Organic Planet

There is a planet only 63 light-years away where I’d say devils fear to tread, much less any starship crew.

The planet in question is so close to its star it completes an orbit in just over two days. Peeking through the cloud tops, the reddish star would be an overwhelming large ball of flames 40 times the angular diameter of the full moon. Temperatures at noon would be a seething temperature of 1,500 degrees Fahrenheit.

The gas giant planet whirls around the obscure red dwarf star HD 189733, tucked away in the obscure southern constellation Vulpecula. The planet is simply called HD 189733 b, though you think by now it would deserve something more imaginative like Abydos, Bedrosia, Mongo, or Zetar.

So why are two of our most powerful space telescopes - Hubble and Spitzer - trained on this foreboding world? Well it has become, in amazingly short order, sort of an interplanetary drugstore to go sniffing around for chemicals relate to life, as we know it. Nothing imaginable lives on this planet, but the atmosphere is laced with the fundamental chemistry used by carbon-based life. So, maybe it should be called Organia.

Astronomers are cutting their teeth on practicing how to isolate and measure the chemical fingerprints in a planet’s atmosphere in preparation for the day organic compounds and other chemistry considered biotracers, can be seen on earthlike exoplanets. Some astronomers thought that such an observational feat was 50 years away, but it’s happening now.

Just yesterday a team of astronomers from NASA’s Jet Propulsion Laboratory reported finding carbon dioxide and carbon monoxide in the atmosphere of HD 189733 b. Seemingly inhospitable worlds like Mars and Venus have predominantly carbon dioxide atmospheres too. But on Earth, plants use carbon dioxide during photosynthesis to make sugars. So carbon dioxide’s presence, in balance with other biotracers, could be telltale evidence for life on another planet.

Earlier this year Hubble astronomers announced finding methane on HD 189733 b. When in the right mix with other biochemistry, this could someday reveal planets with creatures resembling flatulent cows. Add to that mix water vapor, which was found on the planet last year by the Spitzer Space Telescope, and later confirmed by Hubble.

So why is HD 189733 b such a special target? Its orbit is tilted edge-on to Earth so the planet can routinely be seen passing in front of its star (transiting) and passing behind the star (in eclipse). This endless game of peek-a-book allows for unique experiments with modest-sized telescopes, which are not now possible for non-transiting exoplanets.

When HD 189733 b passes in front of its star, Hubble can measure how light passing through the thin sliver of atmosphere on the plant’s limb is filtered by molecules in the atmosphere. Hubble can do this to exquisite precision because it is located in space.

When the planet passes behind its star both Hubble and Spitzer can make infrared measurements. Observations made just before eclipse contain the infrared glow of the planet and star. When the planet ducks behind the star only the infrared glow of the star remains. Subtract the star’s signature are you are left with information about the IR light coming from the planet.

These observations are a dress rehearsal for the day, perhaps not too far in the future, when we find an earthlike planet and want to look for telltale byproducts of life manifest in atmospheric compositon. In all likelihood the first such candidate will be orbiting a red dwarf star, and the planet itself with be a super-Earth, several items the mass of Earth. If we really get lucky the planet will lie in the star’s habitable zone where moderate temperatures allow for a liquid water ocean to exist.

It’s unlikely that the very first super-Earth will be a biochemical clone of our planet. But if the atmosphere has unusual chemical abundances there will be a spirited debate if these are the byproduct of living organisms.

I've gotten some e-mails critical of calling HD 189733 b incapable of life. I almost always use the qualifier "life as we know it" or perhaps better yet, "life as we could recognize it." It's fair to say that life is a condition of the universe. It is the favored form of self expression of matter, and therefore probably manifests itself in ways we can barely imagine. So in looking for extraterrestrial life, we have to begin with what we know.