Not so terribly long ago, the only planets we knew about for certain orbited our Sun. Within the last 20 years, though, astronomers have identified more than a thousand exoplanets: worlds orbiting other stars. And when you consider that all of those are within our galactic neighborhood, it’s pretty clear our galaxy alone is host to billions of planets.
The news of an exoplanet discovery is hardly earthshaking anymore, which is disappointing in a way. Think of it: we’re finding tiny bodies in a distant solar system, so far away that the star is nothing but a single point of light and the planet itself is usually invisible. That’s amazing! If we’re bored by new exoplanet discoveries, it’s because we’ve become used to wealth and happiness, but take a moment and get excited again. I’ll wait.
However, there’s one major goal of exoplanet astronomy we still haven’t achieved: the discovery of another Earth.
But we’re getting closer: on Thursday, a group of researchers announced the discovery of a planet just a little larger than Earth, orbiting in the “habitable zone” of its star. Since liquid water is an essential ingredient for all life on Earth, the habitable zone is a range within a star system where the amount of light from the host star is enough to keep water liquid on a planet’s surface, but not so much that the water would boil away. The size of the zone depends on the temperature of the star: medium-sized yellow stars like the Sun have larger zones, while small red stars have much smaller ones.
Most exoplanets are found by spotting the tiny reduction in the light of its host star, when the planet passes directly in front. The technical term for this is “transiting”, but it’s the same principle as an eclipse of the Sun: the Moon passes between the Sun and Earth, blocking much of the Sun’s light. The difference is that a distant exoplanet only blocks some of its star’s light, and the reduction lets astronomers measure the size of the planet. The size of the planet in turn tells us whether it’s rocky in composition, like Earth, or gaseous like the giant planets (Jupiter, Neptune, etc.).
The planet was found via transit, and it is officially known as Kepler-186f. The name is because it was identified using the Kepler orbiting observatory, while the “f” means it is the fifth planet out from the star. (The star itself is labeled “a,” its first planet “b,” and so forth.) Kepler-186f is about 11 percent larger than Earth in diameter, which means it has nearly 25% more surface area. Kepler-186f is more like our bigger sister than our twin.
Small red stars vastly outnumber their larger cousins, and the new exoplanet is orbiting one of those. That already means Kepler-186f isn’t quite Earth-like: its orbit is smaller than Mercury’s in the Solar System. However, because the star is less than half the diameter of the Sun, it emits a lot less light, meaning the planet only gets around one-third of the light Earth gets. However, it’s enough warmth to place it in the star’s habitable zone.
So does Kepler-186f have liquid water? Could it harbor life?
If the exoplanet has a fairly Earth-like atmosphere to help keep the surface warm (but not too warm), and if it has surface water, then at least some of that water should be liquid.
Those are much trickier questions. Consider the most Earth-like planet in the Solar System (other than Earth itself). Venus orbits the Sun within the habitable zone, and is only slightly smaller than Earth. Yet Venus’ atmosphere is much thicker, composed mostly of the greenhouse gas carbon dioxide, and 100% cloud cover all the time. Combined, those give the planet a year-round surface temperature hot enough to melt lead. Venus is most un-Earth-like, in terms of habitability.
Mars, on the other hand, is significantly smaller with a much thinner carbon dioxide atmosphere, but conditions in the past may have allowed it to have at least some surface water. That’s why we’ve paid a lot more attention to the red planet in the hunt for life, even though it’s much less Earth-like than Venus in many important ways. So far, Mars seems to be lifeless, though researchers haven’t given up hope yet. To make things even more complicated, the best hope for life in the Solar System may be the moons Europa and Enceladus, which harbor liquid oceans beneath their surfaces of ice. These orbit Jupiter and Saturn (respectively), so they lie far outside the habitable zone; their internal heat comes from gravitational energy rather than the Sun.
So, for us to know if Kepler-186f is habitable or not, we have to consider several “ifs”. If the exoplanet has a fairly Earth-like atmosphere to help keep the surface warm (but not too warm), and if it has surface water, then at least some of that water should be liquid. Unfortunately, it’s hard to measure the chemical composition of exoplanet atmospheres, so we don’t know what kind of air (if any) Kepler-186f has.
However, that’s not to say this isn’t an exciting discovery. Science works mostly in small steps, not huge leaps. With every new exoplanet discovery like Kepler-186f, we come closer to finding Earth’s twin—and maybe hints of life elsewhere in the cosmos.