Hundredth Planet Outside Solar System Discovered

Anna Brendle
for National Geographic News
September 17, 2002
Astronomers located the first planets outside our solar system in 1995, and now—just seven years later—an international team of researchers has discovered the 100th "extrasolar" planet. This newest planet, announced today, orbits the star Tau Gruis, 100 light years from Earth, in the southern hemisphere's constellation Grus (the crane).

The Anglo Australian Planet Search Team of U.S., UK, and Australian astronomers uses powerful telescopes in California, Hawaii, and Australia to locate these faraway planets. The team used the Anglo Australian telescope in New South Wales, Australia, to detect this newest planet, the 13th to be viewed from this particular telescope.

"Now our searches have become precise enough to find many planets in orbits like those in our Solar System, we are seeing clues which may help us understand how planets are formed," said UK team leader Hugh Jones of Liverpool John Moores University.

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

The team has been studying the Tau Gruis star since 1998, measuring changes in its velocity to detect any wobbling, said member Chris McCarthy of the Carnegie Institute in Washington, D.C.

"In order to actually detect a planet, we have to see a planet go around its orbit at least once," said McCarthy. He estimated that the new planet weighs at least 1.2 times the mass of Jupiter, based on how much its sun "wobbles" as it reacts to the planet's gravitational pull.

Although scientists have been watching Tau Gruis since 1998, this is the first time that they have been able to confirm the presence of its large planet. This is an indication that there is a considerable distance between the star and the planet.

Soon after the first extrasolar planets were found, beginning in 1995, most planets were found in orbit close to their host stars. Planets closer to their suns orbit at a much faster rate, and therefore take much less time to detect.

"When we first started out, we found planets close in to their parent stars. 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," said McCarthy.

Life Outside Our Solar System?

The 100th planet is located at a distance from Tau Gruis equal to that between the asteroid belt and the sun in our solar system—2.5 Astronomical Units (AU). One AU refers to the distance of Earth to our sun, and is considered the "habitable zone," said McCarthy.

"It's thought that life, at least as we know it, would have liquid water," said McCarthy. The new planet would not be a likely candidate for life because it's probably composed mostly of gas, he said. "Liquid water, if it exists here, would sink."

McCarthy said it was possible that giant planets like the new one might have some moons, and the smaller bodies could harbor liquid water. Astronomers still have no way of detecting extrasolar moons.

Extrasolar Planetary Trends

Astronomers Paul Butler of the Carnegie Institute and Geoffrey Marcy of the University of California in Berkeley have predicted that about 12 percent of the sun-like stars in our galaxy, the Milky Way, have planets orbiting their stars within about 5 AU.

McCarthy said that with 100 extrasolar planets, a trend has emerged that supports the theory that giant planets in solar systems may form at great distances from their stars.

The giant outer planets are loosely packed concentrations of gas, but as they move nearer to their suns they solidify and become more compact—much like the inner planets of our own solar system, including the Earth.

"It is very difficult for a gas giant planet to form close to a star, because the most likely formation process for giant planets involves icy 'proto-planets' colliding, and such ices would likely evaporate close to the star," said McCarthy.

"In order to explain gas giant planets which orbit close to their star," McCarthy continued, "astronomers now believe that these gas giant planets formed in the outer solar system (around 5 AU) then moved inward, under the influence of a nebular disk of material which exerted a 'drag' on their orbit."

As more telescopes and time are being dedicated to the search, the rate of extrasolar planet discovery will increase, predicted McCarthy.

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