What’s more, long-period planets are also largely inaccessible as far as their spectra is concerned. This type of analysis involves measuring the light that passes through a planet’s atmosphere as it transits in from of its star. By measuring its spectra to determine its composition, scientists can characterize the exoplanet’s atmosphere and determine if a planet could in fact be habitable.

To address this, the team suggest that direct detection (aka. direct imaging) will be an more effective method for characterizing the atmospheres of exoplanets. As Dr. Étienne Artigau, a researcher iREX and a co-author on the study, explained to Universe Today via email (translated from French):

“No planet detected for now has been found in “reflected light”. When we see the planets of our solar system, it is because they are illuminated by the Sun that we can see them. In the same way, the planets of the other stars reflect light and it must be possible to detect this light with a sufficiently powerful telescope. The flux ratio between the planets and their star is enormous, of the order of 1 billion, compared to the planets detected by their thermal emission, or this ratio is rather of the order of 1 million.”

At present, direct imaging is the only means of obtaining spectra of non-transiting exoplanets, especially those that are at intermediate and wide distances from their suns. In this case, astronomers obtain spectra from light reflected off of the exoplanet’s atmosphere to determine its composition. Only a handful of exoplanets have been directly imaged thus far, all of which were self-luminous super-Jupiters that orbited their host stars at a distance of hundreds or thousands of AU.

These planets were very young and had temperatures in excess of 500 °C (932 °F), which makes them a rather rare class of planets. As a result, astronomers have no information on the diversity of exoplanet atmospheres, especially when it comes to smaller, rocky planets that have temperatures more akin to that of Earth – where surface temperatures average around 15 °C (58.7 °F).

This is due to the fact that existing telescopes simply do not have the sensitivity to directly image smaller planets that orbit closer to the stars. As they determined in their study, characterizing the atmospheres of planets that are within 5 AU of their stars (where radial velocity surveys have revealed many planets) would require a telescope with a 30-meter aperture combined with advanced adaptive optics, a coronagraph, and suite of spectrometers and imagers.