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How like Earth are alien planets?

New Scientist

AT LAST we are seeing big rewards in the hunt for "super-Earths" - rocky alien worlds a few times more massive than our own. That was the verdict from a landmark meeting of astronomers last week which saw the unveiling of a huge haul of new exoplanets in our galaxy.

"This really changes things," says Sara Seager of the Massachusetts Institute of Technology, who attended the International Astronomical Union meeting in Boston. "It marks the beginning of the detailed exploration of super-Earths." The new discovery of a large number of small planets suggests that they are abundant in our galaxy, and outnumber Jupiter-sized giants by 3 to 1.

This contrasts with the nearly 300 alien planets previously discovered, of which the vast majority are Jupiter-like gas giants. Only a dozen or so are low-mass planets: either Neptune-like ice-worlds or rocky planets like Earth. Now researchers on the High Accuracy Radial Velocity Planet Searcher (HARPS) survey based at the European Southern Observatory in La Silla, Chile, have announced the discovery of 45 more planets in the Milky Way - all of them less than one-tenth of Jupiter's mass.

They spotted them by recording how each planet's gravitational tug makes its parent star wobble. According to Cristophe Lovis of the University of Geneva, Switzerland, a member of the HARPS team, these observations suggest that while many of the new worlds are likely to be "hot Neptunes" - planets composed mainly of water with

a layer of hydrogen and helium on top - it is probable that some will turn out to be more like rocky super-Earths. "These are preliminary numbers," Lovis says of the 45 planets. "But there's no doubt that the majority of them will turn out to be real."

The announcement of this potential haul of super-Earths opens up the exciting prospect that we will be able to glean some detailed information about what these planets are like. For years, astronomers have been waiting for a super-Earth to be found with an orbit that "transits" its parent star: in other words, it passes directly in front of the star as viewed from Earth. This would allow them to deduce many of its characteristics, from its internal structure to the make-up of its atmosphere.

The likelihood of observing such transits is increased when exoplanets have a short orbit around their star. The HARPS planets fit the bill: all orbit in less than 50 days, and some in as little as 10 days. This means that during a relatively short period of observation, the HARPS planets will be much more likely than planets with longer orbits to pass in front of their star.

Though the HARPS team used a different technique to find their planets, looking for transits has proved a highly successful way of discovering alien worlds. Astronomers work on the principle that if you stare at enough stars such events will show up in telescope data as temporary dips in the stars' apparent brightness.

Now they are hoping the observation of transits will tell us much more about distant worlds than simply that they are there. It provides a robust measure of the planet's radius, for example, and by combining this with the mass estimate derived from the planet's gravitational pull on its star it is possible to estimate its density. This in turn can be used to reveal something about the planet's composition and internal structure.

Most exciting of all, says David Charbonneau of the Harvard Smithsonian Center for Astrophysics, transits allow us to study the atmosphere of a planet. The difference in the intensity of infrared radiation when the planet passes behind its star can be used to determine the temperature of the planet's atmosphere.

It is also possible to glean information about the composition of the planet's atmosphere by watching for changes in a star's spectrum as it filters a fraction of the star's light during a transit. Such observations of the star HD 189733 have recently revealed the presence of methane and water vapour in the gaseous atmosphere of a transiting Jupiter-size planet.

Astronomers will now be looking at the stars near the HARPS planets in the hope of spying a transiting super-Earth. The dimming in the star's light will be slight, so such shallow transits are best detected from space rather than with telescopes on the ground, where fluctuations in the Earth's atmosphere can interfere with the observations.

A few satellites are capable of watching for a transit, among them the Canadian MOST satellite. This is particularly well suited to the task, as its instruments can stare at target stars for long periods of time, says Jaymie Matthews of the University of British Columbia, MOST's principal investigator. "If a super-Earth is transiting, we'll see it."

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THIS ARTICLE APPEARS IN NEW SCIENTIST MAGAZINE ISSUE: 24 MAY 2008. EMBARGOED UNTIL WED, 28 MAY 2008, 13:00 HRS EDT (18:00 HRS BST)

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