Raising hope of finding a "second Earth" out there in the cosmos, scientists announced Wednesday that they've found three planets circling a relatively nearby star that could be hospitable enough to support life.
They are among seven planets discovered orbiting the star known as TRAPPIST-1, about 39 light years from Earth in the Milky Way.
The scientists — some from the Space Telescope Science Institute in Baltimore — plan to probe the planets in coming years with advanced telescopes, including the Hubble Space Telescope and its planned successor, the James Webb Space Telescope.
The findings published Wednesday in the journal Nature increase scientific confidence that searching the heavens could turn up signs of life.
"We know there must be many more potential life-bearing planets out there just waiting to be found," said Sara Seager, a professor of planetary science and physics at Massachusetts Institute of Technology.
The TRAPPIST-1 system, which would take 44 million years to reach via jet plane, scientists said, was identified by a Chile-based observatory known as Transiting Planets and Planetesimals Small Telescope, or TRAPPIST.
The observatory announced last spring that there were three planets in the system and astronomers began probing it with NASA's Hubble and Spitzer Space Telescopes.
In research published last summer, Hubble showed two of the planets did not have inhospitable hydrogen- and helium-dominated atmospheres. Nikole Lewis, an astronomer at the Baltimore institute who was among the panelists at NASA's Wednesday press conference, said that raised hope that the planets could support life.
Nearly three weeks of observations from the Spitzer telescope showed TRAPPIST-1 to be different in many ways from Earth's solar system, but nonetheless capable of holding worlds teeming with water and life.
Scientists trained that observatory on the TRAPPIST-1 system using a technique in which they watch objects pass in front of the star, revealing the rest of the system. That showed the four additional planets and their Earth-like qualities.
The three planets that are considered the strongest candidates for habitation receive similar amounts of light from their star as Earth and Mars do, though they are significantly closer to it than any objects are to the sun. TRAPPIST-1 is a red dwarf that is 200 times dimmer than the sun and significantly smaller — if the sun were the size of a basketball, TRAPPIST-1 would be the size of a golf ball.
Two of the planets are about the same size as Earth, while the third is 13 percent larger. Their densities suggest they are rocky, and scientists suspect their surface temperatures could allow them to hold oceans of liquid water.
On all three of the planets, a solar year passes in less than 12 days. A neighboring planet that is closest to TRAPPIST-1 orbits the star once every day and a half.
All seven planets within the system are closer to their star than Mercury, our solar system's innermost planet, is to the sun. But, because TRAPPIST-1 is a weak red dwarf, the three planets likely have surface temperatures similar to Venus, Earth and Mars, the scientists said.
Scientists said they are so close together that a person standing on the surface of any one of them would be able to see the others big and bright in the sky, like the moon is seen from Earth, but with even more beauty.
"The TRAPPIST-1 system has really captured our imagination," Seager said.
Scientists had never before found so many Earth-like planets around a single star.
"The discovery gives us a hint that finding a second Earth is not just a matter of if, but when," said Thomas Zurbuchen, associate administrator of the science mission directorate at NASA.
Work to learn more about the planets and how to detect potential life on them will now ramp up.
Lewis said Hubble will continue to probe the planets to reveal whether they have substantial atmospheres and, if so, what elements they contain.
She also expects the James Webb Space Telescope, which was largely assembled at NASA's Goddard Space Flight Center in Greenbelt and is scheduled to launch next year, will look at the system early in its mission. Both telescopes have spectrographs — instruments that can reveal the composition of the atmospheres.
"It'll take a lot of observations with Hubble and Webb to detect water," Lewis said. "We are certainly looking."
The Spitzer telescope, which NASA launched in 2003, was better suited to detect the planets than TRAPPIST because it observes from space and Hubble because it observes in infrared light. Hubble, launched in 1990, observes in mostly visible light. The view from the TRAPPIST telescope and other ground-based infrared telescopes can be clouded by Earth's own radiation.
Observing in infrared allows scientists to see through dust and to detect relatively cool objects, such as the TRAPPIST-1 star.
"This is the most exciting discovery we've had yet with Spitzer in almost 14 years of observation," said Sean Carey, manager of NASA's Spitzer Science Center in California.
But once the Webb telescope gets up and running, scientists can use it and Hubble together to fine-tune observations and get a clearer picture of the planets, Lewis said.
The Space Telescope institute manages the Hubble's scientific mission from its offices on the Johns Hopkins University's Homewood campus. It will do the same for Webb, in addition to housing its operations center.
"The two have to work in tandem to holistically understand these planets," she said.