Over 100 billion planets likely exist in our galaxy, according to a new study by astronomers using the Kepler Space telescope.
"There's at least 100 billion planets in the galaxy — just our galaxy," said John Johnson, a professor of astronomy at the California Institute of Technology, according to Science Daily. "That's mind-boggling."
"It's a staggering number, if you think about it," adds Jonathan Swift, who was the lead author of the new research paper. "Basically there's one of these planets per star."
Astronomers have been studying a common M-class dwarf star, named Kepler-32, since the Kepler telescope was launched into orbit, four years ago, and they found that there were five planets orbiting around it. The planets were discovered by recording the brightness of the star and watching for 'transits' — dips in the brightness of the star caused by an orbiting planet passing between the star and us. By examining these transits carefully, astronomers can tell the mass of the orbiting planet and how quickly it orbits the star.
The five planets in the Kepler-32 system are all terrestrial planets — rocky planets like the Earth and Mars, as opposed to gas giants like Jupiter and Saturn — and they all orbit within 16 million kilometres from Kepler-32, which is pretty close, considering that Mercury's average distance from our Sun is in the neighbourhood of 50 million kilometres. These close orbits (and thus short orbital periods), coupled with the fact that we view the Kepler-32 system nearly edge-on, allowed astronomers to study all five planets in detail as they whizzed around their parent star.
"I usually try not to call things 'Rosetta stones', but this is as close to a Rosetta stone as anything I've seen," said Johnson. "It’s like unlocking a language that we’re trying to understand — the language of planet formation."
Using the findings from the Kepler-32 system, the research team calculated the odds of viewing such a star system edge-on, and then applied these odds to the other M dwarf stars in our galaxy — which make up around 75 per cent of the billions of stars in the Milky Way. The results gave them a conservative estimate of around 100 billion planets. Considering that number only includes close-orbiting planets around these cool red dwarf stars, there may be far, far more planets out there, both orbiting further out in systems like Kepler-32 and orbiting bigger stars!
The Kepler mission has already found over 2,300 unconfirmed exoplanet candidates and has confirmed the existence of 105 planets orbiting other stars, and it is not the only one looking for them. The MOST (Microvariability and Oscillations of STars) satellite is Canada's contribution to the search and the French Space Agency runs COROT (COnvection ROtation and planetary Transits).
These missions, the astronomers that work with them and citizen scientists like those at PlanetHunters.org (who, along with 42 new candidates, just reported another confirmed planet — a gas giant in its star's habitable zone!) are finding and confirming more and more planets all the time. If these estimates turn out to be right, they all certainly have their work cut out for them, but along with that, the chances of finding more planets like Earth becomes far more likely.
"The Earth isn’t unique, nor the center of the universe,” said astronomer Geoff Marcy, a UC Berkeley professor who has found more than 70 of the confirmed exoplanets. “The diversity of other worlds is greater than depicted in all the science fiction novels and movies. Aristotle would be proud of us for answering some of the most profound philosophical questions about our place in the universe.”