Is Anybody Out There?
Astronomer Phil Muirhead on the search for extraterrestrial life
Our galaxy, the Milky Way, is a vast expanse of stars—hundreds of billions of them—and, as far as we know, we are all alone in it. But studies of our neighboring planets have raised hopes for finding extraterrestrial life. And with more powerful telescopes and new techniques for measuring the light from distant stars, our search is also expanding to an ever-growing list of recently discovered planets—known as exoplanets—far beyond our solar system. Phil Muirhead, an associate professor of astronomy who studies dwarf stars and has detected a number of exoplanets, spoke with arts&sciences to update us on the hunt for life.
arts&sciences: How close are we to finding evidence of extraterrestrial life?
Muirhead: We’ve already found some evidence that there could be life on other planets in the solar system. There was the detection of phosphine in the atmosphere of Venus. Another discovery was methane leaking out of the soil on Mars. One potential explanation is these gases were generated by living things. But there are also abiotic explanations. Methane, for instance, is a relatively simple molecule and you could conceivably get it from naturally occurring processes given the right pressure and temperature.
You study planets orbiting smaller stars in our galaxy. How do you determine if a planet might contain life?
If we are lucky, a planet will pass in between our line of sight and its host star. Some of that starlight will pass through the atmosphere of the planet on its way to us. Some wavelengths will be absorbed and some transmitted depending on the constituent gases of the atmosphere. One thing that would be very exciting would be finding oxygen and methane. Those gases are unstable together; they react very quickly to create by-products like carbon dioxide and water, so if you see them on a planet, it means something is actively and continuously generating both those molecules.
If, in order to get life, you need something like Earth down to the letter, then life would likely be very uncommon. But if all you really need is an Earth-mass planet with an Earth-like radius and a similar distance to its star, then life could be pretty common.
There are billions of stars in our galaxy. How common do you think life is in these other solar systems?
Twenty years ago, we didn’t know the prevalence of planets with a similar mass as Earth that are a similar distance from their stars. Now we know they are common. There is serious debate about what other characteristics of Earth may be necessary for life to exist. For example, our moon largely stabilizes our rotation. This stable rotation creates a more stable climate, which in turn helps life to thrive. We also have water, plate tectonics, and a magnetosphere. If, in order to get life, you need something like Earth down to the letter, then life would likely be very uncommon. But if all you really need is an Earth-mass planet with an Earth-like radius and a similar distance to its star, then life could be pretty common.
Why haven’t we been contacted by aliens yet?
I think the likelihood of receiving a signal transmitted from an extraterrestrial intelligence is low because our civilization has only been able to send those signals for just under a century. And that’s just a tiny, tiny fraction of the history of our planet.
What do you think drives us to search for life beyond Earth?
It’s always good to know who your neighbors are. And it’s always a little scary living somewhere and not knowing anyone around you.