Astronomers have captured enough light from two planets far beyond our own solar system to show details of their chemical make-up.
This breakthrough marks a new phase in the search for extraterrestrial life.

By analyzing the faint glow of one of these alien worlds, they have found tentative evidence of chemicals that play a role in one theory of how life began on Earth.

The chemicals — polycyclic aromatic hydrocarbons — may have helped the formation of ribonucleic acid, the ancestral genetic material of DNA — the building blocks of life.

Although this planet seems to lack water and is at a searing 800 degrees Celsius — too hot for life — three teams announce that they have successfully carried out the feat on this and one other alien world.

Of some 200 alien planets detected so far, 14 pass in front of their parent stars, and two are bright enough to be analyzed by the new method.

This works by detecting the signatures of particles and gases present in a planet’s spectrum, like fingerprints. By reading these fingerprints, researchers can deduce an atmosphere’s composition, the presence of clouds, perhaps even find evidence of life.

A team led by Jeremy Richardson, of the NASA Goddard Space Flight Center in Greenbelt, Md., describe today in the journal Nature how, using the Spitzer Space Telescope, they obtained the infrared spectrum of the extrasolar planet, HD 209458b, nicknamed Osiris, which orbits a Sun-like star in the constellation Pegasus. Mark Swain’s team at the Jet Propulsion Laboratory, using the same data, reaches similar conclusions.

Another spectrum is reported by Carl Grillmair, of the Spitzer Science Center at the California Institute of Technology, and colleagues who studied another exoplanet, HD 189733b. Together, this marks a technical tour de force because light from the star swamps the relatively dim glow of the planets.

Mr. Richardson’s team observed the infrared light spectrum of the planet and parent star together before the planet passed behind the star and again while it was hidden behind the star.

By subtracting their measurements during these two three-hour eclipses, they were able to infer the planet’s own rainbow-like spectrum.
Because the spectrum contains lines corresponding to where chemicals absorb or emit light, forming dark and bright bands, they can begin analyzing its atmosphere.

The planet is a “hot Jupiter” — a gas giant like our own Jupiter that orbits its star some 10 times closer than Mercury does our own Sun. Calculations of what the spectrum of such a planet would look like if it contained water vapor do not fit today’s measurements, suggesting it is dry. According to one emission peak, there is high-altitude silicate dust, a mineral common on Earth. There is also an unidentified feature in the spectrum, a sharper peak at a wavelength of around 7.78 micrometers, which is hard to identify but may correspond to polycyclic aromatic hydrocarbons, “a more exotic possibility.”

Another hot Jupiter, HD 189733b. was studied by a team led by Mr. Grillmair. “In a sense, we’re getting our first sniffs of air from an alien world,” said his colleague, David Charbonneau, of the Harvard-Smithsonian Center for Astrophysics.

Although several attempts have been made from Earth-based observatories, no spectrum has yet been measured for any extrasolar planets until now. “It’s incredibly significant and a fantastic achievement,” said Alan Fitzsimmons of Queen’s University in Belfast.
His colleague, Don Pollacco, said: “This is a momentous observation — it will enable our understanding of their atmospheric structure to be placed on a firm footing for the first time. Amazing.”

Both NASA and the European Space Agency are planning space telescopes that will hunt for Earth-like worlds.