Do Earth-sized “rogue planets” host alien life?

The universe often favors those that play by the rules. Earth and its neighboring planets orbit around the Sun in an orderly fashion, as do other worlds in our galaxy and beyond.

But amidst all this order upheld in the universe, there are also some cosmic rebels.

In a study published Tuesday in the Monthly Notices of the Royal Astronomical Society, astronomers used archival data from NASA’s Kepler space telescope to discover a group of Earth-size rogue planets orbiting untethered to a home star in deep space.

These rebellious planets may have been violently ejected from their star system, but they could still host life beneath their icy cold surfaces. Upcoming space missions could look at them and figure out what they’re like.

WHAT’S NEW — The team of researchers behind the discovery looked at Kepler data collected from late April to early July 2016. The Kepler space telescope was launched to find Earth-like planets orbiting different stars.

Kepler spent nine years scouring the cosmos, searching for subtle dips in the brightness of a star as an orbiting planet transits in front of it.

But this particular team was looking for planets that didn’t orbit stars at all. Out of 27 signals of planets detected in their data set, there were four detected from planets without stars of their own.

“Our observations pointed an elderly, ailing telescope with blurred vision at one of the most densely crowded parts of the sky, where there are already thousands of bright stars that vary in brightness, and thousands of asteroids that skim across our field,” Iain McDonald, an astrophysics professor at the University of Manchester and lead author of the new study, said in a statement.

“From that cacophony, we try to extract tiny, characteristic brightenings caused by planets, and we only have one chance to see a signal before it's gone. It's about as easy as looking for the single blink of a firefly in the middle of a motorway, using only a handheld phone,” he added.

Based on the Kepler observations, the rogue planets are likely to be around the size of Earth and could be rocky.

To detect a planet, Kepler scientists look for the dip in the light of a star as its orbiting planet transits across the surface of the star from the vantage point of a telescope. However, since these planets don’t have stars, the scientists behind the discovery had to get creative.

How they did it — The team used the microlensing technique, which uses light from a background object as a giant cosmic magnifying glass to look at distant objects that are otherwise hard to view.

This technique is important in detecting objects like rogue planets and brown dwarfs, which emit no light of their own.

Scientists have long predicted the existence of free-floating planets, but it wasn’t until 2011 when they found evidence of up to 10 free-floating planets roughly the mass of Jupiter.

Here’s the background — These lone wolves must have started around a host star and were then kicked off during the early, chaotic years of a young star system.

David Bennett, associate professor of physics at the University of Notre Dame, says it may be a somewhat common part of the planetary system formation process.

“There is a stage of planet formation when planetary systems can get unstable,” Bennett tells Inverse. “Planets can get ejected, or they can be captured by another star.”

Without a star to orbit, these planets go about their business on their own, orbiting around the center of a galaxy much as stars do.

“They tell us a bit about sort of the final stages of planet formation where you have this planet scattering,” Bennett says.

During these final stages, things can get a little unstable.

“Planets form in these circumstellar disks, and the gas and the disks counteract some of the forces that would drive instability,” Bennett says. “There's got to be a stage when that gas is dissipated by the radiation from a star; that’s when you would have this sort of planetary instability phase.”

Before they were ejected from their star system, these planets were likely much like Earth and could have possibly had oceans of water or atmospheres capable of sustaining life.

Can free-floating planets support life?

Once they are ejected from their star system, these planets could still possibly sustain their oceans even if their respective suns never shine.

A study published in February 2011 found that an ocean on a planet could stay liquid without the heat of its star. Life on Earth has also originated in deep ocean vents, away from sunlight.

“Those ocean vents are actually getting their energy from the radioactive decay inside the Earth and not from the Sun,” Bennett says.

The Solar System also provides plenty of examples of icy moons such as Saturn’s Enceladus, or Jupiter’s Ganymede that may harbor oceans beneath their icy surface even though these distant moons in the outer solar system don’t see the light of day.

Whatever form of life that may develop on these free-floaters would be entirely nocturnal since they lack stars to provide light and heat.

But Bennett argues that while some of these planets may be considered habitable, they would have 10,000 times less energy than on Earth; therefore, they would be a somewhat unlikely place for complex life to flourish.

WHAT’S NEXT — The Kepler space telescope was retired by NASA in October 2018.

Although Kepler was not designed to look for these rogue planets, other upcoming missions to the cosmos may be capable of finding more of them.

NASA’s Nancy Grace Roman Space Telescope, which will launch in 2025, will use the microlensing technique to find distant, dim objects such as planets with no stars. The European Space Agency is also launching a mission to detect dark matter and dark energy, which have been hard to find thus far, which would make it a good candidate to look for these dark objects.

“Simultaneous observations from those two telescopes would also be a way to measure masses from some of these free-floating planets,” Bennett says.

About a dozen free-floaters have been detected thus far, but scientists suspect there are many more of these rebels going against the cosmic order.

Abstract: We present the first short-duration candidate microlensing events from the Kepler K2 mission. From late April to early July 2016, Campaign 9 of K2 obtained high temporal cadence observations over a 3.7 deg² region of the Galactic bulge. Its primary objectives were to look for evidence of a free-floating planet (FFP) population using microlensing, and demonstrate the feasibility of space-based planetary microlensing surveys. Though Kepler K2 is far from optimal for microlensing, the recently developed MCPM photometric pipeline enables us to identify and model microlensing events. We describe our blind event-selection pipeline in detail and use it to recover 22 short-duration events with effective time-scales t_eff < 10 d previously announced by the OGLE and KMTNet ground-based surveys. We also announce five new candidate events. One of these is a caustic-crossing binary event, modelled in a companion study. The other four have very short durations (t_eff < 0.1 d) typical of an Earth-mass FFP population. Whilst Kepler was not designed for crowded-field photometry, the K2C9 data set clearly demonstrates the feasibility of conducting blind space-based microlensing surveys towards the Galactic bulge