By now you may have heard the report that as many as 1/4 of all the sun-like stars in the Milky Way may have Earth-like worlds. Briefly, astronomers studied 166 stars within 80 light years of Earth, and did a survey of the planets they found orbiting them. What they found is that about 1.5% of the stars have Jupiter-mass planets, 6% have Neptune-mass ones, and about 12% have planets from 3 – 10 times the Earth’s mass.
This sample isn’t complete, and they cannot detect planets smaller than 3 times the Earth’s mass. But using some statistics, they can estimate from the trend that as many as 25% of sun-like stars have earth-mass planets orbiting them!
Now, there’s a very important caveat here: these are planets that have the same mass as Earth, but that doesn’t mean they are very earth-like. The planets the team could find were very close to their parent stars, so they’d be very hot, and uninhabitable. But the good news is that if that trend in mass they saw is correct, the Milky Way is littered with planets the mass of the Earth! If some of them are in the habitable zone of their star… well.
So a funny thing: I was thinking about this very problem a couple of days ago, but from a different angle. How many habitable planets are there in the Milky Way? Not just earth-mass, but also orbiting their star in the so-called Goldilocks Zone, where temperatures are right for liquid water?
There’s a way to estimate it. And it involves the planet recently announced, Gliese 581g. This planet is about 3 times the Earth’s mass, and it orbits its star in the right place. We don’t know what it’s made of, if it has an atmosphere, or really very much about it at all! But given its mass and temperature, it’s potentially habitable.
The distance to the Gliese 581 system is what gets me excited: it’s 20 light years away. That’s close, compared to the vast size of our galaxy. So let’s assume Gliese 581g is the closest potentially habitable planet to us. Given that assumption, we can estimate the number of potentially habitable planets in the entire Milky Way! And the math’s not even that hard.
Astronomers have announced the discovery of a planet with about three times the Earth’s mass orbiting the nearby red dwarf star Gliese 581. That in itself is cool news; a planet like that is very hard to detect.
But the amazing thing is that the planet’s distance from the star puts it in the Goldilocks Zone: the region where liquid water could exist on its surface!
Artist’s drawing (from 2007, before this announcement) of the planetary system of Gliese 581. Credit: ESO
First, a few things: 1) Gliese 581 is a dinky, cool red dwarf about 20 light years away. That’s pretty close as stars go; only a handful are closer. Bear in mind it’s still 200 trillion kilometers (120 trillion miles) away, and that’s still a bit of a drive.
2) The planet is one of six now known to orbit the star [that link goes to a PDF of the journal paper]. Apparently, all the planets have neat, circular orbits, so the system seems to be stable. This new planet takes 37 days to orbit the star once, and orbits at a distance about 1/6 the distance of the Earth from the Sun. As far as we know, it’s the fourth planet from its star.
3) The planets have all been found by the Doppler method: as they orbit the star, they tug on it. This causes a shift in the wavelength of emitted light from the star. The mass of the planet, its distance from the star, and the shape of the orbit all determine how the light shifts, which is how astronomers found those properties of the new planet.
OK, so that’s what we know. Now let me be clear here about stuff we can be fairly sure about.