The Hubble Space Telescope didn’t quite spy little green men waving back at its camera, but it has taken the next step in the search for life beyond our solar system. As astronomers are reporting this afternoon in the journal Nature, the telescope detected an organic molecule in the atmosphere of a planet orbiting another star—the first such detection ever for any of the 277 known “extrasolar” (outside our own solar system) planets.
The molecule is methane. Although best known on Earth as the gas that rises from rotting garbage, it has a much greater allure on other planets. On Neptune and Uranus, the abundant methane in the atmosphere makes the planets look blue-green. But the discovery of methane on the planet named HD 189733b, which orbits a star 63 light years away in the constellation Vulpecula (the fox), goes beyond aesthetics. Under the right conditions of temperature, presence of water and other molecules, methane can be a star player in what is called prebiotic chemistry, the reactions that produce amino acids, nucleic acids and eventually the first living cells. (In 1953 chemist Stanley Miller zapped a mixture of methane, ammonia, hydrogen and water with electricity and got amino acids, the building blocks of proteins.
“This is a crucial stepping stone to eventually characterizing prebiotic molecules on planets where life could exist,” Mark Swain of NASA's Jet Propulsion Laboratory in Pasadena, Calif., who led the team that made the discovery, said in a statement.
The discovery comes from observations made last May with Hubble’s Near Infrared Camera and Multi-Object Spectrometer, which identifies molecules by their telltale spectra. As long as it was aiming at HD 189733b, the spectrometer also confirmed that the planet’s atmosphere contains water molecules, as the Spitzer Space Telescope found last year. With both water and organic molecules, HD 189733b would seem to have the basic ingredients for cooking up something biological.
There is only one problem. Floating out in Vulpecula (the constellation is visible from the north pole to 55 degrees south latitude, with the best visibility is in September), HD 189733b is what’s called a “hot Jupiter.” That means it is massive and so hot—because it is so close to its parent star it whips around in a mere two days—as to be beyond tropical: its atmosphere bursts the mercury at 1,700 degrees Fahrenheit, hot enough to melt silver. That makes HD 189733b too hot for life as we know it: any biological molecules fortunate enough to form would be torn apart by heat before they could do anything interesting.
So consider the discovery a proof of concept. Actually, two concepts. One, it shows that the Hubble can detect interesting molecules on distant planets using spectroscopy. Two, it shows that prebiotic molecules can form on these planets, improving the odds that they also form on planets that orbit in the “habitable zones” around stars, where temperatures are right for water to remain liquid rather than ice or vapor. Consider the measurements an important step toward determining which worlds have the conditions of temperature, pressure and chemistry for life to exist.
This observation is one of the first steps in the search for life on another planet, astrophysicist Marc Kuchner of NASA Goddard Space Flight Center said. “We need to study the chemistry in a planet's atmosphere in order to determine whether the planet could harbor life.