that were in the models. While this story is being spun as another “the existing models do not predict what we observe”, this is not entirely correct: there are pre-existing, well-developed models for hot Jupiter formation that predicted the misaligned planets (notably models that include the Kozai mechanism or planet-planet scattering).

  • bigdan201 April 15, 2010, 16:54

    As others have mentioned, there is a strong sampling bias which turns up lots of hot jupiters. It wouldnt surprise me if a hot jupiter system is unfriendly to terrestrial planets, but this does not concern me. Once we find a wider variety of exoplanet systems, I predict that wel find plenty of terrestrial planets, even if none of them are in a hot jupiter system.

    Also, are you sure that a skewed orbit means that it was captured? It seems to me that an orbit which varies greatly in its distance to the star (ie pluto) is the best evidence of capture. Is it possible that an even orbit that is retrograde and tilted could have occured from natural planetary formation?

  • Duncan Ivry April 15, 2010, 19:37

    djlactin: “The point is not that the planets ROTATE retrograde, but REVOLVE retrograde. Given that a collapsing cloud would have initially evolved around its centre (source of the star?), I’m skeptical that the centre of the cloud would have been rotating in an opposite direction to the overall cloud, turbulence notwithstanding.”

    I’m well aware of the above difference. From what I have seen with respect to turbulent systems over the years, I wouldn’t exclude strange cases — low probability, but not impossible.

    Above that a retrograde rotation of any kind could unfold when a young star resides in a narrow neighborhood with other young stars, among them big, hot ones, and with supernovae occuring from time to time. As far as I know, stars are often born in environments like this.

  • Stevo Darkly April 15, 2010, 21:14

    I’m well aware of the above difference. From what I have seen with respect to turbulent systems over the years, I wouldn’t exclude strange cases — low probability, but not impossible.

    Above that a retrograde rotation of any kind could unfold when a young star resides in a narrow neighborhood with other young stars, among them big, hot ones, and with supernovae occuring from time to time. As far as I know, stars are often born in environments like this.

    You know, taken out of context and understood metaphorically, that could be a psychologist’s explanation for what happens to so many child actors in Hollywood.

  • kzb April 16, 2010, 11:45

    Thomas, the point was, the models were only updated AFTER the hot Jupiters were discovered. They were not predicted before they were found.

    However Andy is saying this is not true and that some models did predict them before they were found. If this is the case then obviously I was wrong.

    A truly useful scientific hypothesis will predict something that later turns out to be true. A hypothesis that has to be amended every time some new observations come to light is not truly much good. I am asking the question to which class of hypotheses these models belong.

  • Mike April 16, 2010, 19:44

    To KZB.
    To the best of my knowledge and an internet search,hot Jupiters were never predicted.Planet migration was also never predicted.
    The more we look the more we see.

  • Colin Weaver April 16, 2010, 23:48

    To Mike – actually it could be argued that Otto Struve predicted the existence of Hot Jupiters in a paper in 1952 :-
    http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1952Obs….72..199S&data_type=PDF_HIGH&whole_paper=YES&type=PRINTER&filetype=.pdf

  • andy April 17, 2010, 5:24

    Just to clarify here, I was referring to theories for the origin of hot Jupiter planets developed after the discovery of 51 Pegasi b but before large numbers of Rossiter-McLaughlin effect measurements had been made.

    Nevertheless, I’m going to have to disagree with the assertion that hot Jupiters were not predicted before the discovery of 51 Pegasi b. Take a look at this proposal for a radial velocity search for planets dating from 1952, which proposes searching for Jupiter-type planets with orbital periods of order 1 day.

    While we’re at it, I’ll also point out that the possibility of planets orbiting around pulsars had also been considered before the actual discoveries, e.g. this paper from 1970. It is worth noting however that the known pulsar planets are inconsistent with the hypothesis of the survival of pre-existing planets presented in the paper.

  • andy April 17, 2010, 9:40

    Oh the perils of the moderation queue…

  • Administrator April 17, 2010, 9:48

    Even so, this information about Otto Struve is news to me, and thanks to both of you. I had thought hot Jupiters were never anticipated by anyone.

  • Mike April 17, 2010, 17:06

    Otto Struve had a great idea,too bad nobody attempted those RV observations back then I think. And yes,this is news to me also.
    Too bad that when the Canadian effort led by Gordon Walker to detect exoplanets by radial velocity in the 1980s they didn’t also have the idea of looking for close orbiting planets.
    That team pioneered the radial velocity method for exoplanet hunting but were unable to confirm any detections. Only later,after the 1995 detections
    did Gordon Walker review his data and found the signatures of hot Jupiters.
    If he had thought to look for them originally his team would have made the first exoplanet detections orbiting main sequence stars.
    I guess almost everybody forgot Otto Struve’s inspired proposal.

  • Duncan Ivry April 17, 2010, 22:10

    @Stevo Darkly

    Nice joke, but besides that, you know, “out of context”, “metaphorically”, “psychologist’s explanation”, and “child actors in Hollywood” is difficult for someone from a different culture and with English as a foreign language ;-)