"These planets are Earth-sized, they are temperate — we can't rule out the fact that they are habitable — and they are well-suited for atmospheric studies," says Julien de Wit, a researcher at MIT. "This is what we know at this stage."
Until recently, he says, some theorists argued that ultracool dwarf stars wouldn't have planets at all. But a group led by Michael Gillon and Emmanuel Jehin of the University of Liege in Belgium took a risk and built a prototype telescope called TRAPPIST to look at 60 nearby ultracool stars.
"Now we actually know that these stars can produce such planets," de Wit says. "Planets around such stars are really great targets for advanced study, like atmospheric characterization."
Researchers have used telescopes to try to study starlight that passes through the atmospheres of other planets outside our solar system, he says, "but these planets were not Earth-sized, these planets were not temperate."
What's more, the fact that this is a small, cool star makes it easier to study the starlight that filters through the planets' atmospheres, de Wit says. He notes they're getting ready to do just that with the Hubble Space Telescope.
Others agree that these particular planets are compelling. "They're all pretty darn interesting," says Marc Kuchner of NASA's Goddard Space Flight Center. He thinks each looks like "another compelling example of a planet that we really ought to spend some quality time getting to know."
He says the planets remind him of GJ 1214 b, a world about twice the size of Earth that scientists have been eager to probe. One recent study found that its atmosphere is dominated by some kind of exotic cloud cover.
"Clouds may be sort of romantic, but it's so hard to tell what they're made of!" says Kuchner, who notes that three planets around their cool star look like the next logical ones for study.
"This is the star with the lowest temperature that seems to have planets around it," he adds.
Researchers hope to study planets beyond our solar system with the help of the soon-to-be-launched James Webb Space Telescope, which can look for things such as water, ozone and carbon dioxide.