The possibility of finding life on other worlds got a huge boost this week with the announcement that seven Earth-sized planets have been found around a nearby star, three of which lie within the so-called habitable zone where water, and therefore life, could exist on their surfaces. Proving that life actually thrives there, however, will be extremely difficult.
The seven planets orbiting the star TRAPPIST-1, form a solar system completely different from our own. The dim dwarf star is much smaller than our sun, casting an orange twilight glow over the entire system. And while the star doesn’t put out much energy, all of its planets are huddled in close like campers around glowing embers of a fire, so they still receive enough light and heat to be warm and comfortable environments for life to thrive. The entire TRAPPIST system would fit inside the orbit of Mercury, and the planets are so close to each other, a person standing on one would be able to see features on some the way we see craters and mountains on our moon.
But to be clear, there is much to be learned about these planets because astronomers have not actually seen them. The alien worlds gave themselves away by passing in front of their star as they orbited it, causing the light from the star to dim slightly from our point of view. This technique has found thousands of extrasolar planets, but all the dimming starlight tells us is the size and orbits of the planets. It doesn’t tell us what kind of atmosphere they might have, or even what colour they are.