Using Nasa's Hubble Space Telescope, a team of astronomers has conducted the first search for atmospheres around Earth-sized exoplanets beyond our solar system and found indications that increase the chances of presence of life on two rocky exoplanets.
The exoplanets TRAPPIST-1b and TRAPPIST-1c - approximately 40 light years away - are unlikely to have puffy, hydrogen-dominated atmospheres usually found on gaseous worlds.
"The lack of a smothering hydrogen-helium envelope increases the chances for habitability on these planets," said team member Nikole Lewis from Space Telescope Science Institute (STScI) in Baltimore.
"If they had a significant hydrogen-helium envelope, there is no chance that either one of them could potentially support life because the dense atmosphere would act like a greenhouse," he added.
The planets orbit a red dwarf star at least 500 million years old, in the constellation of Aquarius.
TRAPPIST-1b completes a circuit around its red dwarf star in 1.5 days and TRAPPIST-1c in 2.4 days.
The planets are between 20 and 100 times closer to their star than the Earth is to the Sun.
Because their star is so much fainter than our sun, researchers think that at least one of the planets, TRAPPIST-1c, may be within the star's habitable zone, where moderate temperatures could allow for liquid water to pool.
Julien de Wit from the Massachusetts Institute of Technology led a team of scientists to observe the planets in near-infrared light using Hubble's Wide Field Camera 3.
They used spectroscopy to decode the light and reveal clues to the chemical makeup of an atmosphere.
"These initial Hubble observations are a promising first step in learning more about these nearby worlds, whether they could be rocky like the Earth, and whether they could sustain life," said Geoff Yoder, acting associate administrator for Nasa's Science Mission Directorate in Washington.
"This is an exciting time for Nasa and exoplanet research," he added.
The researchers hope to use Hubble to conduct follow-up observations to search for thinner atmospheres, composed of elements heavier than hydrogen, like those of the Earth and Venus.
"With more data, we could perhaps detect methane or see water features in the atmospheres, which would give us estimates of the depth of the atmospheres," noted Hannah Wakeford from Nasa's Goddard Space Flight Centre in a paper appeared in the journal Nature.
Observations from future telescopes, including Nasa's James Webb Space Telescope, will help determine the full composition of these atmospheres and hunt for potential bio-signatures.
Webb also will analyse a planet's temperature and surface pressure - key factors in assessing its habitability.