Astronomers spy familiar planetary system

Studying a star in the Big Dipper, astronomers have for the first time found a planetary system that reminds them of home.

Artist’s depiction of the outer of the two planets circling the star 47 Ursae Majoris (bright point of light near upper left). View is from an imagined, volcanically active moon of the outer planet. Arrow indicates the inner planet. 2001 Lynette Cook

The planetary system–actually a pair of planets–orbits 47 Ursae Majoris, a sunlike star that lies just 51 light-years from Earth and is visible to the naked eye. Both planets, like most in our solar system, have nearly circular orbits. Moreover, the location and mass of the outer planet is similar to that of Jupiter in our solar system.

The planets are most likely giant balls of gas, so it’s unlikely either could support life. However, their circular orbits increase the odds that an inner, Earthlike planet could also reside there, the astronomers calculate.

“We seem to be edging closer and closer to the Holy Grail of extrasolar planetary systems” with one or more planets capable of supporting life, comments Alan P. Boss of the Carnegie Institution of Washington (D.C.), a theorist who wasn’t part of the study.

Veteran planet hunters Geoffrey W. Marcy of the University of California, Berkeley and R. Paul Butler of the Carnegie Institution, along with their colleagues including Debra Fischer of Berkeley, announced the finding this week.

The researchers base their results on a now-standard technique for deducing the presence of unseen planets (SN: 8/8/98, p. 88: The tug of an orbiting planet causes a star to wobble ever so slightly. The star’s motion causes the starlight observed on Earth to periodically shift in frequency, a change that astronomers pick up with a sensitive spectrograph. The method most easily detects bodies that exert the greatest tug–massive planets that lie close to their parent star.

Nonetheless, among the nearly 70 extrasolar planets discovered since 1995, astronomers have been surprised to find that many are what they call “hot Jupiters”–massive bodies that come within roasting distance of their parent. It’s as if in our own solar system, Jupiter were close enough to graze the sun’s hot outer atmosphere.

Other extrasolar planets, in longer orbits, seem no less strange. Some, within a period of a few years, regularly careen from a frigid Marslike distance to a hot Venuslike distance from their star.

With improved measuring techniques, Fischer’s team recently began searching for planets in even more distant orbits. It wasn’t clear that these orbits would be circular, like those in our solar system. The new discovery “strongly implies that . . . planetary systems very similar to our own will be revealed, as well as systems we have never seen or imagined before,” says Boss.

In 1996, Marcy and Butler found evidence that a single planet orbits 47 Ursae Majoris (SN: 1/27/96, p. 52). As the researchers and their colleagues accumulated more-precise data on the star, they realized that one orbiting planet couldn’t alone account for its wobble.

After examining 13 years’ of observations gathered by the Marcy-Butler team, Fischer discerned a second, more-distant planet that causes the star to move back and forth 11 meters every second. That’s less than the wobble that Jupiter induces in the sun. The newly found planet takes 7.1 years to circle its star and has a mass at least three-fourths that of Jupiter.

The planet lies 3.73 astronomical units (AU) from its star, where 1 AU is the distance between Earth and the sun. By comparison, Jupiter lies 5 AU from the sun and Saturn, about 9 AU.

The inner planet previously found by Marcy and Butler is at least 2.5 times as massive as Jupiter and lies 2.09 AU from the star. It completes one orbit in 3.0 years.

Intriguingly, notes Fischer, the ratio of the masses of the outer planet to the inner planet is the same as the ratio of Saturn’s mass to Jupiter’s.

Indeed the planets “come the closest of [any of the extrasolar planets] to looking like Jupiter and Saturn in our solar system,” says Boss.

A simulation by team member Gregory P. Laughlin of NASA’s Ames Research Center in Mountain View, Calif., and John E. Chambers, also of Ames, shows that an Earthlike planet would have difficulty forming around 47 Ursae

Majoris within the so-called habitable region, where water would remain liquid. The two massive planets orbiting the star would cause the rocky building blocks of an Earthlike planet to either dive into the sun or be ejected altogether from the system, Laughlin says.

However, Laughlin’s calculations show that once formed in the habitable region, such a planet would last for at least 100 million years.

Planets with circular orbits would protect an inner, Earthlike planet, Boss notes. In contrast, he says, massive planets in highly elongated orbits “can wreak havoc” on inner planets.

Fischer told Science News that 19 other bright, sunlike stars that she and her colleagues have studied for more than a decade also show hints that they may harbor more than one Jupiterlike planet in a circular orbit.

Among 40 or so stars that they’ve studied for a shorter period–and had originally concluded had only one planet–reexamination of the data shows that about half actually have more planets. “I think a lot of the signals we’ve written off as instrument error may actually be real signals,” Fischer says.

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