Kepler study doubles number of known alien worlds

Tuesday, May 10, 2016, 7:11 PM - The list of known alien worlds has now grown by more than 1,200 members thanks to a new analysis technique, more than doubling the total number of confirmed exoplanets we know of in our galaxy.

When the Kepler Space Telescope launched in 2009, it was put out into space with a mission to stare at one specific collection of stars in the sky, and watch closely for any planets around those stars that may wander into view.

After years of observations and followup confirmations, the mission had racked up a total of 4,696 candidate worlds (potential planets that still needed secondary confirmation), and 1,041 planets that had been verified by other observations and studies, and thus labelled as "confirmed" by the mission scientists. Among those 1,041 confirmed exoplanets, only a dozen had been verified to exist in their star's habitable zone - the ring-shaped region around the star that receives just enough radiation from the star so that any planet in the region would be warm enough to have liquid water on its surface.

Now, as of May 10, 2016, scientists at Princeton University and NASA have made a major announcement - their new statistical method of analyzing the Kepler data has now confirmed a total of 1,284 of those candidate worlds. This adds the greatest number of confirmed exoplanets to the list to date, and more than doubles the total number of known, confirmed exoplanets. In addition, it also adds another 9 "habitable zone" planets, bringing the total to 21 alien worlds that are likely our best candidates for finding life.

Number of candidate, confirmed and confirmed habitable zone exoplanets, pre and post May 10th announcement. Credit: NASA/S.Sutherland

The "imposters" led the way

As Kepler stared out into space, it was looking for a very specific signature to find a planet. It's extremely sensitive light-collecting instrument (a photometer) recorded the brightness of all the stars in its field of view, and whenever the light from any particular star dimmed, it represented a transit - when a planet around the star passed in front of it.

We witnessed this exact process on Monday, with the transit of Mercury in our own solar system. As Mercury passed in front of the Sun, it covered up a small portion of the Sun's surface from our perspective. If we recorded the exact brightness of the Sun before, during and after, the transit of Mercury would have caused a slight dip in the overall brightness.

There's one thing, though. Transits aren't the only thing that can cause a star to dim.

Another star can orbit it, forming an eclipsing binary that dims the total light of both stars at times, and even activity on the star itself - star-spots, for example - can mimic a transit signal. The scientists involved with the mission are very aware of this, which is why so many of the planets have remained as candidates. They know that some portion of their database will end up being false positives, and a study performed just last year even said that up to half of the exceptionally massive candidates in the Kepler database may be these false positives.

With this latest surge in confirmed exoplanets, however, it was the false positives that were the key.

Princeton University exoplanet researcher Timothy Morton, the lead author of the study, developed a statistical program, called Vespa. Running the entire Kepler candidate list through Vespa, analyzing each transit signal, comparing the candidate's properties to simulated star systems and known false positives, the program assigned each a likelihood that it wasn't real. If the signal didn't look like what was expected of a planetary transit, or it didn't fit in with the type of star it supposedly orbited, or it was too similar to known false positives, it didn't made the cut.

According to NASA, out of the over 4,000 candidates in the database, Vespa flagged 707 that were very likely false positives, another 1,327 that were more likely than not to be planets, and it even provided independent confirmation for 984 entries that had already been confirmed by other methods.

The remaining 1,284 candidates - which had not yet been verified by any other methods or observations - each had less than a 1 per cent chance of being a false positive. Thus, with such a high probability (greater than 99 per cent) of being real, these entries could be safely added to the list of confirmed exoplanets.

"Vespa is a culmination of a change in attitude about how we deal with these large-data surveys," Morton said in a Princeton press release on Tuesday. "This new problem Kepler created is that we now have thousands of new planet candidates. Astronomers knew we couldn't follow up all of these in the traditional way, but there was nothing to replace it. This result now puts a number on exactly how likely it is that each detected object is a planet."

"They say not to count our chickens before they're hatched, but that's exactly what these results allow us to do based on probabilities that each egg (candidate) will hatch into a chick (bona fide planet)," Natalie Batalha, co-author of the paper and the Kepler mission scientist at NASA's Ames Research Center in Moffett Field, California. "This work will help Kepler reach its full potential by yielding a deeper understanding of the number of stars that harbor potentially habitable, Earth-size planets - a number that's needed to design future missions to search for habitable environments and living worlds."

Of the nine new small habitable zone worlds that have been added to the confirmed list, there are a few that stand out as particularly interesting.

The known small confirmed exoplanets from Kepler, plotted on a graph of the habitable zone of stars at different temperatures. Credits: NASA Ames/N. Batalha and W. Stenzel

The above graph shows the 21 small confirmed exoplanets that are figured to be in their star's habitable zone - the green band, with the light portion being the conservative zone and the dark portion expanding it a bit more optimistically. Earth, Venus and Mars are featured, with their relative position in our Sun's habitable zone. The blue planets are those previously confirmed, while the orange planets represent the newly confirmed worlds.

Kepler-1410b and Kepler-1544b each receive roughly the same amount of energy from their star that Earth receives from the Sun. Their large size, and thus heavier gravity, makes it more likely that they would have a thick atmosphere, however, and thus they have a better chance of being hot worlds that would be inhospitable (or at least uncomfortable) to life as we know it.

Kepler-705b, Kepler-1593b and Kepler-1229b are interesting, though.

Kepler-1593b and Kepler-705b are almost twice as large as Earth, which implies that they are quite heavy. This could mean that they are water worlds, rather than rocky ones like Earth, however they could have the right conditions to support life of some kind.

Kepler-1229b, though, is only 12 per cent larger than Earth, which is well within the range for it to be a rocky terrestrial planet. It's close to the relative position of Mars in the habitable zone, but we have evidence that Mars was likely habitable in the past, and if its atmosphere hadn't been systematically stripped away by the solar wind, it might still be habitable (or at least more habitable) today.

With Kepler-1229b being over twice as big as Mars, it has a much better chance of holding on to a thicker atmosphere than our neighbouring world. If the planet were also to have a magnetic field to protect it from stellar flares, and to prevent its atmosphere from suffering the same fate as Mars', we could be looking at one of the better candidates for life in the Kepler database.

Sources: NASA | Princeton University