Kepler-186f – Earth’s Exoplanet Twin
Astronomers yesterday announced the discovery of the first Earth-sized planet found in the habitable zone of its star. Revelling in the name of Kepler-186f this “twin Earth” was discovered by the Kepler telescope, adding to the 1800 or so exoplanets we’ve already detected.
The Kepler telescope surveys many stars at one time looking at whether the light we receive from those stars dims temporarily. If it does then that could mean its being blocked out by a planet passing across the face of the star. The dip in star light is tiny, a fraction of one percent of the star’s light, but nonetheless we can get a lot of information about the planet and its orbit from this dimming of its parent star.
By measuring how long the star’s light dims for we can work out how fast the planet is going, and therefore how far from the star it is. By the amount of the star’s light that is blocked out we can tell how big the planet is. In fact we can use mathematical techniques to strip out information from a complicated dimming pattern to work out these factors for a family of planets.
And indeed the parent star in this case, Kepler-186, has five planets going round it, named, from closest to furthest, Kepler-186b, -c, -d, -e, and -f. Only the last of these though is orbiting far enough from the parent star to be in the Goldilocks Zone, the region around a star where it is not too hot, not too cold, but just right for liquid water – a prerequisite for life on Earth at least – to exist. And not only that, but the amount of starlight that Kepler-186f blocks out tells us that it’s very similar in size to the Earth, which means it must be a rocky planet like our own rather than a gas planet, as gas planets are much bigger than the Earth.
The parent star Kepler-186 is much smaller than the Sun; it’s a red dwarf star with a mass of 0.48 M☉(solar masses), a radius of 0.47 R☉(solar radii), and a temperature of around 4000°C compared to the Sun’s 6000°C. This means that Kepler-186’s Goldilocks Zone (also known as the habitable zone, or HZ, green above) is much nearer the star than is the case in our solar system. In fact all five of Kepler-186’s planets orbit their star closer than Mercury orbits the Sun, with the most distant Kepler-186f orbiting at a distance of 0.356AU compared to Mercury’s 0.387AU, going round its star every 130 days.
Might there be life?
No one would have believed in the first years of the twenty-first century that this world was being watched keenly and closely by intelligences greater than man’s and yet as mortal as his own; that as men busied themselves about their various concerns they were scrutinised and studied, perhaps almost as narrowly as a man with a microscope might scrutinise the transient creatures that swarm and multiply in a drop of water. With infinite complacency men went to and fro over this globe about their little affairs, serene in their assurance of their empire over matter… Yet across the gulf of space… intellects vast and cool and unsympathetic, regarded this earth with envious eyes, and slowly and surely drew their plans against us.
– an unlikely scenario, borrowed from H.G. Wells’ War of the Worlds
As soon as this planet was discovered (yesterday!) the Search for Extra Terrestrial Intelligence (SETI) trained their Allen Telescope Array on the star in the hope of hearing a message from an intelligent civilisation. So far: nothing. However in order to be detectable to us here on Earth the Keploids would have to be transmitting at 10x the power we do when beaming signals at potential alien civilisations.
Another route to detecting life – any kind of life, not just the intelligent kind – is to use powerful telescopes to study the planet’s atmosphere. If there’s oxygen there then it must be being produced by plant life; if there are industrial pollutants there (like CFCs that don’t occur naturally) then something would have to be making them. However our scopes are not powerful enough to see the atmosphere of Kepler-186f yet, partly because it’s so far away: 490 light years from us.
E.T. Phone Kepler-186f
Even if we did find evidence of intelligent life on this twin Earth, it’s so far away that communicating with it would be terribly slow. Limited as we are in this universe to sending signals at the speed of light, this planet is 490 light years away, and so the conversation would go something like this:
US: “Hello, how are you guys?
[wait 490 years for them to get the signal]
[wait for them to translate the message]
[wait 490 years for their reply to reach us]
THEM: “Fine thanks, how are you?” [980+ years later…]
As you can imagine if it takes light that long to get there, it would take our spaceships even longer. The furthest we’ve ever sent a spacecraft out into space (Voyager 1) is 19 billion km, which sound pretty far, but is only 35 light minutes away. And Voyager 1 has been traveling for 37 years. 37 years for 35 light minutes. That means it would take Voyager 1 around 270 million years to get to Kepler-186f…
Finding Kepler-186 in the sky
Where can you find Kepler-186 in the sky? The short answer is: you can’t. It’s far too distant and faint to be seen with anything other than the most powerful of telescopes, but you can see roughly where it is by looking in the constellation of Cygnus the Swan.
Cygnus is low in the north-east as the sky darkens, rising to high in the east by dawn, and looks like a large cross, with the long leg of the cross representing the swan’s neck, the short leg of the cross being its tail, and the two arms of the cross being its wings. The bright star in the “right wing” (the higher one) is called δ Cygni and Kepler-186 is near this star, towards the tail of the swan.
The discovery if this twin Earth is very exciting, but it’s just the very start of our exploration of exoplanets (planets beyond our solar system). The star that Kepler-186f orbits is a red dwarf, a very typical star. approximately 70% of the 300 hundred billion stars in our galaxy are of this type (called M-type). If only one in a thousand of these stars has a planet like Kepler-186f that still leaves 200 million Earth twins in our galaxy, and some of them might be closer to us, making them easier to study, and perhaps to talk with…