Planets on the move

It was easy to detect planets around pulsars because of their incredibly steady rotation period, but that didn’t work for normal stars, like the Sun, and the astrometric method of watching a star’s position on the sky was not sensitive enough for planet-hunting. Was there another way? In the late 1980s, a few astronomers thought there was.

As a planet orbits a star, the star moves in a smaller circle to counterbalance it. So while the planet is moving toward Earth, the star is moving away, and vice versa. But the star is giving off light, and a moving light source undergoes a Doppler shift.

Various atoms in a star’s atmosphere absorb light at very specific wavelengths. If the star moves, those wavelengths shift a little bit, much like the pitch of a siren changes depending on whether it’s moving toward or away from you. The shift in wavelength is very small. For a planet like Earth, it’s too small to see even with today’s telescopes, but for a big planet like Jupiter, it’s easy to spot.

That was the theory, anyway. This way of looking for planets is called the radial velocity method. For a while, it didn’t turn up anything, until 1995, when astronomers Michel Mayor and Didier Querloz looked at small yellow star called 51 Pegasi.

On paper, 51 Pegasi was a great place to look for planets. It’s about the same size, brightness, and age as the Sun. If solar systems like our own were common in the universe, there was a good chance of finding a planet around 51 Pegasi, and, when Mayor and Querloz looked, they did find a planet–the first ever found around a normal star. Except that, once again, it was in a place where no planet had any business being.

Mercury, the innermost planet of our Solar System, orbits 60 million kilometers from the Sun. This new planet, called 51 Pegasi b, orbits less than 8 million kilometers from its star. Mercury orbits in 88 days. 51 Pegasi b orbits in 4 days. Mercury is as hot as an oven. 51 Pegasi b is as hot as an open flame.

Finally, 51 Pegasi b isn’t a little rocky planet, like Mercury; it’s half the size of Jupiter–a gas giant made mostly of hydrogen. Shouldn’t the intense heat and solar wind have blown away it’s atmosphere long ago? Even if it didn’t, any reasonable theory of planet formation says that the heat would have evaporated the gas and dust that close to the star long before any planets could form there. It was a planet that shouldn’t exist.

Today, we know of many planets like 51 Pegasi b, which we call “hot Jupiters”. Even though they aren’t as common as we once thought, they’re still the easiest to find. The closer a planet is to its star, the faster it moves (and the less time it takes to see it move); and the heavier it is, the more the star will move in response. So it was natural big planets orbiting very close to their parent stars were found first.

51 Pegasi b taught astronomers two things. First, it really was worth it to look for planets. Soon, other astronomers got in the game, and dozens more planets were found, and, today, there are hundreds. Second, not only were planets common in the galaxy, but they were also on the move. 51 Pegasi could never have formed where it is now. They realized that it must have formed farther from its sun and migrated inward. This gave us some clues about the formation of our own Solar System, but that’s another story.

About Alex R. Howe

I'm a full-time astrophysicist and a part-time science fiction writer.
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