Phase III – Stealth Is.

The new search, which began last week, is scanning 86 alien worlds for radio signals that could suggest the presence of an advanced civilization. The extrasolar planets are thought to be the most Earth-like of the 1,235 candidate planets discovered so far by NASA’s prolific Kepler space observatory.

“We’ve picked out the planets with nice temperatures — between zero and 100 degrees Celsius [32 and 212 degrees Fahrenheit] — because they are a lot more likely to harbor life,” said physicist Dan Werthimer of the University of California, Berkeley, in a statement.

Earth owes much of its warmth to carbon dioxide and water vapour in its atmosphere trapping solar heat, but these greenhouse gases freeze at the low temperatures far from the sun. In contrast, hydrogen stays gaseous, and at high pressure it is also an effective greenhouse gas.

Raymond Pierrehumbert at the University of Chicago and Eric Gaidos at the University of Hawaii in Honolulu calculated the warming effect of a hydrogen blanket on Earth-sized planets, as well as on worlds a few times more massive than our own, known as super-Earths. They found that, swaddled in a hydrogen atmosphere a few dozen times thicker than our nitrogen-oxygen one, such a planet could keep warm at up to 15 times Earth’s distance from the sun. And despite the thickness of this alien atmosphere, Pierrehumbert and Gaidos calculate that enough sunlight would reach the planet’s surface to foster photosynthesis.

“It’s a clever idea,” says James Kasting of Pennsylvania State University in University Park, “but I’m sceptical as to whether you can form these planets.” He doubts that an Earth-like planet or super-Earth would pull in so much hydrogen from the cloud of gas surrounding a young star.

Kasting adds that far-out planets will be fainter and harder to see than close-in planets, so finding these distant worlds will be more difficult, as will studying their atmospheres.

Nevertheless, Pierrehumbert and Gaidos point to one known planet that may fit the bill. Named OGLE-05-390Lb, it is about six times as massive as Earth. It orbits a red dwarf – a small, cool, faint star – at 2.6 times Earth’s distance from the sun. A naked planet so far from such a dim star would be a frigid world. But with a thick hydrogen atmosphere it could potentially sustain liquid water at its surface, say the researchers in a study to appear in The Astrophysical Journal Letters.

A manned mission to Europa is somewhat impractical. For starters, it’d take about five years for a rocket to reach the moon, with the same amount of time being required for the return journey. Also, once the astronauts arrived, the radiation in the Jovian system would mean that any trips beyond a thick set of shielding would be somewhat on the toasty side.

But Europa, says Shoer, “ought to be one of the highest-priority exploration targets for robotic space probes”, mainly because it’s “one of perhaps two or three extraterrestrial places in the Solar System where we might hope to find life“.

“It’s a world whose orbital dynamics with Jupiter, its orbital resonances with the other Galilean moons, and its own rigid-body dynamics have a strong hand in creating its surface features,” he adds.

Astronomers believe, based on images from the Galileo mission and magnetometer readings, that Europa has an icy shell over a liquid ocean, with a solid rocky core at the centre. There’s some disagreement between scientists on how thick the ice is — estimates range from 10 to 100,000 metres — but observations have yielded reports of a number of “double-ridges” on the surface that are believed to be cracks in the crust.

These are thought that they’re caused by huge gravitational forces — the Jovian equivalent of tides on Earth, but amplified many times due to Jupiter’s considerably-greater mass. Once a crack forms, it gets squeezed back together and pulled apart again every time the moon rotates, which is approximately once every three and a half Earth days.

The cracks are the most likely place for life to take root. They get sunlight (unlike the rest of the ocean below the ice) and are also subjected to strong currents caused by the aforementioned squeezing and re-widening of the crack. These are likely to be the largest energy sources available to any life that exists on the planet.

Shoer’s plan to investigate these cracks is intricate but clever. A probe would enter orbit around the moon, scanning for these double-ridges. Once located, a lander would be despatched to the inside surface of the ridge, which would then monitor the crack, verify that it’s opening and closing, and work out the exact timing of the cycle.

Then, when the crack is closed, it would inflate cushions around itself and roll down the slope until it comes to rest at the bottom, centred over the crevasse. The cushions deflate, and tethers would be attached to either side of the walls, holding the probe in place. Then, once the crack opens again, a smaller vehicle could be dropped down inside to take measurements. Eventually, it’ll hit the ocean below, and could keep one part at the surface while another section dives as deeply as possible.