Testing for the Presence of a Planet

T Tauri stars are a young class of variable stars. The T Tauri star PTFO 8-8695, which lies around 1,100 light-years away in Orion, has been suspected of harboring a close-in giant planet. A recent study, however, casts doubt on this theory.

Fading Star

Finding a close-in giant planet around a very young (i.e., less than a few million years old) star would be a major discovery. Such a system could tell us about when planets form, how they behave as newly-born planets, and the mechanism that causes planets to migrate inwards to create hot Jupiters. Thus far, the only candidate system of a very young star with a close-in giant planet is PTFO 8-8695.

On top of its normal variability, PTFO 8-8695 was found to exhibit periodic fading events, during which it dims by a few percent for an interval of ~1.8 hours. The leading interpretation of these events was that they are caused by the transit of a planet with a precessing orbit.

Normally we would test this hypothesis by looking for radial-velocity variation of the host star, but stellar activity prevents us from being able to pick out a signal here. So a collaboration of scientists led by Liang Yu (Massachusetts Institute of Technology) set out to systematically test this theory using less conventional approaches.

Failed Tests

Yu and collaborators used three different tests:

  1. They looked for slow changes in the light curve morphology.
    A planet with a precessing orbit would leave a distinctive signature in the appearance of the light curve. The team, however, didn’t find this predicted morphology. Even more importantly, they discovered that the fading events aren’t even strictly periodic — in recent observations, the period has decreased.
  2. They tried to detect the planet’s radiation in infrared.
    Planets emit in infrared, so by observing the system during times when the planet is supposedly passing behind the star, there should be a visible dip in the measured infrared radiation. No dip of the predicted size was found.
  3. They looked for the Rossiter-McLaughlin effect.
    The RM effect is an anomaly that appears in spectroscopic data of a planet’s transit due to stellar rotation. To check for this, the team obtained high-resolution spectroscopy during a fading event. No effect was seen at the predicted level in their data.

Given these three failed tests, and the new evidence that the fading events aren’t strictly periodic, the authors argue that these events aren’t caused by a planet transiting PTFO 8-8695. Instead, the team put forward a few new hypotheses, including starspots, eclipses by circumstellar dust, or occultations of an accretion hotspot. Further observations should help to narrow down the possibilities.

Citation

Liang Yu et al 2015 ApJ 812 48. doi:10.1088/0004-637X/812/1/48

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