Artist's conception of Kepler-452b around its Solar twin star. Image credit: NASA Ames/JPL-Caltech/T. Pyle

Jul 30, 2015 Earth's new potential sibling, Kepler-452b

Sen—"This is just the beginning of a very long journey," said Professor Didier Queloz of Cambridge University, UK, at a NASA press conference July 23 announcing some remarkable discoveries from the Kepler mission: 521 new Kepler planet candidates, a dozen small exoplanets in the habitable zones of their parent stars, and one confirmed planet that has been called the most Earth-like planet ever discovered. Professor Queloz's statement was a fitting remark made on the 20 year anniversary of his co-discovery of the first exoplanet around a Sun-like star, 51 Pegasi b. 

The teleconference was attended by four big names in the field of exoplanet research: John Grunsfeld, associate administrator for NASA's Science Mission Directorate in Washington, D.C.; Jon Jenkins, Kepler data analysis lead at NASA's Ames Research Center in Moffett Field, California; Jeff Coughlin, Kepler research scientist at the SETI Institute in Mountain View, California; and Didier Queloz of Cambridge University.

The announcement included a new release of the Kepler Candidate Catalogue (Catalogue 7) which includes discoveries from all 17 quarters of Kepler data. Catalogue 7 is exciting because in the time since the last catalogue releases there have been tremendous updates to the analysis software that allows for the detection of smaller and longer period planets. Catalogue 7 applies this new software to not only the newly detected Kepler Objects of Interest (KOIs), but to all previously detected KOIs as well. KOIs are objects that produce planet-like transit signals, but have not yet been vetted as planet candidates. Many of the smallest candidates in Catalogue 7 may have been in the Kepler data all along, but were only detectable using the newer software. Catalogue 7 is the first fully automated and uniform assessment of the entire Kepler dataset.

Catalogue 7 adds 521 new planet candidates, bumping the total number of candidates up to 4,696. A good chunk of the new candidates are at long orbital periods, detectable only now that we are using the full Kepler baseline of about four years. Many of the new planet candidates are the Earth(ish)-sized or smaller planets that the new software can detect. Planets that are around 40 per cent larger than Earth or smaller are thought to be mostly made of rock. Planets slightly larger than that are potentially rocky, but could also be gaseous worlds like Neptune.

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The distribution of known exoplanet candidates sorted by their orbital period and size. Previously known candidates are in blue and the newly discovered candidates from Catalogue 7 are in yellow. Solar System planets are shown for reference. Image credit: NASA Ames/W. Stenze

Among the new planet candidates are 12 new planets that are less than twice the size of Earth and are also in the habitable zone (HZ) of their host stars—the region around a star that allows liquid water to exist on a planet's surface. 11 of these are still planet candidates whilst one of them is a confirmed planet. This confirmed planet is named Kepler-452b, and it resides right at the inner edge of its HZ. Kepler-452b brings the total number of confirmed exoplanets discovered by Kepler to 1,030.

Kepler-452b is 60 per cent larger than Earth, orbits its star in 385 days, and is 1,400 lightyears away from Earth. Since Kepler-452b was discovered via the transit method no mass measurement was possible. Measuring the mass of the planet with the radial velocity method is also out of the question with the technology we currently have since the planetary signal it would produce is too small. So, they used a mass-radius relationship derived from other planets to calculate an estimated mass of about five times the mass of Earth, though the mass could be as low as three or as high as seven times Earth's mass.

The star Kepler-452 is also extremely similar to our own Sun. The current size of the star is ten per cent larger than the Sun and it shines 20 per cent brighter. What is exciting is that the star is six billion years old, just 1.5 billion years older than the Sun. Since stars grow larger and brighter as they age, Kepler-452 could represent what our Sun will look like in 1.5 billion years. Even though the planet orbits ten per cent farther from its star than we are from the Sun it receives ten per cent more stellar energy than the Earth does. This places it at the inner edge of the HZ, near the runaway greenhouse/maximum greenhouse limit.

So, what do all these numbers and comparisons mean for the "Earthiness" of Kepler-452b? Well, there's first the issue of whether the planet has a rocky surface like Earth or is a small gaseous world like Neptune. The current determination of a planet's likely composition comes from the same mass-radius relationship used to get the planet's mass. The mass and the size of a planet determines its bulk density, which is then compared to the densities of common planet building material—rocks, gas, and metal. In general, planets 40 per cent larger than Earth or smaller are likely mostly rocky.

Just above that, where Kepler-452b falls, things get a bit muddled. The planet could be rocky, or it could be gaseous, or it could be a combination of both. Jon Jenkins calculates that there is about a 50 per cent chance that the planet has a rocky surface. If it does, then it is indeed very Earth-like. We won't have a definitive measurement of the mass—and thus, the composition—until the next generation of telescopes can target this planet.

Then there is the fact that Kepler-452b lies at the inner edge of the HZ. This could mean that any water that was on the planetary surface has since evaporated into the atmosphere. On Venus, this process led to the eventual loss of all water on the planet. Once the water vapour is in the atmosphere, the water molecules are broken down by ultraviolet (UV) radiation into their component hydrogen and oxygen atoms. Hydrogen, being the lightest, can then achieve escape velocity from the planet and be lost to space, leaving the planet completely devoid of water in a relatively short time.

This very process could be accelerated by the more intense UV light from Kepler-452, meaning that the planet has lost its water and is no longer habitable. However, there is the additional fact that Kepler-452b is nearly five times more massive than Earth. That would mean that the escape velocity for the hydrogen atoms is much faster, and much harder to achieve. The planet could have managed to retain some of its original water during its six billion year life. How can we tell which scenario it is? Well...we can't right now, but the next generation of telescopes, including the James Webb Space Telescope, could find out more information about the planet's atmosphere.

This simulation shows how energy received by both Earth and Kepler-452b changes over time (blue line). The bright green band is the conservative HZ, where surface temperatures are definitely right for liquid water. The dark green band is the optimistic HZ, showing where liquid water could exist given the right atmosphere. The brightness and size of both stars are shown in the inset. Simulation credit: NASA Ames/J. Jenkins

One of the most common questions asked in the Q&A portion of the teleconference was regarding the possibility of life on Kepler-452b, what it would resemble, and whether we could live there (given massive upgrades in spaceflight). Well, to be clear, there have still been no detections of life anywhere else in the Universe outside of the Earth. But there is certainly the possibility that if the planet has liquid water on the surface then life could have evolved on Kepler-452b.

Given that the planet has already spent six billion years in the HZ (with 500 million years to go), if life did develop it could have thrived if it had the right atmosphere. One criticism of previous "Earth-like" planets is that they are in orbit around stars cooler than the Sun that output light more at infrared wavelengths. Plant life that develops in that type of light would be very different than Earth plant life. On Kepler-452b, however, the starlight is almost exactly the same as the Sun, so any plant life that developed there would be very similar to the plant life on Earth. Life that ate that plant life might be similar to the animal life on Earth. We already have one biological experiment with these conditions that produced life, so why not another?

All this being said, is Kepler-452b the best "Earth 2.0" ever discovered? The discovery team believes that this system is the closest Earth/Sun analog ever found. While smaller planets have been discovered with RV detections, most of them are around stars cooler than the Sun and we lack information about their size. But, we should keep in mind that there are still many unknowns with Kepler-452b that are essential to knowing whether the planet is truly Earth-like and habitable.

In the 20 years since the discovery of 51 Pegasi b we have learned that nearly every star in the galaxy has at least one planet. We have learned that between 15 and 20 percent of Sun-like stars should have an Earth-like planet in the HZ, and we improve that number every year. We have found that the exoplanet population is much more diverse than what we see in our Solar System. The Kepler mission has played a very large part in shaping what we know about the larger population of exoplanets and yesterday's announcement has brought us closer to finally learning if our little planet has a sibling out there.

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