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of Another Star The nearest star is 4.3 light years away ... thats about 40 trillion kilometres. All we can see when we look in a telescope is a dot of light ... even in the most powerful Earth-based telescopes. How can astrophysicists detect planets around stars, or any other information for that matter, when they look at a single light smear on a photograph? One way that scientists can determine that a planet is orbiting a distant star is to look for stars that wobble. The gravitational force that holds a planet in orbit around a star causes the star to move as the planet circles around it, and the movement is greater if the planet is more massive. Imagine two skaters on the ice, with a rope, and the heavier person tries to spin the other around in a circle at the end of the rope. Even if the lighter skater being spun is tiny, the person doing the spinning will move a little as he turns, due to the pull on the rope. Telecopes far apart on Earth can be used in pairs to take images of the same star at the same time; the images can be combined to simulate a telescope with a mirror the length of the distance between the two telescopes. This is called interferometry. Another way that astronomers look for and study extrasolar planets is by watching stars, and noting periodic changes in their brightness. The changes are caused by the slight dimming of the star as a planet passes between it and the Earth. We have an animation of this process for you to download ... see the bottom of the page. Now that the Hubble telescope is in orbit in space and working, we can see images clearer that ever before, since there is no layer of atmosphere in the way. The Hubble telescope has been used to take pictures of nearby large stars, including the giant star Betelgeuse. Find out more here. The most important tool an astrophysicist has is the spectrum of light from the star. The light is passed through a device much like a prism, and split into its component wavelengths. The wavelengths that exist in the light are a measure of what elements were undergoing fusion in the star. In addition, any cooler gasses in the upper layers of the star will absorb some of this light, and leave black lines in the spectrum, which will also tell you something about the chemical makeup ... every chemical element leaves a distinct pattern of absorption lines But there's more. Anything that starlight passes through on its way to us will also filter out some of the light, and leave more black lines. The Hubble telescope recently examined the spectrum from the star HD 209458, a faint yellow star about 150 light years away. By looking at the absorption lines in the star's spectrum, particularly those for the element sodium, and noticing how they seemed to vary in intensity in a periodic way, astronomers determined that they were watching a planet with an atmosphere pass in front of the star!  The planet itself was first discovered in 1999 through its slight gravitational tug on the star. The planet was estimated to be 70% the mass of Jupiter ... a fairly large planet. Then astronomers determined that the tilt of the planet's orbit made it pass in front of the star from our viewpoint here on Earth. This planet became the 80th planet of another star to be discovered so far. As the planet passes in front of the star, it causes the star to dim very slightly for the duration of the transit. Observations by the Hubble telescope revealed that the planet is mainly gaseous, rather than liquid or solid, meaning that the planet is a gas giant, like Jupiter and Saturn.  At the right is an artist's conception of that the planet might look like as it orbits star  With some very careful measurements, scientists were also able to determine that the planet has an atmosphere. They did this by carefully analyzing the spectrum: here is a diagram which explains the process.   At the NASA News Release site where we obtained these diagrams, there was also a very well done MPEG video showing an animation of the planet moving in front of the star, and what this does to the brightness. A still from the video is shown at the right. Visit their site and download it! |