40.66 light-years away
TRAPPIST-1
Where are there seven Earth-sized planets around one star?
A star so small it is barely a star at all, with seven roughly Earth-sized planets crowded around it. It is the most thoroughly examined planetary system outside our own.
Seven worlds, and none of them far out
TRAPPIST-1 is an ultra-cool dwarf about the size of Jupiter, though far heavier and hot enough to fuse hydrogen. Between 2016 and 2017 seven planets were found transiting it, all comparable in size to Earth, several of them receiving roughly the amount of light that would allow surface water.
The whole system would fit comfortably inside Mercury's orbit. The planets circle in days rather than years, and their orbital periods form a near-exact chain of ratios — a resonance that indicates they migrated inwards together rather than forming where they now sit.
Why it is studied so heavily
Transiting planets around a small, dim star are the best available case for actually measuring an atmosphere: the planet blocks a large fraction of a faint disc, and starlight filters through whatever air it has on the way to us. That is why TRAPPIST-1 receives so much telescope time.
The early results have been sobering. The innermost planets show no sign of thick atmospheres, which fits the concern that a red dwarf's flares and radiation strip them away. The outer, cooler planets remain open questions, and they are among the most consequential ones in the field.