Worlds that could support life are found practically in the Sun’s backyard

Astronomers have found three Earth-size worlds around a cool star just 40 light-years from the Sun. (credit: ESO/M. Kornmesser)

The star is only about the size of Jupiter and much colder and redder than the Sun. Its luminosity is far less than 1 percent that of our star—so faint that, although the "ultracool" dwarf star called TRAPPIST-1 lies less than 40 light-years from Earth, it can only be seen via relatively powerful telescopes.

Yet it is a star worth looking for. Astronomers using a 60cm telescope designed especially to study such stars, and any planets around them, have found this system to contain some of the most habitable exoplanets discovered to date. As European astronomers looked at TRAPPIST-1 from September through December of last year, they discovered slight, periodic dimming that indicates the presence of three worlds which are close to or inside the system's habitable zone. All have radii of between 1.05 and 1.17 that of Earth's radius.

According to the observations published Monday in the journal Nature, the two inner planets orbit the star every 1.51 days and 2.42 days. The innermost planet, TRAPPIST-1b, likely receives about four times the solar radiation from its star than does Earth, and astronomers estimate its surface temperature is probably closer to the higher end of a range between 11 degrees and 127 degrees Celsius. The next planet, TRAPPIST-1c, receives a little more than two times the solar radiation as does Earth and has a surface temperature likely between -30 degrees and 69 degrees Celsius. The researchers speculate these worlds are likely tidally locked and, therefore, even if they have extreme average temperatures, they may have habitable regions along the terminator or poles.

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