It is believed to be a "mini-Neptune", with an atmosphere rich in hydrogen and water, or a hotter version of Jupiter's moon Europa, which contains twice as much water as Earth under its crust.
"GJ 9827d could be half water, half rock," explained Björn Benneke of the University of Montreal, co-director of the research. 'And there would be a lot of water vapor on top of a smaller rocky body." "Until now we have not been able to directly detect the atmosphere of such a small planet. And we're slowly getting there," he said.