BBC Future

Space Station

ET: The chances of finding alien life

About the author

Richard is a science journalist and presenter of the Space Boffins podcast. He edits Space:UK magazine for the UK Space Agency, commentates on launches for the European Space Agency and is a science presenter for BBC radio. You can also follow him on Twitter or Facebook.

  • Mission statement
    Nasa’s Kepler mission was the US agency’s first craft designed to look for Earth-size and smaller planets orbiting other stars. (Copyright: Flickr/jonf)
  • Dimmer switch
    The telescope is able to spot planets by measuring the tiny dips in the brightness of a star when a planet crosses in front of it. (Copyright: SPL)
  • Stargazer
    It does this using a specially designed 0.95-meter diameter telescope called a photometer or light meter. (Copyright: SPL)
  • Star gazer
    It does this using a specially designed 0.95-meter diameter telescope called a photometer or light meter. (Copyright: SPL)
  • Lights, camera
    The stars are monitored by 42 charge-coupled devices (CCDs), similar to the chips used in digital cameras. (Copyright: Nasa)
  • Countdown
    The telescope, named after German astronomer Johannes Kepler, finally blasted off from Cape Canaveral on 6 March 2009 aboard a Delta II rocket. (Copyright: Nasa)
  • Sundown
    Since then, the telescope has made hundreds of discoveries including a world called 16b, where two suns set over the horizon instead of just one. (Copyright: Nasa)
  • Just right
    However, planet 22b has so far caused the most excitement as it is the first to be found in a star's habitable zone; the region where liquid water could exist. (Copyright: Nasa)
With new "Earth-like" planets being discovered every week, our space columnist Richard Hollingham asks whether it has increased our chances of finding intelligent alien life.

I’ve always imagined that on the wall at the Search for Extraterrestrial Intelligence (Seti) Institute in California there’s a sealed cabinet. Beside it, a hammer and big red letters reading: “In case of aliens, break glass”. Inside will be instructions about what to do next.

In fact Seti does have a special protocol listing the procedures to follow if ET calls. It essentially boils down to this: tell everyone, have meetings, and get nations to agree whether to send a message back. Seti has recently updated this protocol, which may be timely given the number of planetary discoveries being notched up by Nasa’s Kepler mission.

Launched in 2009, the Kepler space telescope is designed to detect planets orbiting stars, other than the Sun, that might support life. So far, it’s found 2,321 possible planets and confirmed 61. Of these, the one everyone has been getting excited about is Kepler 22-b – a rocky planet in the “habitable zone” of its solar system.

But this is likely to be only the first of many.

“We’re going to be flooded with habitable worlds over the next year or two,” says Seth Shostak. As Seti’s senior astronomer, Shostak will be the guy behind the Casio keyboard tapping out the D, E, C, C, G welcoming refrain when the big spaceship lands.

“Kepler has three years worth of data which means it can start discovering cousins of the Earth,” Shostak explains. “So in some sense, this is a predictable tsunami of habitable worlds and – from a big picture point of view – I try not to let my blood pressure rise just because there’s one more.”

Dwarf star

Nevertheless, results from Kepler – helped by a vast volunteer workforce of planet hunters –  has given Seti a new sense of purpose, helping to focus its attention on particular areas of the galaxy. The Institute has already identified more than 2,000 star systems that might have planets and it’s now pointing its antennas at these cosmic addresses.

Shostak reckons there are between one in 100 and one in 1,000 star systems that contain a world that might be habitable. That is a guess based on results from planet hunting missions so far. But, even if you take the more pessimistic end of that spectrum, it suggests there are a billion Earth-like worlds in our galaxy alone. “That’s a big number and makes me more optimistic that we might find something…the statistics are important in motivating and funding the search.”

But just because a planet is habitable, doesn’t mean anyone lives there.

A new analysis by astrophysicists at Princeton University suggests that our expectation of finding life on a “habitable world” is largely based on the assumption that because life evolved on Earth, then it will elsewhere. The conundrum is that until we find life elsewhere, we have no way of knowing how likely life is to evolve.

Still, Seti would be nothing without a bit of optimism. So how can the Institute improve the odds of finding ET?

Ideally it would be useful to know how old the candidate planets are. If, for instance, you were living on a distant world scanning the skies for alien life, then Earth might well be on your list. But if you’d pointed your radio telescope at our habitable planet at almost any time in the last four billion years then you wouldn’t have detected intelligence. Only in the past 120 years, since the invention of radio, would you be able to pick anything up (and that doesn’t even take into account the time it takes for those radio signals to reach you). The same could be happening when we look elsewhere. It takes time for life to originate, evolve and develop technology.

“I’m a big fan of looking at red dwarf stars,” says Shostak. These stars are smaller than our Sun but much longer lived. “If you look at red dwarf star systems where the planets are going to be older, there might be more chances of them evolving something clever.”

But what if alien life was more dim green slime than brainy green men? Dolphins are pretty clever but they’ve not developed radio telescopes. Could the odds of a species evolving to develop electronics, nuclear weapons and America’s Next Top Model be so infinitesimally tiny as to be almost impossible? Shostak argues that as evolution has selected for intelligence on Earth – giving us an evolutionary advantage over our competitors – then it’s fair to suppose that intelligence might be widespread in the universe.

People certainly want to believe. Polls suggest that the majority of Americans think ET exists. (although, as one in three people in the US also believe aliens are already here walking among us, that may tell you more about the people who were polled rather than anything about the odds of alien life) With results from Kepler we should, in the next few years, get a better sense of whether we’re likely to be alone. 

Until then, keep watching the skies.

If you would like to comment on this story or anything else you have seen on Future, head over to our Facebook page or message us on Twitter.