Illustration of the Doppler Wobble Technique. Credit: Copyright (c) Particle Physics and Astronomy Research Council

An international team of astronomers announced Tuesday that they have discovered another extrasolar planet similar to Jupiter, the 100th planet found outside our solar system.

A group led by Hugh Jones of Liverpool John Moores University in the UK announced that they had found a planet orbiting the star Tau 1 Gruis (HD 216435), a Sunlike star 100 light-years away. The discovery, based on data obtained from the Anglo-Australian Telescope in Australia, was announced Tuesday at a conference on the origins of life in Graz, Austria.

Jones' group found that the planet has a minimum mass 1.2 times that of Jupiter and orbits the star in a near circular orbit at an average distance of 2.5 AU (375 million kilometers), approximately where the asteroid belt is in our solar system. Jupiter orbits 5.2 AU (780 million km) from the Sun.

The planet is at least the third extrasolar planet, or exoplanet, similar to Jupiter in both size and orbit found to date. In June a group led by veteran planet hunters Geoff Marcy and Paul Butler - who were also involved in Tuesday's discovery - found a planet four times the mass of Jupiter orbiting 5.5 AU (825 million km) from the star 55 Cancri. Later that same month a Geneva Observatory group led by Michel Mayor, who co-discovered the first exoplanet around a Sunlike star seven years ago, found a planet orbiting 3.65 AU (548 million km) from Gliese 777A with a mass about the same as Jupiter.

According to press releases about the discovery, this planet is the 100th exoplanet found to date. However, the exact number of known exoplanets is not exact, because of differing definitions of exoplanets and the status of borderline or otherwise uncertain discoveries. The Extrasolar Planets Encyclopaedia, one list of exoplanets maintained by the Paris Observatory, lists 101 planets without including the latest discovery. A separate almanac of planets kept by Marcy's group at the University of California Berkeley lists 100 planets, also without this latest discovery.

The image shows an impression by David A. Hardy (c PPARC) of the possible scene from a moon orbiting the extra-solar planet in orbit around the star Tau1 Gruis. The planet has a similar mass to Jupiter and orbits the star in around four years, with a nearly circular orbit of three times the Earth-Sun distance. The star Tau1 Gruis is a 6th magnitude star in the southern constellation Grus, and is slightly brighter though the same temperature as our Sun. The similarity of the appearance of the extra-solar planet to that of Jupiter arises because it has a similar mass. The possible existence of the moons been inferred from our knowledge of the planets in our own Solar System and from theories of planetary formation, they have not actually been detected. Photo credit: David A. Hardy, astroart.org

Regardless of the actual number of planets, this discovery continues the trend of discovering planets that are more similar to those in our own solar system. The first exoplanets discovered were either orbiting within a few million kilometers of their stars or were located in highly elliptical orbits. The discovery of dozens of such worlds raised questions for a time whether solar systems like our own were the exception rather than the rule.

"When we first started out, we found planets close in to their parent stars," said team member Chris McCarthy of the Carnegie Institution of Washington. "But as the planet search program has matured, we're finding more planets farther out and in nearly circular orbits. This means that we are getting closer to detecting more systems that are similar to our own solar system."

"We are seeing a pattern for these planets to be of two types, those very close-in and another set with orbits further out. This Tau 1 Gruis planet builds this second group," explained Jones. "Why are there these two groups? We hope the theorists will be able to explain this."

The large number of exoplanets found to date is allowing astronomers to perform some statistics on the likelihood of planets and the type of planetary systems that may exist. Marcy and Butler's group now believes that 12 percent of the Sunlike stars in our galaxy have planets within 5 AU.

"We are now entering a phase in which the general properties of planetary systems can be compared to those of our solar system," Marcy said in a recent interview. "Do most planetary systems have an architecture like ours, with terrestrial, rocky planets close in and gaseous giant planets far out? We don't know, but we are getting very close to having some strong indications."