Our arm of the Milky Way is filled with older, metal-rich stars. New research suggests these might provide the best conditions for life to form on their planets.
All the life we know of in the entire universe is squeezed onto a tiny rock, floating in a minor arm of the Milky Way. There are billions of other planets with the potential to support life. But what effect does our location have on the prospect of finding it?
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So far, our search for life elsewhere has only scratched the surface. “The bubble of space that we’ve managed to search around our sun is very small compared to the size of the galaxy,” says Jessie Christiansen, an astrophysicist at the California Institute of Technology. Yet we have already found more than 5000 planets circling other stars, called exoplanets. While some of these have been detected across our galaxy – and even in other galaxies – most reside within a few hundred light years of our sun, a stone’s throw in the cosmic scheme of things.
Our galactic neighbourhood
Astronomers have begun looking at different types of stars in our galactic neighbourhood to see how they might affect the chances of habitability on the planets around them. We live in an arm of the Milky Way called Orion, which sits inside the main plane of the galaxy, called the thin disc. We are surrounded by stars within the Orion arm. Further out, on one side we have the overcrowded bulge of the galaxy’s dense core, while on the other we are enveloped by the sparser outer parts of the galaxy’s other arms.
Thin disc stars, like our sun and others in the Orion arm, are generally rich in metals – which means anything heavier than helium – says Jesper Nielsen, an astronomer at the University of Copenhagen, Denmark. “They have more iron, magnesium and so on.”
Stars that reside above and below the plane of our galaxy are said to be in the “thick disc”, while those at the outermost edge of the galaxy are said to be in its halo. These stars are typically more metal-poor as they were some of the first to form in the galaxy, meaning there hadn’t been enough supernovae, the explosions of stars that create heavier elements, to enrich the galaxy yet.
This is good news for the search for life in our vicinity. Nielsen and his colleagues have found it is much harder to form planets around older, metal-poor stars “simply because there’s not enough material around”, he says. On the other hand, metal-rich younger stars – like those that surround us – have such an abundance of material that they are able to form a wider diversity of planets, including rocky worlds like Earth and larger planets.
Communicating with life
Our place in space may not be a barrier to finding life, but it might make it a lot tougher to communicate with it. The first problem we encounter is the distances involved. It is 4.2 light years to our closest star, Proxima Centauri. If there was intelligent life on one of its planets, sending a signal and receiving a reply would take the best part of a decade. It only gets harder for more distant stars.
For now, the search is focused on Earth look-alikes. A telescope being developed by NASA called the Habitable Worlds Observatory (HWO) is set to launch in the 2040s and has a primary goal to image 25 Earth-like worlds around sun-like stars. “I believe, in our lifetime, something like HWO will see a signal in the atmosphere of a rocky planet in the habitable zone of a star like our sun that we think is life,” says Christiansen.
Just as Edwin Hubble’s recognition of Andromeda as a separate galaxy in 1924 shifted our perspective of the universe, so too could the knowledge that we aren’t the only life in the galaxy. “It might start a revolution in life, religion, philosophy and science,” says Christiansen. Or not: “It might be a headline for a day and then everybody just goes back to what they were doing.”