Multi-Planet Solar System Found in Nearby Galaxy

Two teams of astronomers Thursday announced the discovery of the first known solar system of multiple planets around another sun--an eccentric trio of massive planets circling a nearby star in the constellation Andromeda.

Although it is unlikely these planets harbor life, the astronomers said, the triple planet system is the most compelling evidence to date that planets in the Milky Way galaxy may be as common as gravel on the shoals of space. The discovery of other solar systems containing smaller planets like Earth is only a matter of time and technology, the researchers said.

The system of multiple planets was discovered by a team of astronomers led by Geoffrey Marcy at San Francisco State University, working at the Lick Observatory near San Jose, and a team led by Robert Noyes at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., working at the Whipple Observatory in Tucson, Ariz.

“One planet is wild; two planets is even wilder; three is astonishing,” said Marcy.

Traveling in odd, slingshot orbits, the three planets revolve around one of the brightest sun-like stars in the sky, Upsilon Andromedae, which is 44 light years from Earth. When its orbit brings the star back into view from Earth this June, it can easily be seen with the unaided eye in the night sky throughout the Nortern hemisphere.

For their size--the largest has four times the mass of Jupiter--the planets orbit unusually close to their star.

The nearest races in a circular orbit around its star every 4.6 days and is closer than Mercury is to the sun. It has three-quarters the mass of Jupiter. The second, twice the mass of Jupiter, travels in an elliptical path about as close to its sun as Venus. The largest planet takes four years to complete a single elliptical orbit.

SFSU astronomer Deborah Fischer recalled the moment a few weeks ago when, as she analyzed the team’s stellar data on her computer, it dawned on her that the star harbored three planets. “A chill went through me and, all of a sudden, I felt like Jody Foster in [the film] Contact. It was quite chilling and shocking and frightening.

“It implies that planets can form more easily than we ever imagined, and that our Milky Way is teeming with planetary systems,” she said.

The newly found planetary system is so unlike our solar system that astronomers are at a loss to explain how it could have formed. No current theory predicted that so many giant worlds would form around a star. Nor do they know how such huge planets could form so close to a sun.

“I am mystified at how such a system of Jupiter-like planets might have been created,” said Marcy. “This will shake up the theory of planet formation. Is the architecture of our own system unique?”

Several astronomers said the finding was extraordinary.

“It is a very, very good discovery that broadens our perspective,” said astronomer Alexander Wolszczan at Pennsylvania State University. “We have all been expecting” the eventual discovery of a multi-planet solar system. It caps a remarkable string of planetary discoveries since 1995 that has researchers scrutinizing nearby stars with unusual intensity, yielding to date at least 20 extrasolar planets. Until now, however, each new-found planet was a solitary orb circling its own star.

At least twice before, there have been hints of distant star systems containing more than one planet. One system turned out to be the gaseous wreckage of an ancient supernova explosion. The second observatoin has never been confirmed.

The triplets of Upsilon Andromedae are the first planets to reveal solid evidence of a solar system reminiscent of Earth’s, experts said.

For that reason, the astronomers Thursday emphasized that their observations had been conducted independently of each other and that, consequently, when they all reached the same conclusion, their pooled findings have added credibility.

“Extraordinary claims require extraordinary proof,” said Noyes at the Harvard-Smithsonian Center. “We thought it was very important to show that there are two independent sets of data that show the same thing.”

This discovery shows that “other planetary systems exist around sun-like stars,” Noyes said.

Timothy Brown, a member of the discovery team who is based at the High Altitude Observatory in Boulder, Colo., said: “I was personally quite uncertain about the existence of the third planet until I could see it coming out of both data sets. The identification of these objects as planets is really pretty secure.”

Even so, the evidence for the newly found solar system, like that for all the extrasolar planets, is indirect. The planets themselves are not visible from Earth with available technology.

Instead, planet hunters use a technique that measures how the behavior of stars is affected by the gravitational pull of the planets around them. The tug of gravity makes a star wobble slightly in its orbit. Only the largest planets, however, can alter a star’s path enough so that it can be detected across the vast gulfs of space.

In all, the astronomers patiently monitored Upsilon Andromedae for 11 years as part of a detailed survey of 107 star systems thought likely to harbor planets.

By 1996, Marcy and his colleague R. Paul Butler, now a staff astronomer at the Anglo-American Observatory, were sure they had detected the presence of one planet.

But when they announced their discovery that year, Marcy and Butler suspected that Upsilon Andromedae had not yet yielded all its secrets. There was too much residual wobble left over after they had subtracted the gravitational effect of the single planet.

“That made you suspect there was something else in the system,” said Harvard-Smithsonian astronomer Peter Nisenson, who was part of the Noyes discovery team. “It took several years to figure it out.”

Not until early this year had enough observations been made to confirm the presence of a second planet. That explained some of the confusing patterns in the data, but not all of it. Another object still seemed to be tugging on the star.

“We realized [by deduction] that it was obvious there was a third one there,” Nisenson said. “That really made it exciting.”

The astronomers have no shortage of theories about how such a curious system may have formed.

They speculate that the planets simply may have formed close to the host star. Alternatively, the planets may have formed at the outer reaches of the system and then, like balls on a billiard table, may have careened off each other, propelled by the complex interaction of the system’s multiple gravitational forces.

In the process, the mammoth Jupiter-like planets could easily have swept the system clean of any smaller bodies like Earth. If that turns out to be the common mode of planetary formation, it is unlikely that most systems in the galaxy will harbor Earth-like planets.

But Marcy and his colleagues took a different view. The fact they as yet lack the technology to detect any systems with smaller planets makes them hope that these systems may turn out to be quite common.

“The good news is that, of all the stars we have surveyed,, only about 5% harbor these marauding Jupiters,” Marcy said. “The other 95% could harbor Earths in stable orbits.

“Our Milky Way galaxy has 200 billion stars. That is a lot of throws of the dice,” Marcy said. “If even a small percentage harbor Earth-like planets, then our Milky Way galaxy contains billions of Earth-like planets.”

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In a Galaxy Very Nearby

Astronomers have discovered the first system of multiple planets outside our own solar system, orbiting a star called Upsilon Andromedae. It is visible in the Northern Hemisphere from June through February. Similar to the Earth’s sun, the star is bright enough to be seen with the unaided eye on a clear, dark night. It is also fairly close, in astronomical terms, at a distance of 44 light-years (a light-year is the distance time travel in one year, or 5.9 trillion miles).

AU* 0-1

* Astronomical unit, the mean distance from Earth to the sun, or 93 million miles.

* Source: Harvard-Smithsonian Center for Astrophysics