The Search for Planets Around Nearby Stars

by Dr. Richard Terrile

My talk today is about the search for planets around nearby stars. This seems to be one of the fundamental questions in astronomy and, indeed, in science in general. And that is, "Are we alone in the universe?" One way to answer this is to really wonder whether planets can also exist around some of the nearby stars. It's a question that's been wondered about for more than two thousand years, with the Greeks first posing the question as whether inhabited planets might also exist around the stars in the night sky. We are very fortunate because our technology only recently has crossed a threshold which would really allow us to finally answer that question for the first time, to actually go out and look for planets at nearby stars. There are several techniques to do this. Among them are direct techniques and indirect techniques. In the direct techniques, we try and look for the planet directly, to separate its light from the light of the parent star. The problem is that the small amount of light reflected off a planet is only one bil- lionth the light from the parent star. That is like looking at a flash bulb and trying to read the inscrip- tion on the bulb while it is going off in your face. It's a very difficult thing to do. One of the techniques we use to try to do this is to create a miniature stellar eclipse inside the tele- scope using an instrument called a coronagraph. What this coronagraph does is to block out the light from the parent star and allow you to look for very faint objects near the parent star. But we haven't been successful yet in finding planets with this technique because it is a very difficult technique to do from the ground. We'll have to wait until we get a telescope in space should help us do that. From the ground we have had some very interesting results. One of them is in looking at a nearby star called Beta Pictoris with the coronagraph. We found a flattened disk of material. It is seen edge- on, and we believe this flattened disk of material is similar to the disk of material that formed the planets in our solar system. This disk appears to be about 40 times larger than our solar system and contains at its center an area which has been cleared out of material, about the same size as our solar system. One of the reasons we think this material has been cleared out is because we believe that planets may have already formed around Beta Pictoris within this disk. Seeing these disks, particularly around nearby and very common stars makes us believe that the process of planetary formation may be fairly common. Several other techniques used are indirect techniques. What we do is measure the effect of unseen planets on the parent stars. These effects are mostly due to the planet orbiting the star and causing the star to wobble back and forth. If we imagine spinning a very heavy barbell weight with no weight on one end or a very small weight on one end, it will spin near the center of the large weight, but not quite at the center of the large weight. Similarly, as planets orbit our Sun, they pull the center of gravity away from the center of our star out to an area somewhere near its surface. The star over a period of time appears to wobble back and forth as the planets orbit it. Nearby stars will have the same wobbling effect if there are unseen plan- ets orbiting them. So one technique that is used is called astrometry, and it means photographing or measuring the positions of stars over a period of time, over a period of several years. During that time, the positions of the target star are measured with respect to background stars that are much further away and would have much smaller wobbles if they did have planets. When a star is seen to wobble, this alerts us that something unseen or perhaps very exciting is in orbit around that star. That could be another star of smaller size, of less luminosity, less brightness, that we haven't seen; or it could be a planet, a Jupiter-size planet. Several other indirect techniques are also used. These involve measuring the acceleration of the planet as it moves toward us and away from us, again due to this wobble of unseen planets. Those measurements are measured with spectroscopy - looking at the spreading out of light from the star and looking at spectral lines and seeing how they move back and forth. It's called measuring the Doppler shift. Other techniques, still indirect techniques, are measuring the brightness variations of a star. If we are very fortunate, and there is a planet orbiting a star, and we just happen to be in line with the orbit of the planet, then at some time during its orbit it will cross in front of the star. And this will cause a very small decrease in brightness from the parent star over a period of several hours to several days, depending on the planet's orbit. And if we're lucky enough to be able to measure these things, we might also be able to detect planets that way, in that indirect technique. In summary, the search for nearby planets has just begun because our technology's here, and we're using several different approaches - the direct approaches to look for the planet directly and the indirect approaches which look for the effects that these planets have on the parent stars.

Originally published in NASA COTF ASTRONOMY VILLAGE CD-ROM 1995 as The Search for Planets Around Nearby Stars, by Dr. Richard Terrile