Countless Stars Could Be Turning Gas Planets into Habitable Worlds

Neptune might look like a pretty blue marble, but if you traveled there, you'd discover it's a cold, hellish wasteland. And yet across the galaxy, it's possible that frozen gas giants like Neptune are being transformed into balmy, Earth-like worlds, by nothing less than the power of the stars.

That's according to new research from scientists at the University of Washington, who find that powerful tidal forces may be pulling "mini-Neptunes" into the habitable zones of their stars. Once there, stellar radiation can terraform the gas giants into smaller, rocky, and, yes, possibly life-bearing worlds.

The study, which appears in Astrobiology this week, offers yet more evidence to suggest our galaxy may be littered with Earth-like planets.

As the first month of 2015 draws to a close, astronomers have already rolled out a year's worth of exciting exoplanet news. Highlights include the discovery of the two most Earth-like exoplanets ever, Kepler's confirmation of a thousand more, and, for the early pioneers, a series of tantalizing exoplanet travel posters. This is all just the tip of the iceberg because in a few short years, bigger, better telescopes will enable us to uncover thousands more worlds.

Many of the planets we expect to discover in the near future will be mini-Neptunes, consisting of rock-ice cores surrounded by thick balls of hydrogen and helium gas. Sitting outside their stars' habitable zones, it's unlikely these planets will harbor life. But it's possible that some of them will evolve into Earth-like worlds over cosmic time.

"They are initially freezing cold, inhospitable worlds," lead study author Rodrigo Luger said in a press release. "But planets need not always remain in place. Alongside other processes, tidal forces can induce inward planet migration."

In his study, Luger used computer models to show how tidal forces, arising from the strong gravitational pull between a parent star and a planet, can, over time, drag a mini-Neptune into the not-too-hot, not-too-cold habitable zone. Once there, stronger UV and x-ray radiation can heat the planet's upper atmosphere, spawning powerful winds that erode away the thick gaseous shell. Sometimes, this process leaves behind a rocky, hydrogen-free world.

"Such a planet is likely to have abundant surface water, since its core is rich in water ice," said Luger. "Once in the habitable zone, this ice can melt and form oceans," which may lead to life.

Whether any of the potential Neptune-turned-Earths out there will actually end up being habitable depends on a host of factors, including how quickly the planet's hydrogen-helium atmosphere fizzles off into space. If the planet's gassy exterior escapes too quickly, a runaway greenhouse may result, causing the rocky leftovers to bake.

"The bottom line is that this process—the transformation of a mini-Neptune into an Earth-like world—could be a pathway to the formation of habitable worlds," Luger said.

The study adds to the mounting pile of evidence that our little blue planet isn't all that special, meaning that perhaps, life on Earth isn't so special, either. If Earth-like planets turn out to be cosmically common, this is one explanation I'm personally rooting for. There's something damn poetic about star power literally sculpting habitable worlds out of cold, dead balls of gas.