Authors: Jon M. Jenkins , Joseph D. Twicken, Natalie M. Batalha et al.
First Author’s Institution: NASA Ames Research Center, Moffett Field, CA 94035, USA
Last month Nasa’s Kepler Mission announced, in what seems like a roughly annual event, the discovery of “Earth 2.0”. Described as a “Bigger, Older Cousin to Earth”, Kepler 452b is the first small planet (defined here as less than twice the radius of the Earth) to be in a one-year orbit around a Sun-like star.
But is it otherwise that similar to the Earth? Is it potentially habitable? To try and answer that, let’s look at the discovery paper.
Kelper 452b was spotted by Jenkins et al. in the data collected by the Kepler spacecraft. For nearly four years between 2009 and 2013, Kepler stared at a single patch of sky, continuously monitoring over 100 thousand stars. The spacecraft was watching for the tiny drops in brightness caused by a planet passing in front of its star. The mission has been stunningly successful, spotting nearly 5000 exoplanet candidates.
An initial, automatic search through the Kepler data completely missed Kepler 452b. When the data was reprocessed in 2014, with the checks put in to avoid false positives slightly relaxed, the authors spotted a signal in the data from the Sun-like star KIC 8311864 (now known as Kepler 452):
The drop in light from the star, about 0.02% every 384.8 days, was consistent with a roughly Earth-sized planet blocking some of the starlight. But it could also have been a variety of other things, such as variable background stars or larger, less interesting planets, which may have produced the same signal. At 1400 light years from Earth, KIC 8311864 isn’t very bright. Good, high signal-to-noise observations were therefore difficult or impossible to obtain, adding to the potential for confusion.
To weed out the potential false positives, the authors took two main approaches. More powerful telescopes than Kepler, such as the 10-metre Keck telescopes on Hawaii, were used to search the area near KIC 8311864 for objects that might be getting mixed in to the Kepler data.
Finding nothing, they then simulated the light curves of the remaining potential false positives, those that would be too close to the target for the follow-up observations to spot. Problematically, a few of these turned out to be indistinguishable from the observed data. Was the new planet no more?
Not quite. Using the rest of Kepler’s discoveries, the authors worked out the probability of the light curve being produced by a false positive, as opposed to a planet. The odds of the planet hypothesis being correct came out at a comfortable 424:1.
The new planet, christened Kepler 452b, was small, with a roughly one year orbit, around a Sun-like star—already beginning to sound similar to the Earth. But just how similar? Jenkins et al.’s next step was to narrow down the exact properties of this new world.
To do that, they first needed to understand the star. But there was a problem: the characteristics given for KIC 8311864 in the Kepler Input Catalog (KIC) didn’t make sense. Based on our understanding of stars, the stated temperature and size of KIC 8311864 were incompatible. Again, the low brightness of the star was an issue, stopping the use of direct techniques such as asteroseismology to measure its properties.
Jenkins et al. turned to spectroscopy, obtaining data from three telescopes, including the mighty Keck. They could then use models of stellar structure to calculate to properties of the star that would produce the observed spectrum. They found that KIC 8311864 was indeed very similar to the Sun, with the same mass, slightly lower temperature, about a billion years older and about 1.1 times the radius.
The new radius measurement of the star allowed the authors to pin down the size of Kepler 452b—the drop in light in the transit is related to the ratio of the radii of the star and the planet, so if you know one, you know the other. Kepler 452b turned out to have a radius 1.6 times that of the Earth, putting it in a class of worlds known as Super Earths.
Finally, the authors could begin to explore the properties of the planet. First up was its habitability— could life hypothetically exist there? The most popular criteria for habitability is that it orbits in the “habitable zone”, loosely defined as the range of distances from the star within which liquid water can exist on the surface. This depends on the amount of energy the planet receives from its star, known as the insolation. In its one year orbit around a star almost identical to the Sun, Kepler 452b, like the Earth, is quite likely to be in the habitable zone.
To work out just how likely, the authors take an optimistic definition of the habitable zone, where the inner and outer edges are where the insolation is equal to that received by Venus now, and the young, possibly wet Mars respectively. Under this model, the odds of Kepler 452b being in the habitable zone come in at 96.8%.
So far, so good. But just being in the habitable zone doesn’t ensure that you have liquid water. After all, under the definition described above, Venus is just about in the habitable zone, and you wouldn’t want to live there.
Even worse, you may not even have somewhere to stand on Kepler 452b. Planets with 1.6 Earth radii are just on the boundary between being rocky, like the Earth, and gaseous worlds more similar to Neptune. We don’t yet have a full understanding of what determines whether a Super Earth is rocky or gassy. This uncertainty is exacerbated by the fact that there (probably) aren’t any in the Solar System, stopping us from studying an example up-close.
Knowing the mass, and therefore the density, of Kepler 452b, would shed light on its composition, but (surprise, surprise) the system is too dim for such observations. Instead, the authors use theoretical models, based on those few Super Earths for which we do have measurements of both their masses and radii, to work out the probability of different compositions of Kepler 452b. They find that the odds of Kepler 452b having a rocky surface are, depending on what model you use, somewhere between 49% and 62%. And at that point, our ability to learn more about Kepler 452b comes to an end.
So is it right to call it Earth-like? In this astrobiter’s opinion, not really. It is the closest analogue so far to the position of the Earth, in the habitable zone of a Sun-like star. But without more data, we can only say that the planet itself has a chance of being similar to the Earth, and the odds aren’t great. Calling it a second Venus would be just as accurate, and mini-Neptune more likely still.
But this doesn’t stop Kepler 452b being an interesting, unique planet. I think we shouldn’t try too hard to shoehorn our discoveries into narrow categories, and instead celebrate the wonderful variety of all of these strange new worlds.