Scientists used to wonder how common planets were throughout the universe, and now we know. They're extremelycommon. They're so common, in fact, it's possible just to find another one while you're trying to gather data on the one you already knew about. That's what happened when astronomers recently turned their attention to the star K2-18. They found a bonus planet.
The first exoplanet around this star, dubbed K2-18b, was discovered in 2015 by the Kepler Space Observatory. This instrument got a new lease on life when NASA engineers managed to compensate for the failure of its reaction wheels. It couldn't maintain orientation as well as before, but it's “K2” phase still allowed it to spot many exoplanets like K2-18b. Kepler uses the transit method to spot planets — when a world passes in front of its host star, there's a small drop in light. That's enough to strongly suggest a planet, but astronomers always need to follow up.
In the case of K2-18b, astronomers began observing it with the European Southern Observatory's planet-hunting HARPS instrument, which is part of the 3.6-meter telescope in the La Silla Observatory. It appeared to be a super-Earth, a rocky planet more massive than Earth, but Kepler couldn't tell us for sure. HARPS allowed the team to track K2-18's radial velocity as K2-18b orbited it. These fluctuations allowed the team to determine the planet is in the habitable zone of the star and is probably composed of either rock or water and ice. Either way, there's some possibility it could support life.
The HARPS instrument.
While monitoring K2-18b, the team picked up another signal that Kepler missed. It turned out to be another planet much closer to the star. This planet, now known as K2-18c, also looks like a super-Earth. However, it's so close to the star that its temperature exceeds the boiling point of water. That makes it a poor candidate for life as we know it.
It's remarkable we've been able to learn so much about a solar system more than 111 light years away, but this is only the beginning. Scientists are excited to see more planets orbiting red dwarf stars like K2-18. Some other important exoplanet finds have been orbiting red dwarfs, like the one right next door in the Proxima Centauri system. Not only are these stars plentiful and long-lived, but they're also good targets for study with the upcoming James Webb Telescope. That instrument should be online in 2019.
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