Three Earth-size planets — one possibly with liquid water on its surface — have been found around a distant star with help from the Keck Observatory on Mauna Kea.
These worlds are not within practical visiting distance, at 150 light-years away.
Yet they are close enough that it is possible to determine whether their atmospheres are conducive to life, astronomers say.
A paper describing the discovery was submitted Jan. 16 to the Astrophysical Journal.
University of Hawaii astronomer Andrew Howard, one of several co-authors, said the planetary system will help determine how common Earth-like planets truly are.
"We’ve learned in the past year that planets the size and temperature of Earth are common in our Milky Way galaxy," he said in a statement. "We also discovered some Earth-size planets that appear to be made of the same materials as our Earth, mostly rock and iron."
One of the three planets is only 1.5 times the size of Earth and occupies the "Goldilocks zone," an orbit neither too close nor too far from its star. That means water could exist in liquid form.
The star, EPIC 201, is a cool, bright red dwarf about half the size and mass of our sun. Even at its formidable distance, the star is among the top 10 nearest known to have transiting planets, that is, planets that cross in front of the star from Earth’s perspective.
That means it’s bright enough for astronomers to study the planets’ atmospheres to determine whether they are like Earth’s.
Erik Petigura, a University of California at Berkeley graduate student, found the planets Jan. 6 while conducting a computer analysis of data NASA provided from the Kepler Space Telescope.
"Most planets we have found to date are scorched," he said. "This system is the closest star with lukewarm transiting planets. There is a very real possibility that the outermost planet is rocky like Earth, which means this planet could have the right temperature to support liquid water oceans."
After Petigura found the planets, the astronomy team used the Keck Observatory and telescopes in Chile and California to calculate the star’s mass, radius, temperature and age.
The Kepler mission to find habitable planets is currently in its second incarnation, nicknamed K2, after it lost its ability to point at a fixed spot in space. One of its gyroscopic "reaction wheels" failed in July 2012, and a second in May 2013.
But in 2014 engineers were able to stabilize it by pointing it in the plane of Earth’s orbit, which allows some control by sailing on the solar wind. "This discovery shows that Kepler can still do great science," Petigura said.
