The weirdest planets

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.

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About Alex R. Howe

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