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Imagine a single month in which eight new worlds outside our solar system could be discovered. This summer two separate findings were announced by the University of Texas at Austin’s McDonald Observatory and by the European Southern Observatory, thus raising to over 50 the number of planetary candidates to probe in future studies.

Astronomers at the McDonald Observatory discovered a planet orbiting the star epsilon Eridani, a solar system only 10.5 light-years away from Earth. Relatively speaking, this star is a close neighbor, being the tenth closest star to our Sun. Epsilon Eridani is the fifth brightest star in the constellation Eridanus, and bright enough to be seen with the naked eye.The planet orbiting epsilon Eridani has a mass similar to Jupiter’s. Its orbit of seven years is about 60 percent the orbital period of Jupiter. That is the longest orbital period of any extrasolar planet discovered thus far.

In our own solar system, Jupiter exerts a strong gravitational pull that serves as a protective barrier, preventing many asteroids from hitting the Earth. Astrobiologists believe Jupiter’s gravity played an important role in the development of life on Earth, so the discovery of this new planet – quite similar to Jupiter in many respects – has led to some speculation that an Earth-like world could exist in the epsilon Eridani solar system. However, because this planet’s orbit is much more elliptical and eccentric than Jupiter’s orbit, the possibility of an Earth-like planet in that system does not seem very likely.

“Detecting a planet orbiting Epsilon Eridani — a star very similar to our own Sun and only 3.22 parsecs from Earth — is like finding a planet in our own backyard, relatively speaking,” said Dr. William D. Cochran, a research scientist with McDonald and UT Austin’s Department of Astronomy. “Not only is this planet nearby, it lies 478 million kilometers (or 297 million miles) from its central star — roughly the distance from the Sun to the asteroid belt in our own solar system.”

The European Southern Observatory (ESO) announced the discovery of seven new extrasolar planets. Two of these planets are orbiting the same star: HD 83443. This is only the second multi-planet system to be discovered so far (The first was a three planet system orbiting the star upsilon Andromadae). One of the planets in this system is slightly more massive than Saturn (1.17 Saturn masses), the other has half the mass of Saturn. This second planet is the smallest extrasolar planet found to date. Both these planets closely orbit their star in an eccentric yet stable orbit.

The five other planets discovered are all gas giants like Jupiter. The name of the star and the masses of the planets are as follows:

HD 6434 – 0.48 Jupiter masses

HD 19994 – 2.0 Jupiter masses

HD 92788 – 3.8 Jupiter masses

HD 121504 – 0.89 Jupiter masses

HD 190228 – 5.0 Jupiter masses

Such Jupiter analogs are the “Holy Grail” of planet searches, said Alan Boss of the Carnegie Institution of Washington and the NASA Astrobiology Institute. Stars with Jupiter-like worlds in Jupiter-like orbits would be good places to look for even smaller Earth-like planets in orbits that could support life. But most of these planetary candidates announced by the McDonald Observatory/ESO teams are believed to orbit too close to their stars to be able to sustain life.

These recent findings increase the number of known planetary systems around main sequence stars (including our own solar system) to 47. In total, these systems contain at least 59 planets.

What Next?

Future projects for the discovery of extrasolar worlds include NASA’s Space Interferometry Mission, due to be launched in 2005. This satellite will be better able to detect the motions of distant stars. In 2011, NASA hopes to launch the Terrestrial Planet Finder (TPF), which would search for light reflecting off of distant planets, some perhaps as small as Earth.

TPF also will be able to determine a distant planet’s temperature and the composition of its atmosphere. Thus, it may provide the first strong evidence of life beyond our Solar System, a primary goal of astrobiological research. Several members of the NASA Astrobiology Institute are part of the TPF Science Working Group, which provides input into the science goals and design of the spacecraft.

McDonald research scientists on the team included Dr. William Cochran, Dr. Artie P. Hatzes and Barbara McArthur. They worked with Dr. Gordon Walker and Bruce Campbell, University of British Columbia; Dr. Alan Irwin, University of Victoria; Dr. Sallie Baliunas, Harvard-Smithsonian Center for Astrophysics; Dr. Martin Kuerster and Dr. Michel Endl, European Southern Observatory; Dr. Geoffrey Marcy, University of California, Berkeley; Dr. Paul Butler, Carnegie Institution of Washington: Stephenson Yang, University of Victoria; and Sebastian Els, European Southern Observatory Group.