The James Webb Space Telescope (JWST) may be able to give us a tantalising glimpse of the surfaces of rocky planets outside our solar system, as well as their atmospheres.
One way of measuring the atmosphere of a distant planet is to watch as it passes, or transits, in front of its star. As light passes through the atmosphere, molecules will absorb certain wavelengths of light, helping astronomers to figure out its composition.
Measuring the surface of a planet is trickier. Any light reflecting off the surface will also pass through the atmosphere, making it hard to figure out what they are both made from, and the reflected light will also be weak, so noise can be a problem. So far, no one has managed to accurately determine the make-up of an exoplanet鈥檚 surface 鈥� but that could be about to change.
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鈥淲ith JWST, hopefully, the noise floor should be low enough that we鈥檒l be able to disentangle which of those features are coming from the surface of the planet and which are coming from the atmosphere,鈥� says at the University of California, Los Angeles.
In an effort to prepare for JWST鈥檚 exoplanet observations, Whittaker and their colleagues have been practising with data taken by the Spitzer space telescope of LHS 3844b, a planet that is hot like Mercury, as it passed in front of its host star. The heat means the planet鈥檚 surface produces a strong signal unaffected by its sparse atmosphere.
The team created artificial signals from a variety of different surface and atmosphere compositions and then compared them with the telescope data to work out the make-up of the planet鈥檚 surface. The likely scenarios all involved rocky surfaces dominated by iron, with varying amounts of other metals like magnesium and calcium in different forms.
鈥淲hat we can already say just looking at this planet is yes, JWST will be able to characterise some surfaces, but what we鈥檙e finding is that the surfaces that we will be able to characterise are the more reflective ones,鈥� says team member Matej Malik at the University of Maryland.
The planets we can observe might share similarities with bodies in our solar system like Mercury or the moon, which have dark surfaces, but with brighter, spotty regions, says Malik.
The TRAPPIST-1 planetary system, which is around 40 light years from Earth, could be a prime target for this technique. 鈥淚t鈥檚 not something that鈥檚 only applicable to LHS 3844b,鈥� says at the University of Oxford. 鈥淭RAPPIST-1b is a hot planet which is certainly rocky, and potentially a nearly bare rock with a wisp of atmosphere.鈥�
This ability to potentially distinguish the signatures of atmospheres and surfaces is innovative, says Pierrehumbert, and makes observing some exoplanet surfaces within the 鈥渕argins of feasibility鈥�.
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