Wednesday, March 19, 2008

Hubble sees organic molecules in another solar system

Artist's impression of the extrasolar planet with detected methane Image credit: NASA, ESA and G. Bacon

This afternoon, NASA announced that the Hubble Space Telescope detected organic molecules in the atmosphere of a planet circling another star.

First, let me emphasize: Hubble did not discover evidence of life on another planet. Organic molecules (generally molecules made mostly or fully of carbon and hydrogen atoms) are quite common in the Universe. Carbon is made in stars and during supernova explosions, and is one of the most abundant atoms after hydrogen and helium. And the organic molecule found, methane, is quite common in the atmospheres of Jupiter, Saturn, Uranus, Neptune, Saturn's moon Titan, and even in some of the coolest brown dwarf stars.

So, why the big deal? It is because we are detecting the signatures of chemistry on a planet outside of our solar system. We can now begin to probe the chemical make-up of this planet, and from that begin to test our models of giant planets around other stars.

How did Hubble detect the methane (and water) in this planet? The planet in question, which has the extraordinarily boring name of "HD 189733b", is about 63 light-years away toward the constellation Vulpecula, a tiny, forgettable constellation near Cygnus, the Swan, visible during the summer months in the northern hemisphere. The planet is about the size of Jupiter but is located very close to its parent star, completing an orbit in only 2 days. Compare that to 88 days for Mercury to go around the sun, a year for the Earth, or 12 years for Jupiter! So it is a hellishly hot place, and certainly nothing can live on the planet (which is a big gas bag, like Jupiter), or on one of its moons (if it has any).

The planet passes in front of its parent star, as seen from the Earth, once each orbit. Every two days or so, the star appears to get a little fainter as the planet's shadow crosses the face of the star. The planet only blocks out some of the star's light (about 1%), so these little variations are hard to see.

Some of the light from the star passes through the planet's atmosphere on its way to Earth, and any chemicals in the planet's atmosphere will put a fingerprint on that light. This is a very tiny signal, and very hard to see. But, with time, patience, and a lot of hard work, it can be picked out. In the past, astronomers had detected sodium in the atmospheres of a few planets around other stars. Now, we can add methane and water to that list.

The next step is to compare the various chemicals and the amount of those chemicals that we see with theoretical models of what these planets atmosphere's might look like. These models are really just educated guesses -- if we assume the planets are like Jupiter, and have a chemical makeup similar to that of their parent star, and we make some guesses as to how hot the planet might be, we can use concepts of meteorology and chemistry to calculate what the planet's atmosphere should look like. And the very early indications are that the models aren't quite right. The models predict that water and methane and sodium should be there, but in different amounts. This disagreement is a good thing -- it means we don't yet know everything, and that we can learn a lot by doing more studies like this one.

So, congratulations to the team who made this discovery. Good going -- now, let's get back to work and see what more we can learn about these planets!

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