Since the discovery of the first exoplanet in January of 1992, astronomers have identified over 5,500 of them, with nearly a thousand of the systems analyzed having more than one detected planet. It now appears that at least one of the variables in the Drake equation -- fp, the fraction of stars that have planets -- is far higher than anyone might have expected.
What has come as an additional surprise is how varied these worlds are. Having grown up on a steady diet of Lost in Space, Star Trek, and Star Wars, I kind of had exoplanets pictured as mostly Earth-like, with lots of big rocks and maybe an odd-colored sky:
The truth is, every new one we find holds some sort of surprise. Some of the odder ones are:
- TrES-2b, which holds the record as the least-reflective planet yet discovered. It's darker than a charcoal briquet. This led some people to conclude that it's made of dark matter, something I dealt with here at Skeptophilia a while back. (tl:dr -- it's not.)
- CoRoT-7b, one of the hottest exoplanets known. Its composition and size are thought to be fairly Earth-like, but it orbits its star so closely that it has a twenty-day orbital period and surface temperatures around 3000 C. This means that it is likely to be completely liquid, and experience rain made of molten iron and magnesium.
- 55 Cancri e, nicknamed the "diamond planet." Another "hot super-Earth," this one is thought to be carbon rich, and that because of the heat and pressure, much of the carbon could be in the form of diamonds. (Don't tell Dr. Smith.)
- PSR J1719−1438, a planet orbiting a pulsar (the collapsed, rapidly rotating core of a giant star). It has one of the fastest rates of revolution of any orbiting object known, circling its host star in only 2.17 hours.
- V1400 Centauri, a planet with rings that are two hundred times wider than the rings of Saturn. In fact, they dwarf the planet itself -- the whole thing looks a bit like a pea in the middle of a dinner plate.
The reason all this comes up is that we just had a new addition to the "weird exoplanet" list thanks to the James Webb Space Telescope. It's called WASP-107b, and it has a number of bizarre characteristics. First, it is "fluffy" -- that's actually how the astronomers describe it -- having one of the lowest overall densities of any exoplanet known. It has about the mass of Neptune, but a diameter closer to that of Jupiter.
Second, it has a retrograde orbit -- it moves the opposite direction from the rotation of its host star and the revolution of the rest of the planets in the system. Its orbit is highly eccentric (elliptical), and is actually tipped 118 degrees away from the ecliptic (the plane of revolution of the rest of the system). Astrophysicists believe that it got this way because of interaction with the much more massive WASP-107c, but the truth is, they've never seen anything like it, so that's a surmise.
The atmosphere has high quantities of water vapor -- kept gaseous by the high temperatures (the upper atmosphere has an average temperature of 500 C) -- and sulfur dioxide. A bigger surprise was that the "highly dynamic atmosphere" (scientist-speak for "wind speeds that would blow your ass into the middle of next week") creates clouds of superheated silicate sand. The overall result is that being on WASP-107b would be like living inside a permanent pyroclastic flow.
What's shocking is how bizarre some of these new worlds are. It was natural enough to look at the planets in our own Solar System and assume that they kind of ran the gamut of planetary types -- thus the predominance of rocky worlds and gas giants with zillions of moons that you find in early science fiction. What continues to astonish is just how wrong that was. Wherever we look, we see an incredible variety of planets and star systems, and the great likelihood is that despite how many we've found, we've only scratched the surface.
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