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Multiple Planets

Week of April 19, 1999

The big news this week is the discovery of a new solar system outside our own. It isn't the first extrasolar planetary system to be discovered; the first planet outside our own solar system orbiting a normal Sunlike star was discovered in 1995. What makes this new discovery interesting is that it consists of more than one planet.

The star in question is Upsilon Andromedae, a 4th magnitude star in the constellation of Andromeda. It's actually visible to the naked eye for people in the northern hemisphere nearly year round (though it's best in the spring). Even the most powerful telescopes on or off the Earth cannot see the planets circling it though. The apparent distance from the star is so small, and the star so bright, that the feeble glow of the planets is simply too dim to be seen. So astronomers got clever: they know that big planets have enough gravity to tug on the star they orbit. As they orbit the star, they pull the star in a tiny circle. Imagine holding a heavy bucket of water and spinning around; you won't stay in exactly the same place because the bucket will throw off your center of gravity. You'll make a tiny circle on the ground, and the heavier the bucket the larger the circle.

The same is true for planets. The more massive the planet the bigger the circle the star makes. It's still too small to be seen directly, but the motion of the star as it makes that circle can be measured through its Doppler shift. This is the same effect that makes a a car horn change pitch as it goes past you. In this case, the light from the star changes ``pitch''; the pitch of light is actually what we call its color. The astronomers measure the light from a star very precisely, and if it shifts in color (more precisely, wavelength) they know something is going in. They can actually measure the star's change in velocity even if that velocity is only 3 meters per second! As a reference, that's jogging speed for a healthy human.

Sometimes the star itself is pulsating, which is misleading to planet finders. But sometimes the shift is so perfectly periodic and the star so stable that they can rule out anything but a planet. This is the way the first dozen or more planets have been found outside our solar system. Skeptics of this method say that besides pulsation, there may be some unknown factor that might be causing this shift. However, the new system makes this far less likely. The first planet orbiting Upsilon Andromedae was announced in 1997, and orbits its star very closely. That makes it easy to spot, because you can measure the shift of the starlight in only a few days. But after some more months, they noticed that the variation wasn't as regular as they thought. They tried figuring a second planet into the equation, but still got numbers that were difficult to explain. So they tried using a third planet, and suddenly everything made more sense. The variations in the starlight they measured match very well to a system with three giant planets, one closer to Upsilon than Mercury is to the Sun, one at about the same distance Earth is from the Sun, and the third one a bit farther out from Upsilon as Mars is from the Sun.

I have been using the word ``circle'' here, but actually the planets' orbits are quite elliptical. The one farthest out actually dips in towards Upsilon until it is just a bit farther out than Earth is from the Sun. Still, this is the first solar system ever found besides our own to definitely have more than one planet. This is an important discovery: most people all along have been assuming that multiple planet systems exist, but we had never actually seen one. Now we have, and it's a mere 44 light years from the Sun (this fact in itself has already spawned some very Bad Astronomy, as regular readers will not be surprised to find). This bodes well for finding many more systems like this; there are billions of stars like it in the Milky Way, and the Galaxy is 100,000 light years across. Finding one so close means that the Galaxy is probably littered with such systems. Many people like to say this makes life more likely in the Galaxy. I agree, but I am also a bit more conservative in my approach. I don't mind taking it one step at a time, and simply enjoy knowing that there are clever people out there, and the discovery of these systems will each, one by one, help us define our own place in the cosmos.

For more about Upsilon Andromedae's contingent of planets, you can go straight to the horse's mouth: the webpage from the discoverers: Marcy, Butler and Fischer. That page also has a diagram of the system. You can read about their planet searching techniques on their main webpage as well.



©2008 Phil Plait. All Rights Reserved.

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