We’ve developed many techniques for planetary detection since the first discovery of a planet orbiting a main sequence star in 1995, but a recent addition to the repetoire is looking in systems already known to have planets. By studying stars that display a transiting planet — a planet moving in front of the star as seen from Earth — any variation in time between the transits can be detected. From that data, information about any unseen planet perturbing the transits can be inferred.

The new method is called TTV, for ‘transiting timing variations,’ and here’s the most exciting thing about it: a planet comparable to the size of Earth should be detectable using these methods, giving TTV a sensitivity in advance of any other current detection method. And we do expect to find many a multi-planet system out there, with data that can provide insights into the formation of our own Solar System. But to use the method we have to discover and monitor transiting planets.

All of this is summarized in a new paper by Jason H. Steffen and Eric Algol, physicists at the University of Washington. Their work focuses on the TrES-1 planetary system discovered in 2004, and reports on their search for a possible perturbing planet in that system. Found through the efforts of the Trans-Atlantic Exoplanet Survey (TrES), TrES-1 is roughly 500 light years from the Sun in the constellation Lyra. Although Steffen and Algol could find no convincing evidence for a second planet in the system, they were nonetheless able to place constraints on any planet that might be there.

From the paper:

It is unclear what fraction of probable planetary companions are excluded by our results….The sensitivity of TTV to the mass of a perturbing planet renders it ideal for discovering and constraining the presence of additional planets in transiting systems like TrES-1. These studies can help determine the ubiquity of multiple planet systems and resonant systems-including the distribution of mass in those systems. Moreover, TTV analyses of several systems can play a role in identifying the importance of various planet-formation mechanisms. For example, the presence of close-in terrestrial planets favors a sequential-accretion model of planet formation over a gravitational instability model…

So the TrES-1 system may or may not include other planets than the one already detected, but Steffen and Algol argue that TTV methods applied to higher precision observations should be able to create a more detailed map of such systems. TTV thus emerges as another tool in the planet hunter arsenal, complementing radial velocity, planetary microlensing and planetary transit detections. It may be taking us one step closer to the first terrestrial planet ever found orbiting another star.

The paper is Steffen, J. and E. Algol, ” An analysis of the transit times of TrES-1b,” available at the arXiv site via the above link.