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Beyond Solar System, Planets That Look Familiar Findings Suggest Other Earth-Like Worlds

Posted on: Wednesday, 1 September 2004, 06:00 CDT

The universe looked a little more familiar and friendlier on Tuesday.

The roll call of planets beyond the solar system swelled significantly with the announcement of a trio of newly discovered worlds much smaller than any previously discovered around other stars. The masses of these new planets are comparable to those of Neptune or Uranus in our own solar system, ranging from about 14 to 25 times the mass of the Earth.

The previous exoplanets found around living stars have been giants like Jupiter or Saturn, at least 50 times the mass of the Earth, composed of gas at crushing pressures and scorching temperatures and unlikely abodes for life. Astronomers speculated that the new planets might be ice giants like Uranus and Neptune, or even giant hunks of iron and rock called super-Earths.

Like those earlier planets, the new planets are circling too close to their stars to be viable abodes for life. But their discovery, astronomers said, is an encouraging sign that planets are plentiful and varied in the galaxy and that as planet hunting techniques and instruments become more powerful and sensitive, they will find planets as small as the Earth.

"We're getting closer to answering the golden question of whether there is life out there. We're trying to find our own roots, chemically and biologically, in the stars," said Geoff Marcy, an astronomer and a longtime planet hunter at the University of California at Berkeley.

Paul Butler, of the Carnegie Institution of Washington, D.C., said, "We're prepared unexpectedly for the next step in planetary science, finding truly Earth-mass planets."

One of the new planets is part of a system around a star known as 55 Cancri, already known to harbor three other larger planets, making it the first quadruple-planet system to be found beyond the solar system, and a likely target for future research.

Barbara McArthur of the University of Texas said, "We're on the way to finding the first extra-solar planet Earth, and it's an exciting road to be on."

Butler and McArthur were the leaders of two of overlapping teams that announced the discovery of two of the planets at a press conference at NASA headquarters in Washington on Tuesday.

A third team, of European astronomers led by Michel Mayor of the Geneva Observatory in Sauverny, Switzerland, announced the discovery of a third small planet in a press release issued from the European Southern Observatory, a consortium based in Garching, Germany, that operates telescopes in Chile.

Two papers by the American teams have been approved for publication in the Astrophysical Journal in December. The European group has submitted a paper to Astronomy and Astrophysics.

Other astronomers praised the results as an example of how fast research is progressing. David Spergel, an astronomer from Princeton who is involved in a NASA project, said: "This is an exciting result. Given the existence of these 'super-Earths,' I am willing to bet that there are Earth-like planets around nearby stars."

Marcy said that as a result of the new work, he and his longtime collaborator Butler were revamping their planet-searching strategy with the goal of finding planets as small as 10 Earth masses or less, before upcoming space missions to find planets put them out of business.

Butler's team discovered a planet about roughly 20 times the mass of the Earth orbiting the star Gliese 436, a reddish dwarf about 41 light-years away in the constellation Leo, every 2.64 days. It was found as part of a survey of nearby stars using the giant Keck I telescope on Mauna Kea in Hawaii.

The second Neptune discovered, by McArthur's team by combining data obtained from the University of Texas's new 360-inch-diameter (915-centimeter-diameter) Hobby-Eberly Telescope with observations from the Hubble Space Telescope and others is about 18 Earth masses, the astronomers said. It circles the 55 Cancri every 2.8 days, far inside the orbits of the other three Jupiter-like stars.

The European team claimed to have found the lightest planet yet on the record. It orbits the star mu Arae, in the constellation Ara, about 50 light-years from here. It completes a revolution every 9.5 days. But that result has not yet been confirmed by peer review, astronomers said at the press conference, and the European group did not respond to an e-mail message.

The new planets join more than 100 others that have been detected around other stars in the last decade. Like the vast majority of their predecessors, they were discovered by the gravitational tugs they exert on their parent stars. As they go around, they pull their stars alternately toward and away from the Earth, causing the stars' motions relative to Earth to speed up and slow down slightly. That shows up as a periodic shift in the wavelength of light from the star, as measured by high-resolution spectrographs on large telescopes.

The gravitational wobble technique is most sensitive to giant planets orbiting lethally close to their stars; they give the biggest kicks, so it is not surprising that the first planets discovered were in such orbits, astronomers say. Longer observations are required to discern the effects of smaller planets in more comfortable orbits. Indeed, two of the new planets were discovered by continuing to refine the data from stars where giant Jupiter- like planets had already been detected.

Both the mu Arae systems and the 55 Cancri system were already known to have Jupiter-like planets.

The 55 Cancri system has already drawn interest from astronomers because one of its Jupiters has an orbit, about 13 years in duration, similar to that of Jupiter in our solar system. Jupiter seems to serve as a kind of gravitational shield in our own solar system, protecting the inner planets, and life, from cometary impacts. The outermost Jupiter in this system might also provide shelter for the inner orbits, astronomers speculate.

McArthur, who has organized a major campaign to study this system, said: "This is the closest analogue we have to our solar system. All these things make it the premier laboratory for studying planetary systems."

Calculations by theorists have shown that small planets could have stable orbits in the space between the outermost and inner planets in 55 Cancri, but exactly what kind of planets would come to live there is a mystery. Astronomers agree that both the Jupiters and the new Neptune circling close to 55 Cancri must have formed farther out in the system and then migrated in, disturbing or obliterating any smaller planets that were already there. But any new ones that had formed since then, or had moved in themselves, would be secure, said Alan Boss, a planetary theorist at the Carnegie Institution who was not part of the discovery teams but provided commentary at the NASA press conference.

The Gliese 436 discovery is important, astronomers said, because so-called red dwarfs are the most abundant stars in the universe. About 70 percent of the 200 billion stars in the galaxy are red dwarfs, known technically as M dwarfs, but because they are so dim, only a few percent as bright as the Sun, they are hard to study. The new planet is only the second one found around a red dwarf. "If we are finding planets around M dwarfs, that's wonderful news," Boss said.

NASA and the European Space Agency have made the search for extrasolar planets and in particular terrestrial-size planets capable of harboring life a high priority. A series of upcoming satellite missions, including NASA's Kepler in 2007 and the French Corot in 2006, will lead to the Terrestrial Planet Finders, or TPF, sometime in the next decade, to find and study Earth-like planets.

"The detection of these planets is definitely good news for TPF," Spergel said. "The worry has been that we build this exquisitely powerful telescope and then find that there are no Earth-like planets for it to observe."

That worry now seems unnecessary. As Marcy said, "We can't see Earth-like planets yet, but we can see their big brothers."

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