‘Landmark discovery’: Hubble Space Telescope observes water vapour on small planet

In a ‘landmark discovery’, the Hubble Space Telescope has observed the smallest planet outside our solar system to contain water vapour in its atmosphere.

The Hubble Space Telescope. Picture: Handout/NASA/AFP

The Hubble Space Telescope has observed the smallest planet outside our solar system to contain water vapour in its atmosphere, a “landmark discovery” that brings astronomy a step closer to characterising Earth-like worlds.

At about twice Earth’s diameter, planet GJ 9827d orbits a red dwarf star 97 light years away in the constellation Pisces, NASA and the European Space Agency said in statements.

The team behind the finding is examining two scenarios: ­either the planet is a “mini-­Neptune” with a hydrogen-rich atmosphere laced with water, or it’s a warmer version of Jupiter’s moon Europa, which contains twice as much water as Earth under its crust.

“The planet GJ 9827d could be half-water, half-rock,” said Bjorn Benneke of the Universite de Montreal, who co-led research. “And there would be a lot of water vapour on top of some smaller rocky body.

“Until now, we had not been able to directly detect the atmosphere of such a small planet. We’re slowly getting in this regime now.”

Over three years, Hubble observed the planet during 11 transients, or events, in which it crossed in front of its star.

During transients, starlight is filtered through the planet’s atmosphere, allowing astronomers to use Hubble’s instru­ments to analyse patterns of colours (wavelengths), which revealed the telltale signature of water molecules.

Even if GJ 9827d has a water-rich atmosphere, its hot, Venus-like temperature of 425C would make it uninhabitable.

Nevertheless, the Hubble discovery paves the way to future study of GJ 9827d and similar planets, especially by the James Webb Space Telescope, which can use its high resolution infrared images to look for more atmospheric molecules including carbon dioxide, and methane.

“Water on a planet this small is a landmark discovery,” said co-leader Laura Kreidberg of Max Planck Institute for Astronomy.

AFP