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posted by martyb on Friday December 04 2015, @12:36AM   Printer-friendly [Skip to comment(s)]
from the still-leaves-a-lot-of-exoplanets dept.

Over half of the gas giant "exoplanets" spotted by the Kepler telescope may actually be explained by other astrophysical phenomena, such as binary stars and brown dwarf stars:

It's always exciting when Kepler discovers a new exoplanet, and it's generally assumed that there is a relatively low chance of a false positive. But according to a new study, there may be a much higher rate of false positives than we thought with regard to gas giants, possibly up to 55%.

In the study, astronomers from Instituto de Astrofísica e Ciências do Espaço examined a sample of 129 gas planets detected by Kepler through the transit method. The transit method involves extrapolating the existence of a planet from the periodic dimming of a star's light emission that is presumably caused by an exoplanet's orbit. They found that approximately half of them weren't planets at all; rather, the light's dimming was caused by some other astrophysical phenomenon.

Gas giants are particularly vulnerable to false positives, as they can easily be imitated by eclipsing binaries. Eclipsing binaries are binary star systems aligned with the observer's (in this case, Kepler's) line of sight, which causes the larger star to block the light from the smaller. The researchers found that 52.3% of the gas giants were actually eclipsing binaries, while 2.3% were brown dwarfs, or a "failed star" between gas giants that doesn't have enough mass to fuse hydrogen to its core.

Also at the Institute of Astrophysics and Space Sciences.

SOPHIE velocimetry of Kepler transit candidates XVII. The physical properties of giant exoplanets within 400 days of period

Original Submission

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  • (Score: 2) by bzipitidoo on Friday December 04 2015, @12:58AM

    by bzipitidoo (4388) on Friday December 04 2015, @12:58AM (#271638) Journal

    Astronomers have been pretty conservative about declaring that data that suggests the existence of an exoplanet is not caused by something else. This is only one possibility they check before announcing. It can take years, waiting for multiple transits or wobbles, before they feel the evidence is compelling enough to be sure a planet is causing the observed phenomena.

    So I don't know about this article that suggests they got it that wrong. Over half? Really? If true, pretty bad, sloppy astronomical work. Perhaps planet hunting got turned into a competition, with rival teams racing to announce first. Maybe that could explain how over half the results are false positives. Or, maybe the article is full of crap. After all, we recently had the sensationalist speculation that the odd fluctuations in intensity of a recently examined star could be caused by megastructures built by an alien civilization. I'm reserving judgment.

    • (Score: 0) by Anonymous Coward on Friday December 04 2015, @01:05AM

      by Anonymous Coward on Friday December 04 2015, @01:05AM (#271642)

      Given the nature of their report you would think they would be more conservative in the claims:

      We confirm that giant planets receiving a moderate irradiation are not inflated but we find that they are in average smaller than predicted by formation and evolution models. In this regime of low-irradiated giant planets, we find a possible correlation between their bulk density and the Iron abundance of the host star, which needs more detections to be confirmed.

      Really? They can't even see these gas giants, they see cyclical fluctuations in light intensity from stars.

      • (Score: 2) by HiThere on Friday December 04 2015, @03:49AM

        by HiThere (866) on Friday December 04 2015, @03:49AM (#271688) Journal

        You thought that had better evidence? We could get it, but it would cost a lot more that funding sources have been willing to spend. I once saw a proposal that would give really good evidence. A Neptune orbit telescope (for the cold) with a 5-mile diameter mirror. (You could build it out of flat mirrored segments.) That would give you enough resolution to see clouds in motion, possibly in planets in the Magellanic Clouds.

        Even better use a pair of them with an optical link so you could calculate distance by parallax rather than using various inferred measures of distance.

        But as I said, this would be a mite expensive. It would clearly need to be a totally automated system, so you need to design it for repair, but at that distance from the sun it wouldn't need a liquid helium cooler to handle Infra-red.

        Well, that's unrealistic. Nobody's going to fund that before we have a serious space industry. So most of astronomy is done working long chains of inference, often with several weak links. E.g., ever hear of the kind of super-nova called the "standard candle"? How do you know the supernova you're looking at is the right kind of supernova? By inference. Unfortunately, there are other quite similar events that are hard to tell apart. So that's a weak link. Parallax would be much better, but for some purposes even Neptune's orbit is too small a diameter of separation. (The big bang is a long distance away, so when you're looking at things near it, desolution gets difficult. [Actually the big bang happened everywhere if it happened anywhere, but when you're looking back in time you're effectively looking a long distance measured by how much the light has diverged.])

        Caution: I am not an astrophysicist or cosmologist. Don't depend on this comment for anything expensive. But I'm pretty sure everything's right.

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    • (Score: 2) by darkfeline on Friday December 04 2015, @05:36AM

      by darkfeline (1030) on Friday December 04 2015, @05:36AM (#271701) Homepage

      You can't really blame them though. We are talking about trying to detect things extraordinarily far away. No, farther than that. No, even farther than that.

      We're talking about distances where a slight vibration would make an Earth-bound telescope point at a different galaxy.

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    • (Score: 2) by cubancigar11 on Friday December 04 2015, @09:05AM

      by cubancigar11 (330) on Friday December 04 2015, @09:05AM (#271738) Homepage Journal

      Even with >50% false positives, there are too many planets that the chance of an alien life form is still exciting.

      • (Score: 0) by Anonymous Coward on Friday December 04 2015, @09:13AM

        by Anonymous Coward on Friday December 04 2015, @09:13AM (#271741)

        The alleged false positives are for gas giants, which wouldn't be supportive of life anyway. As far as I can see, they don't claim false positives on the smaller rocky planets, which are the ones which possibly bear life.

  • (Score: 0) by Anonymous Coward on Friday December 04 2015, @09:07AM

    by Anonymous Coward on Friday December 04 2015, @09:07AM (#271739)

    What's the difference between a gas giant and a "failed star"?

    • (Score: 2) by sudo rm -rf on Friday December 04 2015, @01:06PM

      by sudo rm -rf (2357) on Friday December 04 2015, @01:06PM (#271774) Journal

      Depends on who you ask. Some say a "failed star" or brown dwarf has experienced fusion in its core at one time in its past, while a gas giant has not. What kind of fusion depends on the mass, but that's only relevant for sub-classification of the body (spectral sequence).
      In this [] [pdf] paper from 2008, the author Adam J. Burgasser (assistant professor of physics at the Massachusetts Institute of Technology in Cambridge) tells us:
      a) Brown dwarf vs. star

      For objects with mass less than about 0.072 M� [that is solar masses, I don't know if the glyph is posted correctly], degeneracy pressure halts contraction before the critical H fusion temperature is reached. Hydrostatic equilibrium, but not thermal equilibrium, is achieved. Such “failed stars” are brown dwarfs.

      and b) Brown dwarf vs gas giant (planet)

      The distinction between hydrogen-fusing stars and brown dwarfs is well defined. But what distinguishes brown dwarfs from planets, given their similar sizes and atmospheric properties? Astronomers vigorously debating that semantic question fall mainly in two camps. One advocates a definition based on formation—a brown dwarf condenses out of giant molecular clouds, whereas a planet forms via core accretion in a circumstellar debris disk. The other focuses on interior physics: A brown dwarf must be heavier than the mass threshold for core fusion of any element, roughly 13 Jupiter masses, or 0.012 M�. Pluto’s recent demotion has focused attention on the ambiguity of the term “planet” in the solar system. Brown dwarfs are forcing us to reexamine a related ambiguity in a galactic context.

      • (Score: 0) by Anonymous Coward on Friday December 04 2015, @02:16PM

        by Anonymous Coward on Friday December 04 2015, @02:16PM (#271787)

        Alright, so

        0.003 Jupiter = rocky planet

        0.05 Jupiter = ice giant planet

        ?? < 1 Jupiter -- 13 Jupiter = gas giant planet

        13 Jupiter -- 78 Jupiter = brown dwarf (failed star)

        78 Jupiter -- ?? = succesful star

        1000 Jupiter = the Sun

        20000 Jupiter = Deneb

        • (Score: 2) by takyon on Friday December 04 2015, @05:13PM

          by takyon (881) <reversethis-{gro ... s} {ta} {noykat}> on Friday December 04 2015, @05:13PM (#271860) Journal

          That looks right. Apparently there is a little more going on [] above 60-65 Jupiter masses.

          They can fuse lithium above 65 Jupiter masses:

          They occupy the mass range between the heaviest gas giants and the lightest stars, with an upper limit around 75 to 80 Jupiter masses (MJ). Brown dwarfs heavier than about 13 MJ are thought to fuse deuterium and those above ~65 MJ, fuse lithium as well.

          In theory, a brown dwarf below 65 MJ is unable to burn lithium by thermonuclear fusion at any time during its evolution. This fact is one of the lithium test principles to examine the substellar nature in low-luminosity and low-surface-temperature astronomical bodies.

          The presence of lithium is a good test for whether a heavier object is a brown dwarf and not a low-mass star:

          Lithium is generally present in brown dwarfs and not in low-mass stars. Stars, which reach the high temperature necessary for fusing hydrogen, rapidly deplete their lithium. This occurs by a collision of lithium-7 and a proton producing two helium-4 nuclei. The temperature necessary for this reaction is just below the temperature necessary for hydrogen fusion. Convection in low-mass stars ensures that lithium in the whole volume of the star is depleted. Therefore, the presence of the lithium line in a candidate brown dwarf's spectrum is a strong indicator that it is indeed substellar. The use of lithium to distinguish candidate brown dwarfs from low-mass stars is commonly referred to as the lithium test, and was pioneered by Rafael Rebolo, Eduardo Martín and Antonio Magazzu. However, lithium is also seen in very young stars, which have not yet had enough time to burn it all. Heavier stars, like the Sun, can retain lithium in their outer atmospheres, which never get hot enough for lithium depletion, but those are distinguishable from brown dwarfs by their size. On the contrary, brown dwarfs at the high end of their mass range can be hot enough to deplete their lithium when they are young. Dwarfs of mass greater than 65 MJ can burn off their lithium by the time they are half a billion years old, thus this test is not perfect.

          But they could be too cool to fuse lithium:

          A remarkable property of brown dwarfs is that they are all roughly the same radius as Jupiter. At the high end of their mass range (60–90 MJ), the volume of a brown dwarf is governed primarily by electron-degeneracy pressure, as it is in white dwarfs; at the low end of the range (10 MJ), their volume is governed primarily by Coulomb pressure, as it is in planets. The net result is that the radii of brown dwarfs vary by only 10–15% over the range of possible masses. This can make distinguishing them from planets difficult.

          In addition, many brown dwarfs undergo no fusion; those at the low end of the mass range (under 13 MJ) are never hot enough to fuse even deuterium, and even those at the high end of the mass range (over 60 MJ) cool quickly enough that they no longer undergo fusion after a period of time on the order of 10 million years.

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    • (Score: 0) by Anonymous Coward on Friday December 04 2015, @09:13PM

      by Anonymous Coward on Friday December 04 2015, @09:13PM (#271951)

      Goatse and Paris Hilton?

      Or is one of those a dwarf planet? hmm. Well at least Paris's stardom is gone.

  • (Score: 0) by Anonymous Coward on Friday December 04 2015, @08:41PM

    by Anonymous Coward on Friday December 04 2015, @08:41PM (#271940)

    Astronomy error: finding 50% wrong, errors corrected within months, cost ~0 - let's say $1M, end of story.
    WMD in Iraq: finding 100% wrong, errors disputed endlessless, cost multiple $Trillion, on-going, country-destroying conflicts.