There is something terribly exciting about actually seeing an exoplanet. While we have discovered over 4,000 planets outside the solar system, the majority of these worlds have been identified through their influence on their star, either via a dimming of the star’s light as the planet transits across its surface, or the wobble of the star from the planet’s gravitational pull. These are incredibly powerful techniques for planet hunting, but neither allow us to actually lay eyes on the planet itself.
The method to actually see a planet is known as “direct imaging” and it is a tricky process, as the star’s light can easily overwhelm any radiation coming from the smaller, cooler planet. Exoplanet imagining has therefore focused on young Jupiter-sized worlds orbiting far from the powerful lighthouse of the star. These planets are large and their recent formation has left them packed with heat, with temperatures around 1340°F (727°C). Such hot houses emit thermal radiation at wavelengths around 5 microns, so most of the instruments dedicated to capturing planet pictures operate around this wavelength range.
However, these wavelengths are a bad choice if you want to try imaging an Earth-like world. As an evolved planet on a temperate orbit, thermal emission from a planet like our own is longer at about 10 – 20 microns. This is an awkward wavelength for observations from the Earth, as the Earth’s own thermal emission can swamp the distant signal of the planet.
Yet, being able to directly image temperate planets is an important technique for studying possible habitable worlds. As you move away from the star, the chances of the planet’s orbit transiting across the star’s surface from our view from Earth decreases. For a planet on a similar orbit to the Earth around a sun-like star, the probability is less than 0.5%. The only way to study many of these worlds may be if we can see them directly, and space-based observatories have been generally seen as the path to this kind of imaging.
But as described in a recent paper in Nature Communications, a team led by Kevin Wagner at the University of Arizona decided to give temperate planet imaging from the ground a shot. The results offer a promising glimpse of the future of imaging planets, and they may have identified a new planet in the habitable zone to boot.
The experiment was part of Breakthrough Watch, which receives funding from Breakthrough Initiatives founded by the billionaire Yuri Milner, and has the goal to search for Earth-sized planets around nearby stars. This project is named NEAR: New Earths in the α Centauri Region, which, as the name suggests, focuses on the binary star system, α Centauri. These twin stars have a similar size to the sun and, together with Proxima Centauri, are the nearest stars to our own system; a distance that helps to maximize the team’s chances of being able to capture a snapshot of a temperate planet.
Despite multiple attempts, no planets have so far been confirmed to be orbiting either α Centauri A or B. But the stars are of strong interest, as their proximity would make any planet one of the best opportunities for collecting data on a different planetary system.
Situated at 1.3 pc (4.3 light years) away from Earth, the habitable zone (the temperate region around the star where the Earth could support surface water) for the α Centauri stars is just resolvable with the current class of 8-meter telescopes. The team used an infrared camera, upgraded by Breakthrough Watch for the project, mounted on the European Southern Observatory’s (ESO) Very Large Telescope (VLT) that is situated in the Atacama Desert in Chile.
Despite using one of the top instruments currently available, the observation was a long one with around 100 hours of data collected. However, the effort was rewarded by a possible planet candidate situated just 10% further from α Centauri A than the Earth is from the sun. The team note that this is not definitely a planet, as the observation result could match that of an exozodical dust disc circling the star — that is, scattered light from the star that is re-emitted in the infrared by interplanetary dust particles. If it is a planet, it sits within the star’s habitable zone and is between Neptune and Saturn in size. This is too large to be rocky, but its discovery indicates that the α Centauri may harbor planets we have not yet seen, and also shows the potential to image those temperate worlds directly.
While on the edge of what can currently be achieved, the upcoming generation of 30-meter class of telescopes should have an easier time. Europe’s planned ELT (Extremely Large Telescope) should be able to detect Earth-sized planets within the habitable zone of α Centauri A within a few hours. This suggests that being able to stare at other rocky worlds from ground-based observatories is a medium-term possibility and one that could be quite important.