The possibility of life on other worlds has fueled our imagination for centuries. Over the past 20 years, the discoveries of planets orbiting other stars has stepped up the search for worlds like Earth that could sustain life. NASA's Hubble Space Telescope has made significant and unique contributions to the planet hunt. Astronomers used Hubble, for example, to make the first measurements of the atmospheric composition of extrasolar planets.
Now, astronomers are using Hubble to conduct the first search for atmospheres around temperate, Earth-sized planets beyond our solar system, uncovering clues that increase the chances of habitability on two exoplanets.
They discovered that 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. Those dense atmospheres act like a greenhouse, smothering any potential life. Observations from NASA's James Webb Space Telescope (set to launch in 2018) will help determine the full composition of these atmospheres and hunt for potential biosignatures, such as carbon dioxide and ozone, and methane. Webb also will analyze a planet's temperature and surface pressure — key factors in assessing its habitability.
"The lack of a smothering hydrogen-helium envelope increases the chances for habitability on these planets," said Nikole Lewis of the Space Telescope Science Institute (STScI) in Baltimore, Maryland. "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."
Scientists observed the planets in near-infrared light using Hubble's Wide Field Camera 3. Using spectroscopy, they were able to decode the light and reveal clues to the chemical makeup of the atmosphere. While the total content of the atmospheres is unknown and will have to await further observations, the low concentration of hydrogen and helium has scientists excited about the implications.
"These initial Hubble observations are a promising first step in learning more about these nearby worlds - whether they could be rocky like Earth, and whether they could sustain life," said Geoff Yoder, acting associate administrator for NASA's Science Mission Directorate in Washington, D.C. "This is an exciting time for NASA and exoplanet research."
The planets orbit a red dwarf star at least 500 million years old, in the constellation of Aquarius. They were discovered in late 2015 through a series of observations by the Transiting Planets and Planetesimals Small Telescope (TRAPPIST), a Belgian robotic telescope located at the European Southern Observatory’s (ESO’s) La Silla Observatory in Chile.
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 Earth is to the sun. Because their star is so much fainter than our sun, researchers think that at least one of the planets, or possibly both, may be within the star's habitable zone, where moderate temperatures could allow for liquid water to pool.
In May, astronomers took advantage of a rare simultaneous transit, when both planets crossed the face of their star within minutes of each other, to measure starlight as it filtered through any existing atmosphere. This double-transit, which occurs only every two years, provided a combined signal that offered simultaneous indicators of the atmospheric characteristics of the planets.
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 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," said Hannah Wakeford at NASA's Goddard Space Flight Center in Greenbelt, Maryland.
These planets are the first Earth-sized worlds found in the Search for Habitable Planets Eclipsing Ultra-Cool Stars (SPECULOOS) survey, which will search more than 1,000 nearby red dwarf stars for Earth-sized worlds. So far, the survey has analyzed only 15 of those stars.