Tuorla Observatory News 20 Feb 2009

Sensitive technique reveals a new exoplanet

Exoplanets, planets going around other stars than our Sun, were first discovered just over a decade ago. The progress since then has been rapid, with hundreds of such planets now known. Most of these planets are found by searching for the small changes in the radial velocity of the host star as the planet orbits it.

In the case of a single exoplanet orbiting a host star, these changes in the radial velocity are very smooth and regularly periodic, and ordinary methods for searching for periodicities in the data are effective. But as the number of planets around a host star increases, the periodicities become harder to glean from the data, and traditional methods may fail to detect the extra planets alltogether, or give biased results for the planets which are detected.

Now, Mikko Tuomi (Department of Mathematics and Statistics, University of Helsinki) and Samuli Kotiranta (Tuorla Observatory, University of Turku), have applied a method developed in the 1950s for computing statistical mechanical properites of molecules, to the task of finding planets hidden in radial velocity data. Their technique takes advantage of the fact that planets must move in particular orbits around their host star governed by Newtonian gravity and Kepler's laws of planetary motion. Adding these relevant physical constraints to the search greatly improves the method's sensitivity.

The two researchers, who are both PhD students at their respective universities, have applied the method to the star HD 11506, which is known to host at least one exoplanet -- and has been suspected to host a second planet. The team not only recovers the known planet, but shows that there is indeed a second planet around this star with high probability. The second planet has a mass which is about half that of the mass of Jupiter, and a period around its host star of about 6 months. If confirmed, this will be the first planet discovered by a Finnish team -- with the promise of many more to come, as there are many other host stars to which the technique can be applied.

The study has been accepted for publication in Astronomy and Astrophysics Letters, and a preprint is available here.

The radial velocity curve for the star HD11506, showing the velocity of the star as a function of time. Measurements of the star's velocity are shown as circles, while the line represents how the radial velocity would varies for the best fitting case of two exoplanets circling the star. The line is a remarkably good fit to the data, and considerably better than the single planet case. The two planets have estimated masses of about 3 and 0.5 times the mass of Jupiter, and periods of 3.5 and 0.5 years respectively. A larger version of the figure is available here.

This page was last modified by  Chris Flynn  on  20/02/2009 15:30  astroweb@utu.fi