What sets newly found super Earth apart? It's simple as night and day. (+video)
Scientists say they have found a planet seven times more massive than Earth orbiting in a star's habitable zone 42 light-years away. It could have seas, and perhaps just as important, it could have an Earth-like climate because it has a day-night cycle.
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Confirmation will be a challenge without space-based telescopes designed to image planets directly, the team acknowledges. HD 40307g could be the first planet spotted by any such future observatories. Because it is so far from its host star, HD 40307g's reflected light is less likely to be swamped by the light from its star.Skip to next paragraph
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Yet even without the prospect of immediate confirmation, the team is confident it has picked up signatures from bona fide planets.
With the team's confirmation of the star's inner three planets and the discovery of three more candidates, "I see no reason to doubt the existence of these planets," says Mikko Tuomi, a researcher at the University of Hertfordshire in Britain and a team leader. The system wouldn't exist if the planets' true masses weren't close to those the team calculates as minimums, he explains. Otherwise the system would have been unstable.
The discovery of these three additional planets highlights the often unsung roll data-analysis software plays in the planet-detection business, astronomers say.
Other astronomers had taken spectra of HD 40307 using an exquisitely sensitive spectrometer bolted to the back of a 3.5 meter telescope the observatory operates high in the Chilean desert. In 2008 they announced the discovery of the inner three planets.
They detected the planets by the wobble they impart to the host star as they orbit. This wobble shows up in a back and forth shift in the spectra from the star.
But based on what astronomers have been learning about planetary systems outside the solar system, where there were three, there were likely to be more.
Armed with a newly crafted software package, Toumi and colleagues went back and reanalyzed the data. They not only detected the original three, but they added spotted three more – and by an odd technique: by throwing data out.
Typically, Toumi explains, the intensity of the wobble in a star's spectrum as planets orbit appears up and down the various colors, or wavelengths of light. But natural changes to a star's surface on time scales ranging from days to years can inject faux wobbles as well.
If the planets are close in, their signatures will be stronger than those of the faux wobbles. If a planet is farther out, however, its signature is weaker because its gravitational influence on the star is weaker. At some point the faux wobbles can be as large or larger than those imparted by a distance planet.
Fortunately, those faux wobbles tend to manifest themselves toward the blue end of the spectrum, Toumi says. So the team focused its analysis on the red end. That, plus a fresh approach to other aspects of the analysis, pulled the newest planets out of the data.
"Rather counterintuitively, throwing some data away led to an increased precision," which yielded the detections, Toumi writes in an e-mail.
[Editor's note: In the original version of this story, the distance from Earth to a recently discovered Earth-like planet was misstated in the summary.]