The Rossiter-McLaughlin effect is an evolving tool for exoplanet research, one that has already begun to pay off. We recently looked at a paper studying whether this quirk of radial velocity methods could help in the detection of a terrestrial-class planet. The effect causes a distortion in radial velocity data during a planetary transit, one that seems to indicate a change in the velocity of the star under study. But in reality there is no change — what observers see is the effect of the transiting planet on the starlight, as shown in the diagram below.
It turns out the effect might be useful in finding planets larger than two Earth radii, but perhaps less so with smaller worlds. However, new work by a Japanese/American team using the Subaru Telescope points to a different observational capability. Observing the extrasolar system TrES-1, the team has been able to measure the angle between the parent star’s spin axis and the planet’s orbital axis, only the third time such an alignment has been measured.
Image: The Rossiter-McLaughlin effect is defined as the radial velocity anomaly during a transit from the known Keplerian orbit caused by the partial occultation of the rotating stellar disk. For example, if a planet occults part of the blue-shifted (approaching) half of the stellar disk, then the radial velocity of the star will appear to be slightly red-shifted, and vice-versa. The radial velocity anomaly depends on the trajectory of the planet across the disk of the host star, and in particular on the spin-orbit alignment of the system. Thus by monitoring the Rossiter-McLaughlin effect one can measure the spin-orbit alignment. Credit: National Astronomical Observatory of Japan.
TrES-1, a K0 star with a visual magnitude of 12, is the faintest target ever used in such work; its planet TrES-1b is a ‘hot Jupiter’ with an orbital period of three days. The outcome is welcome considering that ongoing transit surveys often work with faint stars indeed. In fact, we’re getting more and more interested in M-dwarfs in a variety of ways (especially in an astrobiological sense), knowing that the dim red objects may help us snare transits of objects of Earth’s size and even smaller. Now we have a way to measure spin/orbit alignments, allowing the collection of further data by which to study planet formation.
Ponder this: Given the preponderance of Jupiter-class worlds close to their parent stars that has emerged from early radial-velocity work, we’d like to know more about how they formed. Migration inward through the early solar system seems a reasonable assumption. Planets that form and migrate like this ought to show relatively minor misalignments between the stellar spin axis and the planet’s orbital axis.
Image: An illustration of the concept of the spin-orbit alignment (described by lambda) in an exoplanetary system. Credit: National Astronomical Observatory of Japan.
But planets dispersed by the gravitational effects of giant planets (‘planet-planet scattering’) through a protoplanetary disk should show tilts different from the original orbital axis. If such mechanisms are in play in a given system, measuring the spin/orbit alignment through the Rossiter-McLaughlin effect should reveal the resultant misalignment. That makes this tiny observational twist a genuine diagnostic tool in the continuing development of planet formation theories.
What’s next is surely to tighten up the numbers. The current study can only show an alignment angle of 30 degrees with an error of plus or minus 21 degrees. These are useful contraints (enough to demonstrate the prograde orbital motion of TrES-1b), but they’ll be refined with future work. The paper is Narita et al., “”Measurement of the Rossiter–McLaughlin Effect in the Transiting Exoplanetary System TrES-1,” Publications of Astronomical Society of Japan, Vol 59, No. 4 (August 25, 2007), pp. 763-770, available here.