Astronomers using data from the Keck I Observatory on top of Mauna Kea mountain in Hawaii have discovered an exoplanet in a triple star system. The planet, only slightly larger than Jupiter, is very close to the central star in the system, completing one orbit in just 3.3 days. The other two stars orbit each other very closely and their gravitational center of mass orbits the central star every 25.7 years. The discovery calls into question several theories of planetary formation, including the idea that multiple star systems are too disruptive for planetary formation. Planets may turn out to be even more common than previously predicted if this latest work is confirmed.

The new planet has been designated HD 188753 Ab because it orbits the central “A” star of the HD 188753 stellar system. The stellar system resides about 149 light-years away. The planet was not directly imaged. Instead, astronomers used the radial velocity technique that attempts to detect a so-called wobble in the star’s movement caused by a planetary companion. This wobble is actually the star speeding up or slowing down as the orbiting planet gravitationally tugs on it over a repeating period of time that matches the planet’s orbital period. If a graph of the data shows just such a wobble, then it is a good bet that an unseen planet is responsible, and therefore has been indirectly identified.

This procedure is much more difficult to apply to multiple star systems like HD 188753 than to single star systems. Astronomers needed to separate out the light of the central star from the other two stars. To do so they tried various mathematical models of the light in the system until they came up with one that seemed to fit their real observations the best. They were then able to delete the light contribution from the outer two stars and focus in on the data from the central star. They tried something similar on 20 different multiple star systems, but only HD 188753 revealed an obvious companion planet.

On the planet HD 188753 Ab or on a theoretically moon there might be days when there are three sunsets. For the new release, artists drew conceptual drawings of the HD 188753 system and created animations of a possible triple sunset scenario. However, just as the conceptual drawings and animations created by artists for the Deep Impact mission did not match the real event all that closely, the HD 188753 system concepts provide only a very general idea of what might be going on there. The rapid progress in optical technology may yet lead to a day when such planets can be directly imaged. Techniques like the radial velocity method can tell us where to look when these advanced telescopes become available in the next few decades.

According to the Jet Propulsion Laboratory’s PlanetQuest website, 154 exoplanets have been discovered so far. The site provides a wealth of information about exoplanetary research, including an interactive and visual atlas of the known stellar systems with planets, news updates about the latest discoveries, and proposed missions to discover even more planets.

More Information

• PlanetQuest
  • Press Release, July 13, 2005 – “NASA scientist discovers world with triple sunsets”
    • Images and animations page
  • New Worlds Atlas