The discovery that planets can form around a variety of stars — and not just specific types, as previously thought — might open the floodgates on the search for habitable worlds in the galaxy.
The hunt for life elsewhere in the galaxy starts with the search for habitable planets — which scientists may have just made a little easier. Astronomers have found that relatively small planets can form around a wide variety of stellar types, observations that imply there may be more stars hosting habitable worlds than we ever imagined.Planets are born in disks of gas and dust that surround their parent stars, so both the planets and the star pull their raw material from the same source. For this reason, the earliest stars — those composed only of hydrogen and helium — probably didn’t host planets, as the heavier elements required to form planets didn’t yet exist. Instead, planets began to form as the star population matured and built up stores of carbon, silicon, and oxygen, elements now found in rocky planets.
Two decades of ground-based observations had suggested that metal-rich stars (“metal” is the astronomical term for elements other than hydrogen or helium) were more likely to harbor planets — particularly gas giants like Jupiter — than metal-poor ones. But today, a team led by Lars Buchhave (Niels Bohr Institute), working with results from the planet-hunting Kepler satellite, announced that smaller planets can form around stars with a wide range of metallicities, including those with as little as a quarter the heavy-element content of the Sun.
In a nutshell, this means that the stellar requirements for forming smallish planets are not particularly stringent. This implies two things: First, that planets may have started forming early in the evolution of the Milky Way, giving those worlds a long time to evolve life; and, second, that far more planets might be hiding out there than previously thought.The discovery comes at an opportune time. NASA’s Kepler mission, originally scheduled to end in November of this year, has been given a four-year extension.
Most of the 226 planet candidates in the new study orbit their stars closely. Such tight orbits are probably outside the “habitable zone” where liquid water, and therefore life, could potentially form on a planet’s surface. But with Kepler’s search extended, scientists will have the opportunity to examine planets with larger orbits and greater chances of having environments conducive to life.
“This really opens up the possibility for searches for habitable planets,” says Guillermo Torres (Harvard-Smithsonian Center for Astrophysics), a coauthor of the paper, which will appear in an upcoming issue of Nature. “We can now look at basically any star, and there’s a chance it will have a small planet … I am more optimistic about [the search for life] now, and I think many other people will be as well.”