Thursday, December 06, 2007

Shadow worlds


Image credit: McDonald Observatory

We now know of well over 200 planets around other stars. Most of these planets are known only by the gravitational pull they exert on their parent star. And for that reason, we know almost nothing about these planets other than their mass and how far away from their parent star they are.

A small fraction of planets actually pass in front of their parent star as seen from the Earth. When they do so, we see the star's light get fainter as the planet blocks a little light from the star. What we "see" is a shadow. From the amount of light blocked, we can learn about the size of the planet. And, from some physics and knowledge of the planets in our Solar System, we can guess what the planet is made of, but this is just a guess.

To truly understand other worlds around other stars, we need to look for chemical signatures of their chemical composition. There are several ways to do this, but one way is to look for how the spectrum (the spread of colors in light) of a star changes when the planet is in front of it. This is because the light from the star has to pass through the planet's atmosphere, and any atoms in the planet's atmosphere can absorb light of specific colors. For example, when we look at sunlight that has passed through our atmosphere, we can see ozone, water, and carbon dioxide quite easily, as these molecules absorb unique colors of sunlight.

Several years ago, astronomers used a spectrograph on the Hubble Space Telescope to do this, and they detected sodium in the atmosphere of one planet. This is not surprising -- there is a lot of sodium in the Universe (that's one reason why there's a lot of salt on Earth), and sodium likes to absorb and emit light at two very distinct colors (which is why sodium street lights look orange). Then the spectrograph on Hubble quit working (it was old).

Over the past few years, Seth Redfield, a postdoc here at the University of Texas (that's Seth's picture above), decided to use our large Hobby-Eberly Telescope to look for sodium in some other planets outside our Solar System as they went in front of their parent star. His target: a planet 20% more massive than Jupiter around the star HD 189733. This star is in the little-known constellation Vulpecula (the "little fox"), near the summer constellations of Cygnus the Swan and Aquila the Eagle. The star itself is just 1/4 degree away from the famous Dumbbell Nebula -- in fact, in the linked picture, you can see the star (about halfway between the nebula and the right edge of the frame, and the brightest star in that part of the picture).

Many astronomers were sceptical that the project would work. From the ground, there are all kinds of difficulties that the Hubble Telescope didn't have. Earth's atmosphere is the biggest problem -- it has sodium, and the amount of sodium can change with time. It's possible, but very hard, to account for this precisely. Also, as the Earth moves around the Sun, the precise colors of sodium in the other planet's atmosphere will change slightly, but measurably, due to the Doppler Shift. The star itself likely has starspots (like sunspots), and these may cause changes in the sodium line. And even effects that wouldn't seem important, like exactly where across the star the planet moves (near the middle? toward one edge?) matters in the detection.

But Seth and his team did manage to work through all of these, and they detected sodium in the atmosphere of the planet. They did a lot of computer simulations, double-checking, second-guessing, but their detection stood up to all the tests. Sometimes experiments succeed because of luck; in this case, the team succeeded because of lots of sweat and toil.

Finding sodium is not surprising, but the fact that they did it means that, even without Hubble's spectrograph (which may be repaired by astronauts next summer), astronomers have a hope of detecting atmospheres of planets in other Solar Systems, allowing us to determine what the atmospheres are made of.

And, although there is a long way to go, Seth's work may eventually lead astronomers to find evidence of life on other planets -- ozone and chlorophyll are two signatures of life some astronomers think we should be able to see.

So, congratulations Seth on your discovery!

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