Did Astronomers Catch Sight of an Exoplanet in Its Death Throes?

By Eliza Strickland | August 27, 2009 10:42 am

hot jupiterIn a star system 330 light years away from Earth, astronomers have spotted a giant planet that booms around its parent star in tight, fast circles, completing an orbit (the planet’s “year”) in less than one Earth day. The exoplanet, known as Wasp-18b, is so close to its star that researchers say it appears to be spiraling inwards to its fiery doom. But the odds of seeing a planet in its death throes are so low that researchers are searching for alternate explanations, and say the planet could force scientists to rethink established ideas about planetary forces known as tidal interactions [National Geographic News].

The planet is known as a “hot Jupiter,” meaning that it’s a massive gas giant like our own solar system’s Jupiter, but it orbits in close proximity to its star. Current theories say that such a massive planet so close to its star should be pulling on the host star, creating a tidal effect similar to the moon’s pull on Earth. At that range the planet’s pull would be so strong that it would drain energy from its orbit, causing the planet to rapidly fall into the star [National Geographic News]. But if that’s the case, the planet would meet its death in less than a million years. Since the star system is thought to be about 1 billion years old, the odds of catching the planet in its last stages are one in a thousand.

In the study, published in Nature, researchers list other explanations for the odd state of affairs. One possibility is that Wasp-18, a sunlike, medium-sized star, is a thousand times less energetic than would be expected. That would mean it produces much less friction on the planet than normal. This orbital drag, which scientists call the “tidal dissipation factor,” slows a planet each time it circles its star [Los Angeles Times]. If the star is producing less friction, it may not be slowing the planet’s orbit as much as expected. However, this would suggest that sun-like stars can have characteristics that scientists have never seen before.

Astronomer Douglas Hamilton, who wasn’t involved in the research, says the final possibility is that “we’re just missing something — there is some property of stars or tides that we just don’t understand.”… An answer could be coming in just a few years. According to [lead researcher Coel] Hellier, if the orbit of Wasp-18b really is decaying at the expected rate, the effects should be measurable within the next decade [Los Angeles Times].

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Image: C. Carreau / ESA

  • YouRang

    The odds are 1 in a thousand, so what? There are a hundred billion stars in the galaxy. And 999/1000 ^100,000,000,000 is almost zero probability that there aren’t planets at the appropriate stage to be found. (And that’s ignoring the preferential method of finding planets (both for raising the probability and lowering it).)

  • amphiox

    Agree with YouRang above. Given that we’ve already discovered several hundred exoplanets, and our methods are prejudiced towards large, close-in ones, odds of 1/1000 aren’t actually all that bad. We’d have close to a 50-50 chance of having found one already. Maybe this is it.

  • Brian

    Adding to prior postings only.

    The odds would only be 1/1,000 for any single planet (in fact, for this specific system, not just any random system). But we don’t know just one exoplanet! We know, what, 400 odd? That’s not so far from a thousand. I’d hesitate to say that our odds of finding such an end-stage exoplant at 1/1, but they are certainly falling, and pretty rapidly I’d guess.

    Also, our detection methods are still biased towards “hot Jupiters” so far as I know, and we’ve detected lots of those. So many that the planetary formation theorists have had to incorporate some new ideas to make room for the new data.

    Finally, an orbital year of one (Earth) day–obviously that’s fast. Really, really fast! But is it so improbable? Consider: HAT-P-7b, orbital period 2.2 days. HD 209458 b, orbital period 3.5 days. HD189733b, orbital period 2.2 days.

    It doesn’t sound so improbable when put that way.

  • Ryan

    Saying that the odds are one in a billion is an overstatement. What are the odds of a star system having an infalling gas giant in its first million years? Doesn’t it stand to reason that the chances of this phenomenon go up as a star system ages (to a point at least).

  • http://www.science27.com Bjarne Lorenzen

    Mathematical Breakthrough
    It is now mathematical proven that the decelerating force that affected the Pioneer probes and the accelerating force that had caused many Fly-by anomalies:
    1.) Both affect the Earth (and the planets) as well, – and with full force.
    2.) Automatically equalize each other (when affecting the planets).
    3.) This explains the cause of the WASP-18b mystery and all the probes anomalies as well.


  • http://www.science27.com Bjarne Lorenzen

    New calculations shows that WASP-18b probably not will be stable in its present orbit, but will be thrown away with from the star, with a velocity 3 times higher than the velocity astronomers today believe it “should” approach the star.

    This certainly will be a big chock, and something we should be able to confirm within 1 to 2 years.

    The idea that also planets are affected of the same accelerating force that space probes (by fly by) support this new theory and make it also possible to understand the cause of the 4 following mysteries:

    1.)Why gas-planets can be found very closed to starts ( with their atmosphere intact)
    2.)Why huge planets can be found more as 15 billion km. from their mother stars.
    3.)How Jupiter’s was created (longer away from the Sun) and what brought it closer to the Sun.
    4.)How water came to earth.




  • http://www.science27.com Bjarne Lorenzen

    The conclusion in my previous post was wrong.
    It’s not enough only to consider the possible force that can be pass by, – based on the angel velocity as a result of a astronomic bodies rotation (centrifugal force) affecting the planet (WASP-18b).

    It’s also necessary calculating the possible energy requirement and compare that to the (rotation) energy available due to the stars rotation..
    This shows that even though if enough energy fast “could” be transmitted to Wasp-18b, it’s simply not available.
    Wasp-18b will therefore approach the star, and will not only be affected by the tidal effect (expected 2.8 s. per year) but will probably also be effected by too weak rotation energy (+ 2 s per year) Total 4.8 s. per year.

    However Wasp-18b should have been dismissed for several hundred million years ago. How could it “hide it self” from the tidal force so long time?
    The answer seems to be “written in the stars”.
    By comparing with other solar systems we can easy understand the logic. (The site is now updated and much better now) >


    Sorry for the confusion…

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