New method for finding Earth-like planets
A new technique for reducing light interference during telescope observations enables imaging of Earth-like planets in nearby solar systems. An international team of astronomers and physicists, including researchers from Uppsala University, have used a diamond coronagraph to look for planets in the solar system Alpha Centauri, just 4.4 light years away. The method has demonstrated promising results.
Because of current technical limitations, it has been difficult to image exoplanets, i.e., planets outside of our solar system. Those images we do have are in most cases giant planets, much larger than Jupiter, which orbit very young stars. Often, these planets are also far from the habitable zone, where liquid water is found on the planet surface and creates the conditions for life.
One way to discover Earth-like planets in the habitable zone around stars similar to our sun in both size and age is to observe the sky in the infrared. These types of planets shine brightest at these wavelengths. But one problem is that both the atmosphere and the actual telescope emit infrared light, causing interference.
Developed at Ångström Laboratory
A new system has been developed to enable capturing images of smaller exoplanets by observing them over long periods, about 100 hours of observation time, in the middle of the infrared spectrum. The system has been used at the European Southern Observatory’s (ESA) Very Large Telescope (VLT) in Chile and has attained unmatched sensitivity using a diamond coronagraph that was produced at Ångström Laboratory at Uppsala University. Coronagraphs are used to reduced blinding light from a star and, in this way, accentuate the weaker light from any planets orbiting it.
“The method could, in a best case scenario, capture images of planets about 3 times the diameter of Earth in the habitable zone of Alpha Centauri A. That is equivalent to an improvement by a factor of 10 compared to existing systems for direct observations,” says Mikael Karlsson, the head of the research team within diamond optics at Uppsala University.
Studied a solar system 4.4 light years away
Alpha Centauri is our closest solar system and is just 4.4 light years away. From the southern hemisphere, it is visible with the naked eye and looks like a single bright star. But a telescope reveals it is actually two stars: Alpha Centauri A and B, which are both similar in size and age to our own sun. They orbit each other and are therefore classified as a binary star. A small, cool red dwarf called Proxima Centauri, in turns, orbits these stars. So far, two planets have been confirmed in this triple system. Both orbit Proxima Centauri.
“This system was the best candidate to test our method, since Alpha Centauri A and B are similar to our sun, but we do not know whether there are any planets orbiting either of them,” says Pontus Forsberg, a researcher at Uppsala University and the creator of the diamond coronagraph.
After the researchers used their method to eliminate interference that was confirmed to come from the telescope and instruments, a point of light emerged on the image. This could potentially be a planet ranging in size between Neptune and Saturn and located about the same distance from Alpha Centauri A as Earth is from the Sun, i.e., in the habitable zone.
“At this point, without further observation campaigns, we cannot exclude the possibility that the light comes from our instruments or possibly from an asymmetric dust cloud instead of a planet,” says Mikael Karlsson.
Refining the design
The researchers are continuing to refine the design and manufacture diamond coronagraphs so that the Extremely Large Telescope (ELT) currently being built in Chile can be equipped with similar components within five to eight years. An important mission for the ELT is searching for exoplanets. With this new technology, it will be possible to image planets as small as Earth in the habitable zones of Alpha Centauri and other nearby stars.
The diamond coronagraphs are being developed together with the Université de Liège and are the result of more than 10 years of technical development with funding from the European Research Council (ERC). The study is part of Breakthrough Initiatives, a global space science programme searching for Earth-like planets around nearby stars.
The study has been published in the scientific journal Nature Communications. Reference: K Wagner et al. (2021), Imaging low-mass planets within the habitable zone of α Centauri, Nature Communications. DOI: 10.1038/s41467-021-21176-6