NASA’s Transiting Exoplanet Survey Satellite (TESS) has only been scanning the skies for about a year, but it has already identified several new candidate exoplanets. As astronomers were working to confirm one recent sighting, they happened upon something unexpected. That solar system, known as GJ 357, hosts not one but three exoplanets. What’s more, one of those planets is a super-Earth in the habitable region of the star.
TESS uses the transit method to spot exoplanets with its array of cameras, similar to the dearly departed Kepler satellite. It can scan multiple stars at the same time, watching for the telltale dips in light that indicate an exoplanet has passed in front of its host star. That’s what TESS saw in GJ 357, which is only 31 light-years away. To confirm the existence of the exoplanet dubbed GJ 357 b, astronomers used ground-based telescopes to measure the star’s radial velocity. This alternative method looks for tiny changes in light caused when a star “wobbles” in response to the gravity of orbiting planets.
The team confirmed GJ 357 b, but they also found two more exoplanets now known as GJ 357 c and GJ 357 d. GJ 357 is a small M-type dwarf star, which is 40 percent cooler than the sun. GJ 357 b orbits the star in just 3.9 days and is 22 percent larger than Earth. The surface equilibrium temperature (a measure of solar radiation only) is 490 degrees Fahrenheit (254 degrees Celsius), ruling out life as we know it. GJ 357 c is a bit farther out, but still too hot for life. It’s 3.4 times as massive as Earth and orbits ever 9.1 Earth days. The temperature here is 260 degrees F (127 degrees C).
Astronomers are most interested in GJ 357 d, which sits near the outer edge of the system’s habitable zone with a 55.7-day orbit around the star. It’s 6.1 times more massive than Earth, meaning it could be rocky or gaseous. If it’s a rocky planet, it would be about twice as large as Earth.
According to the team, GJ 357 d has an equilibrium temperature of -64 degrees F (-53 degrees C). That sounds too low to be even potentially habitable, but this is only a measure of solar radiation. Earth’s equilibrium temperature is -1 degree F (-18 degrees C), but the atmosphere increases the surface temperature. GJ 357 d gets about as much solar radiation as Mars, and a sufficiently dense atmosphere could allow liquid water to flow on its surface. Astronomers around the world plan to take a closer look at this nearby super-Earth over the coming years.
For most of human history, people wondered if there were other planets out there in the unfathomable reaches of space. It turns out there are some very close to Earth, though. A team of astronomers has revived interest in Barnard’s Star, which hangs in space just six light years away. After dismissing evidence for an exoplanet orbiting this star in the 1970s, the team now says there’s a very good chance Barnard’s Star does indeed host a super-Earth.
Barnard’s Star is a red dwarf and one of the closest stars to Earth. Only the Centauri system is closer, and we know there’s at least one exoplanet there. Astronomers announced the discovery of Proxima Centauri b with great fanfare in 2016. At just 4.4 light years away, this planet will make an excellent target for future observation. The planet orbiting Barnard’s Star is just a little bit more distant, but it’s still close enough to be key to understanding exoplanets.
The tale of planet hunting around Barnard’s Star starts in the 1960s when astronomer Peter van de Kemp released compelling data supporting the existence of an exoplanet. However, other teams were unable to confirm, and by the 1970s we knew the discovery was merely a product of defective instruments. Scientists kept looking at the star, and now astronomers from the Institute of Space Studies of Catalonia and the Institute of Space Sciences in Spain have assembled 20 years of data to reveal Barnard’s Star b.
The newly published study lays out the basics of the exoplanet. Barnard’s Star b completes an orbit of the star every 233 Earth days. It’s about as far away from Barnard’s Star as Mercury is from Earth, but Barnard’s Star is much cooler. So, it’s distinctly outside the habitable zone with a surface temperature of -170 degrees Celsius (-274 degrees Fahrenheit). It’s the equivalent of a planet orbiting between Mars and Jupiter in our solar system. Barnard’s Star b has a mass about three times that of Earth, but we don’t know its size or composition yet. We suspect it’s rocky, though.
This image shows an artist’s impression of the planet’s surface.
Barnard’s Star b does not transit in front of its host star from Earth’s perspective — that would have made detection vastly simpler. Instead, the team used the radial velocity method. They watched for small counter movements in the star that indicate a massive body (a planet) is in orbit. A smaller star like Barnard’s Star wobbles more than a more massive one, so even a small-ish planet like this one had some detectable effect.
The team claims high confidence that Barnard’s Star b is real, but other teams will need to confirm. We don’t want another van de Kemp scenario.
Spotting new exoplanets with NASA’s Kepler space telescope is nothing new. Google even managed to automate the process with machine learning. Just add them to the pile with the 3,700 other exoplanets, right? Sometimes researchers still spot a planet of particular interest in the data that deserves additional study. Such is the case with one of 15 planets just uncovered by Japanese astronomers. It’s Earth-like and potentially habitable.
Kepler has been in operation for nearly a decade, scanning large numbers of stars in order to spot small dips in brightness. That dip could signify a planet is orbiting the star and passed in front of it from Kepler’s perspective. However, someone needs to go through the data to determine if the signal really does give away the presence of a planet.
Researchers from the Tokyo Institute of Technology spotted the latest exoplanets hiding in the data when examining readings collected by Kepler in the K2 phase. K2 started several years ago when reaction wheel problems limited the effectiveness of the satellite’s camera. Even in this limited mode, Kepler has found hundreds of new exoplanets to add to the total of more than 2,500 discovered since it started operating. Three of the 15 newly discovered planets fall into the Super-Earth category, meaning they’re rocky and between 1.5 and two times larger than Earth.
Of the three Super-Earths, one known as K2-155d is a tantalizing target for future study. It orbits a red dwarf star about 200 light years away, which is called K2-155. Red dwarfs are very long-lived stars, so life would have plenty of time to develop. They’re smaller and cooler than the sun, but an exoplanet orbiting close enough could still be warm enough to have liquid water on its surface. The flip side is that radiation would be more intense at that distance.
K2-155d is about 1.6 times the size of Earth, but we can’t say anything for sure about its density. Therefore, it may have more gravity than Earth or it could be very similar because of a lower density. Based on simulations of the exoplanet, the team believes it could have an atmosphere similar to Earth’s. Additional observations will help nail down the exoplanet’s properties and determine if it would be habitable by our standards.
NASA is set to launch the Transiting Exoplanet Survey Satellite (TESS) next month. This spacecraft will look at the brightest stars near Earth over a two-year period to gather data about exoplanets. As the brightest red dwarf visible from Earth, K2-155 could be on the list.
Scientists used to wonder how common planets were throughout the universe, and now we know. They’re extremely common. 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.