Despite the fact that helium is one of the most abundant elements in the universe, its existence wasn't known until 1868, when Pierre Janssen—a french physicist and mathematician—accidently stumbled across strange spectral lines in the Sun's light during a solar eclipse. Now, nearly 150 years later, using the same technique, astronomers are able to determine many characteristics of stellar objects—like stars and planets—by merely breaking their light apart.
However, since planets do not generate their own light, this information is normally acquired during a transit—when a planet passes in front of its parent star relative to Earth. The alignment ultimately affords us an opportunity to study wavelengths of light a planet's atmosphere absorbs as starlight trickles through the atmosphere temporarily; when a constituent wavelength is singled out, it corresponds to a certain element. then, by ultimately removing the noise (like, say, the transmission spectrum of a host star), we can deduce the atmospheric composition of a planet—and at infrared wavelengths, its temperature— but little else, as the information that can be garnered during a transit is limited.
Now, an international team of researchers—led by Jorge Martins from the Instituto de Astrofísica e Ciências do Espaço (IA) and the Universidade do Porto—have, for the very first time, obtained the spectral lines of a non-transiting exoplanet in visible light.
The planet—51 Pegasi b, a hot-Jupiter exoplanet found approximately 50 light-years from Earth in the Pegasus constellation—was well known long before their landmark observations, being that it holds the title of being the very first planet found orbiting a Sun-like star. Given its close proximity, it's an ideal target for this novel application.
The team describes their technique:
Jorge Martins expands: "This type of detection technique is of great scientific importance, as it allows us to measure the planet's real mass and orbital inclination, which is essential to more fully understand the system. It also allows us to estimate the planet's reflectivity, or albedo, which can be used to infer the composition of both the planet's surface and atmosphere."
"It also allows us to estimate the planet’s reflectivity, or albedo, which can be used to infer the composition of both the planet’s surface and atmosphere," Martins added.
In the coming years, the Very Large Telescope's ESPRESSO spectrograph will go online, and it will hopefully shed more light on potentially habitable exoplanets. It may even help us find Earth 2.0.
“These encouraging results clearly show a bright future for this type of studies when next-generation instruments (e.g., ESPRESSO at the VLT) and telescopes (e.g., ESO’s E-ELT) become available to the community,” the researchers conclude. “The sheer increase in precision and collecting power will allow for the detection of reflected light from smaller planets, planets on orbits with longer periods, or an increase in detail for larger planets like 51 Peg b."