Slashdot Mirror

← Back to Stories (view on slashdot.org)

Several Extrasolar Planets May Be Optical Illusions

Posted by Roblimo on from the not-everything-is-what-it-seems-to-be dept.

person-0.9a writes "Seems that a few of the extrasolar planets detected via their sun's wobble might be written off according to a CNN article. You can also read it about it in New Scientist."

2 of 136 comments (clear)

  1. Re:Anyone who knows more care to elaborate? by renard · · Score: 5, Informative
    Sun spots can masquerade as Doppler shifts by altering the spectrum of the star systematically as the star rotates. You think it's the planet's period, but it's actually the star's rotation (at the latitude of the sunspot). That's why Greg Henry's discovery of photometric variability with the same period as the "planet" is so damning.

    Planetary transit searches will be subject to sunspots as a noise source (star's light changing by small amounts, erratically), but are unlikely to be fooled by them. A planetary transit causes the star's light to dim in a distinct flat-bottomed way that a sunspot cannot mimic.

    Note also that no one has discovered a planet yet using the transit-search technique. The transiting planet of Henry & Charbonneau was known to exist already thanks to the spectroscopic surveys.

    -Renard

  2. Re:Anyone who knows more care to elaborate? by TMB · · Score: 5, Informative

    (preface: yes, I Am An Astronomer)

    The important thing to realize is that you're measuring the spectrum integrated over the entire surface of the star WEIGHTED BY THE FLUX (read: brightness) AT THAT PART OF THE SURFACE.

    The star is rotating, so part of the star is moving away from us and is red-shifted, while part is moving toward us and is blue-shifted. If the surface were all radiating at the same level, then we'd always see part of the spectrum blue-shifted and part red-shifted.

    Now stick a big starspot on. The starspot is fainter than the rest of the star. When the starspot is on the approaching side of the star, there is less blue-shifted light and so the net spectrum appears slightly red-shifted. Half a period later, the starspot is on the receding side of the star so there is less red-shifted light, and the net spectrum appears slightly blue-shifted.

    The end result is a spectrum that systematically shifts back and forth, very similar to that of a star with an orbiting planet.

    [TMB]