Published online 15 December 2008 | Nature | doi:10.1038/news.2008.1307


Finding distant Earths faster

Habitable extrasolar planets could be found easier than ever thanks to new technology.

ExoplanetAccurate measurements of an exoplanet's radius are vital for determining its habitability.NASA/JPL

Nearly a decade after astronomers first detected an extrasolar gas giant moving across the face of its parent star, a team of astronomers report that a new camera is giving them the sensitivity they need to spot planets nearly as small as Earth.

The team observed a planet — WASP-10b — around three times the mass of Jupiter orbiting the star WASP-10, about 300 light years from Earth and measured precisely how much the star dimmed as the planet passed in front of it.

It is similar to watching a solar eclipse when the moon blocks out the sun along our line of sight. But instead of a total eclipse, during an occultation a planet about the size of Jupiter would block out just 1% of a star's perceived luminosity – a change that's been difficult and time-consuming to detect.

However, in a paper currently in press with Astrophysical Journal Letters1, a team led by John Johnson, an astronomer at the University of Hawaii in Honolulu, report that they were able to detect a dimming of less than 0.05% during a single transit of 2.2 hours. In the process, they were able to both prove their technology and revise downwards the planet's radius to about that of Jupiter — 16% lower than previous estimates.

"This instrument provides us with the first realistic chance to detect a transiting Earth-like planet," says Johnson. "In the past, you'd have to spend maybe 10 nights on the telescope to get this precision, but now we are doing it in a single night."

Cutting the noise

David Charbonneau of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, who was part of one of the first teams to confirm the occultation of a Jupiter-mass planet circling the star HD 209482, calls the quality of the new data "exquisite".

"Other methods typically give you information about the planet's orbit and mass," says Johnson. "But they don't give you information about the planet's interior structure. By measuring these transits to very high precision, we were able to actually infer that it has a core surrounded by a gaseous atmosphere."

"The real key to these impressive observations is they have managed to almost eliminate systematic noise," says Don Pollacco of Queens University in Belfast, part of the UK-based Super Wide Angle Search for Planets (SuperWASP) team that first detected WASP-10b last year.

The latest feat was achieved in large part by using a new type of orthogonal detector array and OPTIC (the Orthogonal Parallel Transfer Imaging Camera), a US$300,000 camera mounted on the University of Hawaii's 2.2-metre telescope atop Mauna Kea.

It's all a blur

Co-author Josh Winn, a physicist at the Massachusetts Institute of Technology in Cambridge, says that to avoid pixel saturation and better register each incoming photon, the starlight is intentionally electronically blurred over a large square of pixels.


"We like blurry images," says Winn. "If you want to measure the brightness of something as precisely as possible, you want a really blurry image so that you can spread the light out over lots of pixels and average all these variations from pixel to pixel. Then, if we can measure exactly how much the starlight dips, we can get the planet's radius."

An estimated 30% of stars that resemble the Sun are thought to harbour super-Earths – planets larger than the Earth but smaller than a gas giant. But at the moment, only 50 of roughly 330 detected extrasolar planets are known to transit their stars along our line of sight.

But Johnson says it is too soon to label just any super-Earth habitable. "That's making huge assumptions about the composition of these things," he says. "Are they gaseous? Are they waterworlds? Or do they have solid surfaces? Until we measure their radii and do the internal modelling, we won't know."

More data on planetary radii should be forthcoming thanks to OPTIC II, a more advanced version of the current camera which is expected to be up and running in about a year. 

  • References

    1. Johnson, J. A., Winn, J. N., Cabrera, N. E. & Carter, J. A. Preprint at (2008).
    2. Charbonneau, D., Brown, T. M., Latham, D. W. & Mayer, M. Preprint at (1999).
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