Astronomers say three recently discovered planets similar to Earth's size and temperature may have conditions that could sustain life.

An international team observed the three planets orbiting a reddish, ultracool dwarf star, once thought too dim to anchor a solar system. Their research, published in the journal Nature on Monday, said these are the first planets ever seen orbiting an ultracool dwarf star.

"Systems around these tiny stars are the only places where we can detect life on an Earth-sized exoplanet with our current technology," co-author Michael Gillon, of the University of Liege in Belgium, said in a statement. "So if we want to find life elsewhere in the universe, this is where we should start to look."

Previously, scientists have only found exoplanets -- planets that do not orbit our sun -- with conditions unlike Earth's. In November, for instance, a rocky, Earth-sized planet was found 39 light years away, but its temperature was estimated at 300 degrees to 600 degrees. The discovery of the three potentially habitable planets may encourage researchers to look more closely at the huge numbers of ultracool dwarf stars.

The three planets orbit a star in the Aquarius constellations named Trappist-1, which is about the size of Jupiter . But the planets are close enough to the star to have "temperate" conditions on their surface, MIT researcher Julien De Wit told NPR .