New discoveries were announced based on observations made with the W.M. Keck Observatory (top) and the La Silla Observatory (bottom).

Four ways to find a planet Worldwide Planet Searches

(PLANETQUEST) -- The past four weeks have been heady ones in the planet-finding world: Three teams of astronomers announced the discovery of 12 previously unknown worlds, bringing the total count of planets outside our solar system to 145.

Just a decade ago, scientists knew of only the nine planets - those in our local solar system. In 1995, improved detection techniques produced the first solid evidence of a planet circling another star. A proliferation of discoveries followed, and now dozens of ongoing search efforts around the globe add steadily to the roster of worlds. Most of these planets differ markedly from the planets in our own solar system. They are more similar to Jupiter or Saturn than to Earth, and are considered unlikely to support life as we know it.

The news of the past four weeks has included:

• The discovery of six new gas-giant planets by two teams of European planet-hunters was announced this week. Two of these planets are similar in mass to Saturn; three belong to a class known as "hot jupiters" because of their close proximity to the host stars. The sixth is a gas giant at least four-and-a-half times the mass of Jupiter.

  All were discovered as part of the High Accuracy Radial velocity Planet Search (HARPS), an ongoing search program based at La Silla Observatory in Chile.

• On January 20, a paper posted in the online edition of the Astrophysical Journal described five new gas-giant type planets detected by a team of U.S. astronomers. These planets provide further statistical information about the distribution and properties of planetary systems, according to the paper.

  The U.S. team based its finding on observations obtained at the W.M. Keck Observatory in Hawaii, which is jointly operated by the University of California and Caltech. Observation time was granted by both NASA and the University of California.

• Last week, Penn State's Alex Wolszczan and Caltech's Maciej Konacki announced the discovery of the smallest planet-like body detected beyond our solar system. The object belongs to a strange class known as "pulsar planets." It is about one-fifth the size of Pluto and orbits a rapidly spinning neutron star, called a pulsar.

A pulsar is a dense and compact star that forms from the collapsing core left over from the death of a massive star. The new pulsar planet is the fourth to be discovered; all orbit the same pulsar, named PSR B1257+12.

Because the planets around the pulsar are continually strafed by high-energy radiation, they are considered extremely inhospitable to life. (Note: The current planet count posted on this website includes only planets around normal stars.)

Two methods of detection

The pulsar planet was discovered by observing the neutron star's pulse arrival times, called pulsar timing. Variations in these pulses give astronomers an extremely precise method for detecting the phenomena that occur within a pulsar's environment.

The gas-giant planets were detected using the radial velocity method, which infers the presence of an unseen companion because of the back-and-forth movement induced in the host star. This movement is detectable as a periodic red shift and blue shift in the star's spectral lines. (For more about this method, see the article Finding Planets.)

The names of the new planets around main sequence stars are:

• HD 2638 b
• HD 27894 b
• HD 63454 b
• HD 102117 b
• HD 93083 b
• HD 142022A b
• HD 45350 b
• HD 99492 b
• HD 117207 b
• HD 183263 b
• HD 188015 b

Written by Randal Jackson/PlanetQuest