A gallery of six exoplanets that have retrograde orbits (artist concepts). ESO/A. C. Cameron

Dropping a Bomb About Exoplanets

Article Updated: 24 Dec , 2015


Not all exoplanets are created equal, and new discoveries about the orbits of newly found extra solar planets could challenge the current theories of planet formation. The discoveries also suggest that systems with exoplanets of the type known as hot Jupiters are unlikely to contain Earth-like planets. “This is a real bomb we are dropping into the field of exoplanets,” said Amaury Triaud, a PhD student at the Geneva Observatory who led an observational campaign from several observatories.

Six exoplanets out of twenty-seven were found to be orbiting in the opposite direction to the rotation of their host star — the exact reverse of what is seen in our own Solar System. The team announced the discovery of nine new planets orbiting other stars, and combined their results with earlier observations. Besides the surprising abundance of retrograde orbits, the astronomers also found that more than half of all the so-called “hot Jupiters” in their survey have orbits that are misaligned with the rotation axis of their parent stars.

Hot Jupiters are planets orbiting other stars that have masses similar to or greater than Jupiter, but which orbit their parent stars much more closely.

Planets are thought to form in the disc of gas and dust encircling a young star, and since this proto-planetary disc rotates in the same direction as the star itself, it was expected that planets that form from the disc would all orbit in more or less the same plane, and that they would move along their orbits in the same direction as the star’s rotation.

“The new results really challenge the conventional wisdom that planets should always orbit in the same direction as their stars spin,” said Andrew Cameron of the University of St Andrews, who presented the new results at the RAS National Astronomy Meeting (NAM2010) in Glasgow, Scotland this week.

Artist’s impression of an exoplanet in a retrograde orbit. Credit: ESO

At this writing, 454 planets have been found orbiting other stars, and in the 15 years since the first hot Jupiters were discovered, astronomers have been puzzled by their origin. The cores of giant planets are thought to form from a mix of rock and ice particles found only in the cold outer reaches of planetary systems. Hot Jupiters must therefore form far from their star and subsequently migrate inwards to orbits much closer to the parent star. Many astronomers believed this was due to gravitational interactions with the disc of dust from which they formed. This scenario takes place over a few million years and results in an orbit aligned with the rotation axis of the parent star. It would also allow Earth-like rocky planets to form subsequently, but unfortunately it cannot account for the new observations.

To account for the new retrograde exoplanets an alternative migration theory suggests that the proximity of hot Jupiters to their stars is not due to interactions with the dust disc at all, but to a slower evolution process involving a gravitational tug-of-war with more distant planetary or stellar companions over hundreds of millions of years. After these disturbances have bounced a giant exoplanet into a tilted and elongated orbit it would suffer tidal friction, losing energy every time it swung close to the star. It would eventually become parked in a near circular, but randomly tilted, orbit close to the star. “A dramatic side-effect of this process is that it would wipe out any other smaller Earth-like planet in these systems,” says Didier Queloz of Geneva Observatory.

The observatories for this survey included the Wide Angle Search for Planets (WASP), the HARPS spectrograph on the 3.6-metre ESO telescope at the La Silla observatory in Chile, and the Swiss Euler telescope, also at La Silla. Data from other telescopes to confirm the discoveries.

Source: ESO


8 Responses

  1. Sirius_Alpha_1 says:

    Minor and irrelevant nitpick. The artist impression of WASP-5 shows a prograde orbit, not a retrograde one.

    There seems to be two populations of hot Jupiters as far as the orbital alignments go. One group is nice and well aligned, the other is seemingly random and/or retrograde.

    Perhaps planet migration via interaction with the disk may explain the first population, and planet-planet scattering may explain the later.

    Not sure why only one mechanism is needed.

  2. Sirius_Alpha_1 says:

    The sixth retrograde planet is HAT-P-7 b, by the way. WASP-5 b’s orbit is prograde.

  3. Dark Gnat says:

    I agree that there probably shouldn’t be a “one size fits all” apporach. There are too many variables.

    If a star forms in a gas cloud, then it’s reasonable that other stars would form nearby, and their gravity would have an effect.

    As for retrograde orbits:

    A: Could the star have simply “rolled over”?

    B: Could the planets be captured?

  4. Torbjorn Larsson OM says:

    The discoveries also suggest that systems with exoplanets of the type known as hot Jupiters are unlikely to contain Earth-like planets.

    “Whether a significant fraction of hot Jupiter systems showing misalignment implies that a significant fraction of planetary systems are misaligned is another matter though. Hot Jupiters appear to be pretty rare: we find lots of them because they are easy to find, but apparently the proportion of stars that have them is about 1/300. This contrasts markedly with results from microlensing surveys that suggest systems with long period “cold Jupiters” like the one in our solar system have about 15% occurrence rate.”

    Quoted from andy @ Bad Astronomy.

    So ~ 20 % of 1/300 or ~ 0.7 % of systems definitely doesn’t have Earth analogs. While ~ 20 times as many systems are potential whole hog solar system analogs.

    OK, color me optimistic.

  5. Lawrence B. Crowell says:

    Some years ago I studied the catalogue of extrasolar systems. The most frequent place for Jovians I found to be around 1AU. This too precludes terrestrial planets, for their orbits would be too chaotic.

    The distribution of jovians pretty much scratches the old idea of solar system formation. This had stellar radiation clearing out the inner regions of gas and pushing it to the outside where jovians form.


  6. Jorge says:

    We’re finally getting to understand that solar systems (and even the planets themselves) form through a multiplicity of ways. Here’s another possibility:

    All theories of planetary formation I heard of look at the systems where planets form as more or less closed systems. The exception is this one we’re seing now, where perturbations caused by nearby stars are being used to explain “misbehaved” planets such as these retrogrades.

    We are also now realizing that planetary formation must produce a rather large popupation of free-floating planets, ejected from their systems in various phases of their evolution. These castaways are usually forgotten, as if they stayed for ever in interstellar space, not interacting with anything.

    However, they would interact with everything they happened to float close to. They could capture and lose satellites, for instance, they could hurl small bodies into “normal” planetary systems. They could do all sorts of things, and the bigger they are to start with, the better.

    And what would happen if they dived into a dense molecular cloud? Did anyone simulate that yet? I foresee that, under adecuate circumstances, one such rogue planet could very well become a seed for a new star. If it had a faraway satellite (and Hill spheres, in interstellar space, are HUGE), it could also grow and maybe become a planet. Any accretion disc that would form around the primary would likely not follow the rotation plane of the planet-to-be-star, nor of the satellite-to-be-planet. And so on, and so forth.

    Such situations would probably be excedingly rare, but I’d bet there are systems formed this way, somewhere in this vast universe of ours. Maybe one population of such retrogrades gor originated by this kind of mechanism.

    Just a little late-night thought experiment. What do you say?

  7. KultiVator says:


    There’s plenty of logic in your comments.

    A planet set adrift may eventually find itself in a neighbourhood full of gas and other matter that might make for a nice planet-swelling snack!

    Perhaps we might one day ovserve rogue (starless) gas giants and maybe even the odd rock giant out there, not so far from home. (I’m not talking about ‘Planet X’ here by the way)

  8. Polaris93 says:

    Is it possible that the reason that those hot Jupiters in retrograde orbits were not originally part of the systems we observe them in, but instead were captured by those stars? Triton (http://en.wikipedia.org/wiki/Triton_(moon)), Neptune’s largest moon, is in a retrograde orbit about its primary, one of the reasons it is believed it was captured by Neptune from the Kuyper Belt, where it originally had its own orbit about the Sun. So why couldn’t a hot Jupiter have been captured by the star it currently orbits from somewhere outside that star’s system?

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