Science Meets Fiction

Astronomers have discovered a handful of planets orbiting the collapsed stars known as pulsars. Almost all of these confirmed or purported pulsar planets are small, only a few times the mass of Earth, and are thought to have formed out of the gas and dust left over from the supernova explosion that created the pulsar. However, PSR B1620-26 b, discovered in 1993, did not fit that pattern.

PSR B1620-26 b was several times as massive as Jupiter, and that was not the weirdest thing about it. It was the first planet discovered orbiting a binary star: a pulsar and a white dwarf, it has a very wide orbit that lasts 100 years, and, strangest of all, it resides in the globular cluster M4, yet another place where planets were not expected.

A globular cluster can have over a million stars in a space less than 100 light-years across. With that many stars buzzing around, one would expect that their gravity would knock any planets out of orbit. In fact, we think that is what happened here.

Astronomers believe that PSR B1620-26 b first formed around a solitary normal star at the same time that the cluster formed–12 billion years ago–which also makes it the oldest known planet. Millions of years later, its original sun passed close to a binary star: the pulsar we now see and another normal star now lost to history. Through a complex gravitational interaction, the pulsar’s original partner was kicked out, and the planet’s original sun replaced it, bringing the planet with it. Eventually, that sun aged and died, becoming the white dwarf we now observe.

Meanwhile, the planet lives on. Not having been present for the supernova that created the pulsar, it still retains its thick atmosphere, and, if it avoids the dense center of the star cluster, it may continue circling its old sun and its newer companion for billions of years to come.

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.

At this writing, 838 planets outside our Solar System—exoplanets—have been discovered, with thousands more possible planets waiting to be investigated. Yet the study of exoplanets is one of the youngest fields in astronomy and in all of science. In fact, just over 20 years ago, the number of known exoplanets was zero, and it was considered a little silly even to look for them.

Why? In the 1980s we learned that some young stars, like Beta Pictoris, probably were forming planets when huge disks of gas and dust were found to be orbiting them. Over millions of years, the gas and dust would condense into planets, but those planets, it was thought, would be invisible. Our own Sun is a billion times brighter than the planets that orbit it. From interstellar distances, any planets should be invisible in the glare of their parent star.

So by the early 1990s, very few serious astronomers were looking for exoplanets because even if planets were common in the galaxy, there should be no way to see them. Worse, the scientific landscape was littered with dozens of false alarms all the way back to 1855. Every time some reported finding an exoplanet, it was soon discredited or at least strongly questioned.

Still, a few brave souls kept looking. One idea was not to look for the planets directly, but instead to look for its effect on its parent star. As the planet orbited in a large circle, the star would also move in a small circle, like a counterweight, and it might be possible to observe the changing position of the star on the sky. This method worked great for binary stars, but it didn’t turn up any planets. Then, in 1992, a pair of planets did appear, in a place even the planet hunters did not expect.

PSR B1257+12 is a millisecond pulsar—a collapsed star that underwent a supernova at the end of its life. A pulsar packs more mass than the Sun into a space the size of Los Angeles and emits intense beams of radio wave and x-rays. A millisecond pulsar in particular spins incredibly fast at a nearly constant rate. Astronomers found that PSR B1257+12 was spinning at over 9,000 rpm, and its rate of spin was so steady that they could measure it to one part in a trillion.

Except it wasn’t quite steady: sometimes the pulses of radio waves would reach Earth a little earlier, meaning that it was closer to Earth, and sometimes a little later, meaning that it was farther away. That meant the pulsar was moving in circles, tugged by the masses of two unseen companion. Many pulsars have companion stars that are not detectable with visible light, but when astronomers Aleksander Wolszczan and Dale Frail calculated the masses of these companions they found that they were much too small to be stars. They were planets—the first ever discovered outside our Solar System!

Not only were they exoplanets, but they were tiny, only a few times the mass of Earth. Two years later, a third planet was discovered orbiting the pulsar, which is only the size of our Moon and is still the smallest confirmed exoplanet ever found.

But these planets were weird. It’s weird enough that they’re orbiting a dead star and are being constantly roasted by x-rays, but they shouldn’t have existed at all. The supernova explosion that created the pulsar should have disintegrated any planets it had to start with. So how did they get there? We now believe that these three planets formed after the supernova, condensing out of the debris that was left over.

So the field of planet hunting was partially vindicated in 1992, but these weren’t normal planets. They were weird, and they didn’t form like normal planets. It would be three more years before astronomers discovered a planet orbiting a star like our Sun, which I’ll talk about in a future post, but even then, the weirdness continued. It continues still to this day, so that now we’re starting to wonder…if maybe we’re the weird ones.