This planet has three suns!

A triple-star planetary system

Going one better than Luke Skywalker's home planet Tatooine with its two suns, astronomer Maciej Konacki of the California Institute of Technology and NASA, has reported in Nature the detection of the first extrasolar (outside our solar system) planet with three suns!

The triple-star system is named HD 188753 in the constellation Cygnus (The Swan), about 149 light-years from Earth. The primary star is like our Sun, weighing 1.06 solar masses. The other two smaller stars, a larger orange one and a smaller red one, form a tightly bound pair, totalling 1.63 solar masses, and orbit around each other at a distance similar to that between Saturn and our Sun.

Dr. Konacki detected the planet using a new technique he had developed for precisely measuring velocities of all objects in multiple-star systems that might reveal the presence of planets.

The planet is a gas giant slightly larger than Jupiter, and orbits the main star, every 3.35 Earth days. Such planets are called hot Jupiters because of their nearness to their star, and they have been found around 30 other stars.

Challenge to model of how planets form

In the current model of giant planet formation, the heat from its star prevents the initial stage - the formation of a planetary core. Therefore, hot Jupiters are thought to form about three AUs (astronomical units - the distance between the Sun and the Earth) farther out, beyond the "snow line" and have then migrated inwards to be close to their sun. However, this planet had to contend with the competing gravitational pull of three stars, and the pair of smaller suns should have sheared off much of the planet-making material in the disk around the primary star in its youth, and so it should not exist!

More evidence that our solar system is special

This discovery means that planets may exist in double and triple star systems that have been largely ignored. It is not that triple-star systems are rare, in fact in 2002 an astronomer looking for signs of planet formation around young, double-star systems found a third star in about half the cases. While the implication is that there may be more planets out there than previously thought, if that were case it would be  further evidence that our Solar System is even more special!

References

"Scientists find planet with 3 suns," CNN, July 14, 2005.

"This planet has three suns," ABC, 15 July 2005.

Britt, R.R. "Search for Planets Instead Yields Bevy of Stellar Triplets," SPACE.com, 10 January 2002.

Konacki, M., "An extrasolar giant planet in a close triple-star system," Nature, 436, July 14, 2005, pp.230-233.

Peplow, M., "The triple sunset that should not exist: Astronomer spies improbable world with three suns," News@Nature, 13 July 2005.

Schirber, M., "Triple Sunset: Planet Discovered in 3-Star System ," SPACE.com, 13 July 2005.

Wilford, J.N., "Discovery of a First: A World With 3 Suns," The New York Times, July 15, 2005.

Stephen E. Jones, BSc (Biol)
"Problems of Evolution"