Our knowledge of exoplanets has expanded exponentially over the last 20 years, thanks to sophisticated search techniques and telescopes like Kepler. One of the problems with our data, however, has been that it’s much easier to find large gas giants or huge rocky worlds than it is to find Earth-like planets. Our planetary search techniques rely on measuring variations in gravity or a star’s apparent magnitude as a planet transits across it, and smaller planets are much harder to find with these methods. Today, however, scientists have announced the location of a relatively nearby planet that may be the best candidate we’ve ever found for supporting life.
The planet, Ross 128b, orbits a red dwarf star. These stars output a fraction of the luminescence as our sun, have a much lower surface temperature, and often flare in ways that could sterilize any life on the surface of their planets. The flare up problem is why the closest Earth-size planet at Proxima Centauri is considered a relatively poor candidate for life. Ross 128b, on the other hand, has several factors that could recommend it as a target of further investigation.
This artist’s impression shows the temperate planet Ross 128 b, with its red dwarf parent star in the background. This planet, which lies only 11 light-years from Earth, was found by a team using ESO’s unique planet-hunting HARPS instrument. Ross 128 b will be a prime target for ESO’s Extremely Large Telescope, which will be able to search for biomarkers in the planet’s atmosphere. Image credit: ESO/M. Kornmesser
First, the star it orbits is relatively quiet. While it still qualifies as a flare star, it’s much more quiescent than Proxima Centauri. Second, while Ross 128b orbits its star every 10 days and at 0.049 AU, the star itself is 280 times less luminous then the sun. Much depends on how high the planetary albedo is — the equilibrium temperature of the planet could range from -60C to 20.85C depending on what’s on the surface. (Earth’s average surface temperature is ~15C, to put the comparison in perspective.)
Third, the planet may sit within the habitable zone of its host star. There’s always some play in these numbers — since we haven’t found a guaranteed-habitable planet yet, we can’t say for certain if the habitable zone is properly calibrated. Depending on how you run the numbers, Ross 128b could be outside, inside, or at the inner edge of its habitable zone. The image below shows the habitable zone calculations for an A-type star (a star much more luminous than Sol), a sun-like star, and an M-class star (red dwarf). The habitable zones are always approximates, but the image makes the point: Venus sits slightly too close to the sun to be habitable, while Mars is believed to be slightly too far away. But a planet either too close or too far from its star could be habitable, if it possessed other characteristics that tilted the scale in the opposite direction.
Two things to keep in mind: We don’t know yet if Ross 128b has an atmosphere or, if it has one, whether that atmosphere has the proper composition to support life as we know it. But Ross 128b’s relatively rare flare ups make it a tantalizing possibility. Frequent solar flares can blast the atmosphere right off a planet, as can ordinary solar wind. Flares from Sol may have blasted the atmosphere off Mars, and solar wind continues to abrade it to this day. But at 10.89 light years away, this may be the best candidate we’ve yet found for viable life — and it’s parked in our own backyard.
Feature image by Y. Beletsky (LCO)/ESO