The discovery of a Jupiter-like planet around one star and a cloud of comets around another are the latest clues in the puzzle regarding how solar systems form and how many extrasolar systems may be like our own.

A team of astronomers announced Wednesday that they had discovered a planet orbiting the star 47 Ursae Majoris that is the most similar to Jupiter of the more than 70 extrasolar planets discovered to date. A paper announcing the discovery has been submitted to the Astrophysical Journal.

A second Jupiter-sized planet has been detected orbiting 47 Ursae Majoris, a star similar to the sun. In a striking parallel to our solar system (bottom), both Jupiter-size planets are in nearly circular orbits around their star (top). The diameters of the sun, the star and the planet orbits are not to the same scale. Credit: Kirk Woellert/NSF

The planet, the second found orbiting the Sun-like star 51 light-years from Earth, has a minimum mass three-quarters that of Jupiter in a 7.1-year orbit that, if placed in our solar system, would lie in the asteroid belt between Mars and Jupiter. Both it and a larger inner planet, found in 1996 by veteran planet hunters Geoff Marcy and Paul Butler, have nearly circular orbits.

It's the combination of two planets in relatively ordinary orbits that makes this discovery noteworthy, astronomers said. "For the first time we have detected two planets in nearly circular orbits around the same star," said Debra Fischer, a University of California Berkeley astronomer who, along with Marcy, Butler, and others, discovered the new planet.

Most past extrasolar discoveries could be classified in one of two groups. One group of planets, with masses as small as one-third that of Jupiter, were in orbits just several million kilometers from their parent stars and with orbital periods as little as three days. Another group of planets, whose masses range up to several times that of Jupiter, exists farther from their stars but in highly eccentric orbits. Neither set of worlds bares any resemblance to any planets in our own solar system.

While some have questioned whether our own solar system is an oddball, rather than those discovered to date, a more likely explanation is observational bias. Nearly all the planets found to date have been indirectly identified by measuring the gravitational tug, or wobble, they exert on the stars they orbit, causing a Doppler shift in spectral lines observed by telescopes on Earth. The biggest wobbles -- which are the easiest to detect -- are created by planets that are very massive and/or orbit close to their parent stars.

The situation is beginning to change thanks to better instruments that are able to detect smaller wobbles. "As our sensitivity improves we are finally seeing planets with longer orbital periods, planetary systems that look more like our solar system," said Fischer. Larger volumes of data collected over longer periods of time also help astronomers detect planets with long-period orbits: the observations used in this discovery span 13 years.

The amplitude of the wobble created by this new planet is actually less than the wobble that Jupiter induces in the Sun, proving to astronomers that can discover more Jupiter-like worlds. "Our long range goal remains the detection of true Jupiter analogs -- bona fide solar system analogs -- to allow us to compare our solar system to other planetary systems," said Butler.

"Every new planetary system reveals some new quirk that we didn't expect," said Marcy. "With 47 Ursae Majoris, it's heartwarming to find a planetary system that finally reminds us of our solar system."