Starlight Absorption

Starlight Yields Clues To Exoplanets’ Atmospheres

Astronomers used Hubble to analyze light from the nearby star TRAPPIST-1 that passed through the atmospheres of four Earth-sized planets in the star’s habitable zone. This zone is a region at a distance from the star where liquid water, the key to life as we know it, could exist on the planet's surfaces. The astronomers were looking for the signatures of certain gases, including hydrogen, in the atmospheres that were imprinted on the starlight.

The graphic at top shows a model spectrum containing the signatures of gases the astronomers would expect to see if the exoplanets’ atmospheres were puffy and dominated by primordial hydrogen from the distant worlds’ formation.

The Hubble Space Telescope observations, however, revealed that the planets do not have hydrogen-dominated atmospheres. The flat spectrum shown in the illustration at bottom indicates that Hubble did not spot any traces of water or methane, which are abundant in hydrogen-rich atmospheres.

The researchers concluded that the atmospheres are composed of heavier elements residing at much lower altitudes than could be measured by the Hubble observations.

Hydrogen is a greenhouse gas, which smothers a planet orbiting close to its star, making it hot and inhospitable to life. The results, instead, favor more compact atmospheres like those of Earth, Venus, and Mars.

The exoplanets are members of a system of seven Earth-size worlds orbiting TRAPPIST-1, located 40 light-years away.