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.
That task is before giant new telescopes currently in development, such at the James Webb Space Telescope and the Thirty Metre Telescope on the ground. They should be able to see the planets and tell us much more about actual conditions on their surfaces.
Even with our new giant eyes in the sky, life is very difficult to detect. Take Earth for example. Astronauts aboard the International Space Station can look down and see continents, oceans, forests and even cities, especially at night.
But the space station is only 400 kilometres above. Apollo astronauts who went to the moon could still see continents and oceans, but once you get farther away than that, all the detail on the Earth's surface disappears. The most distant image of Earth was taken in 1990 by the Voyager 1 spacecraft from the edge of our solar system, just beyond the orbit of Neptune. Taking up only a few pixels in the camera frame, Earth was, as famous astronomer Carl Sagan put it, "a pale blue dot."
Could you tell whether there is any life, let alone intelligent life on that blue dot?
The fact that it is blue is a good clue, because that is the colour of liquid water. You would know by measuring Earth's distance from the sun that it would be the right temperature for water to exist. On the other hand, the planet Neptune is also blue for a completely different reason. It has methane in its atmosphere, which happens to absorb red light from the sun leaving blue to reflect back. Neptune is not a water world, and not very hospitable to life as we know it.
Speaking of methane, it is referred to as a bio-molecule, because it, along with others such as oxygen, and water vapour can be produced by living organisms. But they can also be produced by volcanoes and chemical reactions, so they may not conclusively prove life.
If we pointed a radio receiver at Earth, we could hear all the babble from radio and television programs broadcasting through the air. When the first three planets were found around TRAPPIST-1 in 2016, the SETI Institute, which searches for intelligent signals from space, pointed its telescopes towards the system and heard nothing. Then again, radio technology is a recent invention on Earth and is rapidly disappearing as we go digital, so perhaps the aliens on the other planets have already outgrown it.
If detecting life on Earth would be difficult from the edge of our solar system, finding it around a star system 39 light-years away will be a much greater challenge. Until we develop starships that travel at tremendous speeds so we can visit these worlds and look down on them as our astronauts do on Earth, we are left with pale dots of whatever colour exist there.
But here is another thought: if we find no life on these planets, that doesn't mean it couldn't evolve there in the future. Red dwarf stars don't put out much energy because they are slow burners, living much longer than stars like our sun. So there is a chance that life could appear, evolve, become intelligent, develop telescopes, look for planets around other stars and stumble upon our pale blue dot.
Will they be asking the same questions we are asking now?
Will anyone be here to answer?