Artist's Concept: NASA and JPL/Caltech
Science: NASA, ESA, J. de Wit (MIT), H. Wakeford (University of Exeter/STScI), and N. Lewis (STScI)

Hubble Probes Atmospheres of Exoplanets in TRAPPIST-1 Habitable Zone

Scientists have made the first steps towards understanding what makes up the atmospheres of several Earth-sized exoplanets orbiting the red dwarf star TRAPPIST-1.

An international team of astronomers, including Hannah Wakeford from the University of Exeter, have conducted the first spectroscopic survey of the far-flung worlds, found 40 light years away from Earth.

Using NASA’s Hubble Space Telescope, the team have found three of the exoplanets do not appear to contain puffy, hydrogen-rich atmospheres similar to gaseous planets such as Neptune.

The results indicate the atmospheres may be shallower and rich in heavier gases like those found in Mars, Venus, or Earth’s atmosphere, such as carbon dioxide, methane, and water.

The findings will pave the way for NASA’s James Webb Space Telescope, scheduled to launch in 2019, to probe deeper into the planetary atmospheres to search for the presence of these elements, which could offer hints of whether the planet were habitable, or life could be present.

The research is published in leading scientific journal, Nature Astronomy, on Monday, February 5 2018.

Hannah Wakeford, from Exeter’s Physics and Astronomy department and also the Space Telescope Science Institute (STScI) in Baltimore, Maryland said: “No one ever would have expected to find a system like this. They’ve all experienced the same stellar history because they orbit the same star. It’s a goldmine for the characterization of Earth-sized worlds.

“One of these four could be a water world -  one could be an exo-Venus, and another could be an exo-Mars. It’s interesting because we have four planets that are at different distances from the star. So we can learn a little bit more about our own diverse solar system, because we’re learning about how the TRAPPIST star has impacted its array of planets.”

The planets orbit a red dwarf star that is much smaller and cooler than our Sun. The four alien worlds are members of a seven-planet system around TRAPPIST-1. All seven of the planetary orbits are closer to their host star than Mercury is to our Sun. Despite the planets’ close proximity to TRAPPIST-1, the star is so much cooler than our Sun that liquid water could exist on the planets’ surfaces.

Two of the planets were discovered in 2016 by TRAPPIST (the Transiting Planets and Planetesimals Small Telescope) in Chile. NASA’s Spitzer Space Telescope and several ground-based telescopes uncovered five additional ones, increasing the total number to seven.

The Hubble observations took advantage of the fact that the planets cross in front of their star every few days. Using the Wide Field Camera 3, astronomers made spectroscopic observations in infrared light, looking for the signature of hydrogen that would filter through a puffy, extended atmosphere, if it were present.

“Hubble is doing the preliminary reconnaissance work so that astronomers using Webb know where to start,” said Nikole Lewis of the STScI and co-leader of the Hubble study.

“Eliminating one possible scenario for the makeup of these atmospheres allows the Webb telescope astronomers to plan their observation programs to look for other possible scenarios for the composition of these atmospheres.”

Although Hubble did not find evidence of hydrogen, the researchers suspect the planetary atmospheres could have contained this lightweight gaseous element when they first formed. The planets may have formed farther away from their parent star in a colder region of the gaseous protostellar disk that once encircled the infant star.

“The system is dynamically stable now, but the planets could not have formed in this tight pack,” Lewis said. “They’re too close together now, so they must have migrated to where we see them. Their primordial atmospheres, largely composed of hydrogen, could have boiled away as they got closer to the star, and then the planets formed secondary atmospheres.”

In contrast, the rocky planets in our solar system likely formed in the hotter, dryer region closer to the Sun.

Hannah Wakeford added: “There are no analogs in our solar system for these planets. One of the things researchers are finding is that many of the more common exoplanets don’t have analogs in our solar system. So the Hubble observations are a unique opportunity to probe an unusual set of worlds.”

The Hubble team plans to conduct follow-up observations in ultraviolet light to search for trace hydrogen escaping the planets’ atmospheres, produced from processes involving water or methane lower in their atmospheres.

Astronomers will then use the Webb telescope to help them better characterize those planetary atmospheres. The exoplanets may possess a range of atmospheres, just like the terrestrial planets in our solar system.

The team’s results will appear in the Feb. 5 issue of Nature Astronomy.

The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, Inc., in Washington, D.C.

Date: 5 February 2018

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