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These Are the Two Most Earth-Like Planets Ever Discovered

Introducing Kepler-438b and Kepler-442b, the most precise Earth analogs on the books.
​Concept drawing of exoplanet surface. Image: IAU/L. Calçada.

A group of Harvard astronomers has announced the discovery of eight new exoplanets located in the "Goldilocks" zone of their stars—meaning that these planets might support liquid water. And, potentially, life.

As if that isn't exciting enough, it turns out that this new batch includes the two most Earth-like planets ever found. So move over, previous record-holders Kepler-186f and Kepler-62f, because Kepler-438b and Kepler-442b are now the planets most likely to resemble our own.

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These candidate planets were evaluated with main factors in mind: their size and the amount of sunlight they receive from their stars. "These are the two conditions that we think are required for a planet to be habitable," Harvard astrophysicist Guillermo Torres, lead author of the study, told me in a phone interview.

Both of the new exoplanets appear to pass these metrics: Kepler-438b is about 12 percent larger than Earth and receives 40 percent more light, while Kepler-442b is about a third larger than Earth, and receives two-thirds as much light.

Though the planets' orbital periods are much shorter than Earth's orbit—measuring 35 and 112 days respectively—they receive comparable amounts of sunlight because they orbit red dwarf stars, which are much dimmer and colder than the Sun. Accordingly, the habitable zone in these extrasolar systems are located much closer to their stars than in our own.

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Rooting out these rare "Goldilocks" exoplanets is an incredibly challenging endeavor. As I wrote yesterday regarding yet another Harvard exoplanet study, it is possible to characterize Earth-sized planets in detail when they are in very tight orbits around their stars, but these planets are too hot to be considered habitable. It's also possible to nail down the specs of large, Jupiter-sized planets orbiting in the Goldilocks zone, but once again, life as we know it is unlikely to exist on these gas giants.

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The combination of looking for small, rocky planets with larger orbits presents numerous challenges for astronomers. Because they are small, these exoplanets induce almost imperceptible dips in brightness when they transit in front of their stars. And because they have large orbits, they don't transit very often, so basically, there's not a lot of data to work with.

Moreover, there are many other cosmic events that can produce a similar effect to a planetary transit, so it's difficult to be sure that a dip in the light curve—which is the brightness of a star as a function of time—is actually caused by a planet.

Torres confronted this problem by developing a computer program called BLENDER, which has since become a major tool in exoplanet research.

"When we have this light curve with a little dip, the first reaction is to say 'oh, that's caused by a planet in front of the star,'" he told me. "But it turns out that there are many other phenomena that can produce a similar dip and have nothing to do with a small planet. It's a really nasty thing to deal with, because it's very hard to distinguish the real planetary transit from one of these false positives."

"And this is the issue," he added. "This is why I wrote the BLENDER code."

BLENDER is essentially a massive simulator, capable of running about a billion different scenarios for a light curve dip. By comparing these simulations to the real light curves observed by the Kepler space telescope, the program calculates the probability that a planetary transit caused the dip.

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"It's actually a trial and error exercise," said Torres, "except that the parameter space for these types of objects that can mimic the light curves is so large that we have to use a supercomputer at NASA Ames to do this. We use the Pleiades supercomputer to test approximately a billion different scenarios for each candidate from Kepler."

"For these eight planets that we are announcing," he continued, "the probability [that they are planets] is always greater than 99.7 percent."

Torres's study, forthcoming from The Astrophysical Journal, is a fresh example of how sophisticated exoplanet research has become in just two decades, and how rapidly the field continues to mature.

Indeed, several giant telescopes currently in construction may even be able to characterize the atmospheric composition of Earth-like worlds, which could contain clues about the presence of extraterrestrial life.

"That's going to be the future," he said. "It's mind-boggling to think about it. We didn't know of any planets outside the solar system twenty years ago. Now, we have Earth-like planets, and we're talking about looking at their atmospheres for biomarkers. That's really exciting in my mind."