An Earth-Sized World Orbiting in its Star’s Habitable Zone Was Found in Older Kepler Data

To date, astronomers have confirmed the existence of 4,144 extrasolar planets in 3,074 systems, with a further 5,094 candidates awaiting confirmation. The majority of these planets were found by the Kepler Space Telescope, which spent nine years (between May of 2009 and February of 2018) monitoring distant stars for transit signals – where a planet passing in front of a star causes a dip in brightness.

And yet, even though it is now defunct, the data that Kepler accumulated over the years continues to lead to new discoveries. For instance, a transatlantic team of researchers recently found a signal in Kepler‘s archival data that eluded detection before. This signal indicates that there is a second planet orbiting Kepler-1649, an M-type red dwarf star located 302 light-years away.

What is especially exciting about this find is that this planet (Kepler-1649c) is the most Earth-like planet ever found by Kepler in terms of size and temperature! As they state in their study, which recently appeared in The Astrophysical Journal, the team confirmed that the planet was 1.06 Earth masses and receives about 75% as much light as our planet does from the Sun (meaning it may have similar temperatures).

This planet orbits its star much closer than Earth does the Sun, which results in an orbital period of just 19.5 Earth days. This is to be expected with planets that orbit within the habitable zones of lower-mass red dwarf stars. And while no flares have been detecting in this system, red dwarfs are also known for their flare activity, which could make the planet’s environment challenging for potential life.

As Thomas Zurbuchen – the associate administrator of NASA’s Science Mission Directorate – said in a recent NASA press release:

“This intriguing, distant world gives us even greater hope that a second Earth lies among the stars, waiting to be found. The data gathered by missions like Kepler and our Transiting Exoplanet Survey Satellite (TESS) will continue to yield amazing discoveries as the science community refines its abilities to look for promising planets year after year.”

As noted, Kepler searched for exoplanets by monitoring stars for periodic dips in brightness, which can be the result of planets passing in front of them relative to the observer (aka. the Transit Method). However, these dips can be caused by a number of phenomena (ranging from natural changes to other objects passing in front of them) and are only the result of exoplanets an estimated 12% of the time.

Previously, Kepler mission scientists developed a special computer algorithm called Robovetter to search through the volumes of data collected by the telescope, which is managed by NASA’s Ames Research Center. This algorithm is tasked with distinguishing between signals produced by planetary transits with the various other phenomena known to cause dips in a star’s brightness.

Signatures that are determined to be from other sources by Robovetter are labeled “false positives,” which is what Kepler-1639c was originally thought to be. Luckily, scientists knew in advance that the algorithm’s results would need to be double-checked because of the sheer amount of volume it had to sort through – which drastically increases the likelihood of errors.

This task is the job of the Kepler False Positive Working Group, which reviews all the signals vetted by the algorithm to see if they could actually be transit signals. What they found was that Kelper-1649c had been mislabeled and that it was one of the most unique Earth-analog planets discovered to date.

Granted, there is a lot that remains unknown about Kepler-1649c, including some key parameters that will help astronomers determine just how likely it is to be habitable. These include its atmospheric composition, which has a significant effect on the planet’s ability to be warm enough for liquid water to exist. Due to the distance between Kepler-1649c and Earth, the current size estimates have a significant margin of error.

There are also questions related to the planet’s mass, which would indicate whether or not it is truly rocky or potentially a “water world” (where water makes up a significant portion of its mass). These questions, in addition to what astronomers already know about Kepler-1649c, make it an extremely worthy candidate for follow-up investigations.

While there are other exoplanets that are believed to be closer to Earth in terms of size and temperature, Kepler-1649c has the added benefit of orbiting within its star’s habitable zone. Said Dr. Andrew Vanderburg – a NASA Sagan Fellow at the University of Texas at Austin and the lead author on the study – said in a recent NASA press release:

“Out of all the mislabeled planets we’ve recovered, this one’s particularly exciting – not just because it’s in the habitable zone and Earth-size, but because of how it might interact with this neighboring planet. If we hadn’t looked over the algorithm’s work by hand, we would have missed it.”

Also noted already is the fact that this star system contains another confirmed exoplanet that is also similar in size to Earth – Kepler-1649b. This planet orbits the star at about half the distance as Kepler-1649c (and is therefore considered to be analogous to Venus) which leads astronomers to suspect that the star may have a third planet orbiting it.

With an orbital period of 8.7 days, Kepler-1649b completes four orbits of its star for every nine orbits completed by Kepler-1649c. This kind of stable orbital ratio is often indicative of an orbital resonance, which is what three of Jupiter’s largest moons (Io, Europa, and Ganymede) experience. Their orbital periods – 42 hours, 85 hours, and 172 hours days, respectively – conform to a nearly perfect ratio of 1:2:3.

In short, the orbital periods of the Kepler-1649 system could be indicative of a middle planet in the system (with an orbital period of about fourteen days) that helps to stabilize and synchronize their orbits. Or it could be that this latest discovery shows that there are systems in the Universe with 4:9 resonances, whereas most others are 1:2 or 2:3.

To date, no third planet has been found, which could be because it is simply too small or its orbit is too skewed for Kepler to have detected it using the Transit Method. In the end, this question provides astronomers with even more incentive to conduct follow-up studies of Kepler-1649. And of course, the system provides yet another example of how common rocky planets are around red dwarf stars.

“The more data we get, the more signs we see pointing to the notion that potentially habitable and Earth-size exoplanets are common around these kinds of stars,” said Vanderburg. “With red dwarfs almost everywhere around our galaxy, and these small, potentially habitable and rocky planets around them, the chance one of them isn’t too different than our Earth looks a bit brighter.”

Since red dwarf stars are the most common in the Universe (accounting for 75% of stars in our galaxy alone), it stands to reason that planets like this one could be more common than previously thought. As we find more Earth analogs orbiting nearby red dwarfs, the chances for determining if they could be habitable greatly increases.

This discovery also shows that even as scientists work towards automated the analytic process, oversight and double-checking are extremely valuable.

The research team also included members from The Open University, the NASA Ames Research Center, the NASA Goddard Space Flight Center, the SETI Institute, the Harvard-Smithsonian Center for Astrophysics (CfA), and the Space Telescope Science Institute (STSI).

Further Reading: NASA, The Astrophysical Journal Letters