An international team of astronomers today announced that they have found a faint cool star that is surrounded by a system of seven planets, each of which resemble the Earth in size. Three of the planets orbits in what we call the "habitable zone" where water can be liquid and temperatures might be right for life.
The star, located about 40 light years away, is so faint and cool, it doesn't have a name like bright stars do. It's referred to by the name of the telescope that discovered it and given a number (TRAPPIST 1). Each of the planets is then given a letter from b to h. (See the diagram above.)
Note that the planets are all very close to their dim star, taking from 1.5 days to about 20 days to orbit it. (By comparison, the closest planet to the Sun, Mercury, takes 88 days to orbit.) Planets e, f, and g are the ones where the combination of a cool little star and close-by planet work out to make the temperatures potentially reasonable for life. This star enters the record books as the one with the largest number of Earth-like planets, and the largest number of candidate planets in the habitable zone.
Astronomers caution that the kind of star these planets live around (called an "ultra-cool red dwarf") tends to have a lot of "activity" on its surface when they are young. Great flares of energy and particles are given off in this kind of activity, which might flood the nearby planets with high-energy radiation. That might not be so healthy for the formation of life there until the star settles down to a more stable adult existence.
On the other hand, such low-mass stars (this one contains only 8% of the "stuff" our Sun has) tend to live much much longer than a star like the Sun, so there may eventually be a much longer opportunity for the planets to evolve their surfaces and atmospheres and give birth to life.
Another complication for planets so very close to their star is that their motion probably resembles that of our Moon in a crucial way. The Moon (and these planets) take the same time to orbit as to spin, which means they keep the same face toward the object they go around. So one side of each planet always faces their star and the other side is always in darkness. There is no day and night cycle on these worlds -- you either live on the star-facing side and have perpetual day or you live on the other side and have perpetual night. Only a significant atmosphere might make such a world more bearable and astronomers are using a variety of telescopes to probe whether these planets are surrounded by an air layer and how much and what kind of air they have.
Just to put the discovery in context, astronomers now know over 3,000 planets orbiting other stars, ranging from balls of gas and liquid much bigger than Jupiter, down to rocky balls smaller than Venus. Experts now estimate that perhaps half of the stars in our Milky Way Galaxy may possess planets, and many stars will have more than one planet, just like TRAPPIST 1 and the Sun do. The universe seems rich with planets of all kinds, making it more likely than ever that we are not the only form of semi-intelligent life in the cosmos.
Below is a little poster NASA created to show how the other planets in this system would look from the surface of one of them.