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Possible Earth-like planet could hold water

Published: Monday, Nov. 12, 2012 9:24 a.m. CDT

(MCT) — LOS ANGELES — By taking a fresh look at old data, an international team of astronomers has discovered a possible super-Earth planet relatively nearby that could potentially hold liquid water, scientists announced this week.

The research, released Wednesday by the journal Astronomy & Astrophysics, used a novel technique to analyze previous measurements of a nearby star. The paper drew some praise even as other experts in the field expressed caution about the results.

The finding adds three planet candidates around the dwarf star HD 40307 about 42 light-years away. Combined with three others that were discovered in 2008, they bring the total to six. Five are clustered close to the star, nearer than Mercury’s orbit around the sun.

But one of the three new finds lies far enough away to be in what’s known as the habitable zone, where a planet could support liquid water — and perhaps life.

That candidate planet, dubbed HD 40307 g, has seven times the mass of Earth, the scientists reported.

“It’s likely that it’s sufficient in mass that it does have an atmosphere,” said coauthor Hugh Jones, an astronomer at the University of Hertfordshire in England. “But we don’t know.”

It’s difficult to draw extensive conclusions about such super-Earths because no such mid-range planet exists in our own solar system. But Jones did say that HD 40307 g is far enough way from its star to be able to rotate freely and possibly have a proper night and day, making it even more likely to have an Earth-like climate. The closer-in planets are gravitationally locked into the star’s motion, just as the moon is locked into Earth’s, which is why we only see one side of it.

Although the planet appears somewhat closer to its star than Earth is to the sun, the dwarf star is also smaller and dimmer than the one at the center of our solar system.

Jones and his collaborators found the new planet by looking through old measurements collected by the 3.6-meter telescope at La Silla Observatory in Chile. They used a technique called the radial velocity method, which takes advantage of the Doppler effect.

A planet’s gravity tugs on its star, causing the star to wobble a bit. When the star moves slightly toward us, it squeezes the light, making it bluer; when the star moves away, the wavelengths of light stretch out, making it redder. The scientists measure that distortion in the light and figure out whether a planet is causing it. The wider the wobble, the more massive the planet.

Scientists need to separate that signal from a lot of “noise” caused by the star’s own activity as dark spots grow and shrink on its surface, affecting the light it emits. At such great distances, making that distinction is exceedingly difficult.

But the members of Jones’ team had noticed a key fact: Stellar activity seems to register more in the blue half of the starlight, making it a bit of a mess to read. The red half of a star’s fingerprint, however, is relatively undisturbed. As a result, when the team looked only at the red side of the light, the planetary signals suddenly emerged.

“The interesting thing is that if they threw out some of their data, they got more precise measurements,” said Eric Ford, an astronomer at the University of Florida, Gainesville, who was not involved in the discovery.

The finding has yet to be confirmed by other analysis or observations. Not everyone is convinced this particular planet exists.

Francesco Pepe of the Geneva Observatory said his team already had seen the signals described in the new paper. In fact, they originally produced the data in question.

“These signals are, however, at the edge of detectability and some doubts remain(ed) on their planetary nature,” Pepe wrote in an email. “It is our policy to exclude any possible other explanation and to collect sufficient data to confirm the (possible) additional planets.”

Planet-hunting scientists have become increasingly antsy about false positives. Ford recalled the 2010 announcement that two “Earth-like” planets were found circling Gliese 581. Their existence — particularly of Gliese 581 g, which is in the system’s habitable zone — was almost immediately thrown into question, launching a flurry of quarreling papers in the astronomy community. Their status is still somewhat in dispute.

The leaders of that research, Steven Vogt of the University of California, Santa Cruz and Paul Butler of the Carnegie Institute of Washington, worked with Jones on the new paper. And Pepe was among those who challenged the earlier finding.

“There’s the ideal science and there’s the science that involves humans,” Ford said.

In an ideal world, who collects and analyzes the data wouldn’t matter, Ford said. But after such protracted battles, “people probably don’t forget things like that.”

For his part, he said the new findings were promising: “I suspect all these planets will probably turn out to be real.”

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