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posted by martyb on Friday December 04 2015, @12:36AM   Printer-friendly
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

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  • (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.

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  • (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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