About 14 light-years away from Earth, a small red dwarf star only a quarter the size of the Sun sheds a gloomy, crimson radiance on its little retinue of worlds—three planets, ranging from a smidge larger than Earth to over five times its size.
So far, so typical. But here’s where things get interesting: a nearer look discloses a few surprises about this little star and its miniature system. About a billion years younger than our Sun, the dwarf star is wracked by immense magnetic sunspots that blacken its ruddy face; from time to time it sends out arcing prominences and flares that scorch its closest child—a planet like Venus’ bigger, nastier brother, with a hellish atmosphere that precipitates liquid metals, and a surface convulsed by tidally-induced volcanism. Nothing lives in this nightmarish place.
Further out, a giant blue world—five times the size of Earth—plies a chilly orbit in the weird, dim glow of its scarlet primary; it’s like a mini-Neptune, its atmosphere of cold hydrocarbon smog concealing an immense world-ocean of hyper-pressurized H2O and exotic ices. Any life here would be rudimentary—at best.
But between these two extremes swings a little planet only slightly larger than Earth; its atmosphere is dense but clear, its surface largely drowned in a rust-red, shallow ocean constellated with island chains and microcontinents. A closer examination shows that these tiny landmasses, and the shallow coastal seas surrounding them, are dotted with curious “forests” of black, plant-like things swaying gently in the wind; not plants at all, they’re really a kind of commensal pseudo-bacteria that grow in great colonies like the stromatolites of early Earth. Their broad, black leaf-like surfaces absorb and metabolize their sun’s plentiful infrared light, while their metabolic wastes filter down to be consumed by the non-phototrophic organisms at the base of the colony.
These biomes have lived for only a few days. In a few more, the planet’s nearness to its sun will be unbearable; all the colonies will die off, leaving behind dormant spores to grow again in the brief fall. A glacial period of bitter cold and spreading ice will follow, gripping the planet for a few days and unleashing a ferocious “ice age” ere the return of spring. This is just one year in the life of the planet—a year spanning a mere 17 terrestrial days.
Meanwhile, this swift drama of life and death and changing seasons accelerates the pace of evolution beyond anything experienced on Earth; the planet has been habitable for less than half a billion years. Give it another half billion…who knows what might evolve?
The planet is called Wolf 1061c, and it belongs to one of the nearest planetary systems to the Earth. The above scenario is a fantasy—but it’s an informed and plausible fantasy, founded on the knowledge we have that Wolf 1061c orbits within the habitable zone (HZ) of its red dwarf sun. And as our technology becomes more sophisticated, and our understanding of exoplanets grows, we may someday learn whether such scenarios are really possible.
Our knowledge of the Wolf 1061 system, and its habitable characteristics, comes courtesy of astronomers like Stephen Kane, of San Francisco State University, who is working to understand the complex interplay of orbital dynamics, stellar life-cycles, and exoplanetology that determines an alien star’s HZ.
Deciphering the parameters of extrasolar HZs is proving to be more difficult than expected; there’s a lot to juggle, and, with only one example before us, we’re not exactly in a position to speak authoritatively on the subject. One important point to keep in mind is that a star’s HZ changes in time as well as space; as a star ages, and grows hotter, its HZ sweeps further outward, desolating formerly habitable worlds and making a paradise of formerly uninhabitable ones.
Three billion years ago, Venus was a pleasant ocean world; observe it today—it’s about as inviting as the interior of a self-cleaning oven. In two billion years or perhaps even less, Earth will recapitulate its sister’s horrible fate; meanwhile, a planet that was never in the HZ, like Mars, had great oceans and a moist atmosphere billions of years ago, for intrinsic reasons entirely unrelated to solar irradiance.
So there are many variables that can skew any assessment of a distant star’s potential to host habitable planets: the mass, composition, and geology of a planet; the size, temperature, and age of a star; the presence of any companion suns; the eccentricity of a planet’s orbit; proximity to the galactic core—and others too numerous to list.
About the best astronomers can do is try to gather as much information as they can through their imperfect telescopes, and extrapolate from there about a system’s life-sustaining potential. For instance, Kane’s team at SFSU—together with collaborators from Tennessee State University and Geneva, Switzerland—was able to determine that Wolf 1061c orbits a little too close to the inner edge of the star’s HZ, but that its swift orbital changes could keep the planet’s conditions from reaching Venus-like extremes.
“The Wolf 1061 system is important because it is so close and that gives other opportunities to do follow-up studies to see if it does indeed have life.” —Stephen Kane, SFSU
That’s a start, and we’ll do better in the future. The James Webb Space Telescope, Hubble’s successor, is slated to launch next year; it will have the requisite resolving power to disentangle the atmospheric composition of distant worlds like Wolf 1061c, and possibly even detect the traces of biogenic chemistry.
And there are other future space telescope architectures on the drawing board—missions like TESS (Transiting Exoplanet Survey Satellite) and the projected ATLAST space telescope. These, we hope, will pinpoint transits, image planets directly, and study atmospheric spectra with greater precision—all of which will help us calibrate our understanding of habitable conditions around alien stars.
In the meantime, we’ll continue to scan the skies and explore the planets of alien solar systems with our increasingly more powerful instruments. We’ll compile a list of stars that are the most likely to host life-bearing planets, and someday, perhaps, we’ll finally spot a distant world—whether Wolf 1061c or someplace yet undiscovered—that discloses the first thrilling biosignatures in its atmospheric spectrum.