A new way to search for life-sustaining planets

It is now possible to capture images of planets that could potentially sustain life around stars in the area, thanks to advances reported by an international team of astronomers in the journal Nature communication.

Using a newly developed system for middle-infrared exoplanet imaging, combined with a very long observation time, the authors of the study say they can now use ground-based telescopes to take images of planets about three times the size of Earth. directly within the habitable zones. of stars in the area.

Attempts to image exoplanets directly – planets outside our solar system – have been hampered by technological constraints, which have led to the detection of easily visible planets much larger than Jupiter and around many young stars and far beyond the habitable zone – the ‘sweet spot’ in which a planet can sustain liquid water. If astronomers want to find alien life, they should look elsewhere.

“If we want to find planets with living conditions as we know them, we need to look for rocky planets that are about the size of Earth, within the habitable zones around older, sun-like stars,” said Kevin’s first author. Wagner, a Sagan Fellow in NASA’s Hubble Fellowship Program at the Steward Observatory of the University of Arizona.

The method described in the article offers more than a tenfold improvement of the existing capabilities to observe exoplanets directly, Wagner said. Most studies on exoplanet imaging have looked at infrared wavelengths of less than 10 microns, reaching just less than the wavelengths where such planets shine most brightly, Wagner said.

“There’s a good reason for that, because the earth itself shines at you at those wavelengths,” Wagner said. “Infrared emissions from the sky, the camera and the telescope itself essentially numb your signal. But the good reason to focus on these wavelengths is that this is where an earthy planet in the habitable area around a sunny star the brightest is going to shine. “

The team used the Very Large Telescope, or VLT, from the European Southern Observatory in Chile to observe our nearest neighboring system: Alpha Centauri, just 4.4 light-years away. Alpha Centauri is a triple galaxy; it consists of two stars – Alpha Centauri A and B – which are similar in size and age to the sun and orbit each other as a binary system. The third star, Alpha Centauri C, better known as Proxima Centauri, is a much smaller red dwarf orbiting its two siblings at a great distance.

A planet that is not quite twice the size of Earth and orbits in the habitable zone around Proxima Centauri has already been detected indirectly by observations of the radial velocity variation of the star, or by the small pendulum that a star below the tugboat of the invisible planet displayed. According to the authors of the study, Alpha Centauri A and B may present similar planets, but indirect detection methods are not yet sensitive enough to find rocky planets in their more proven areas, Wagner explained.

“With direct imagery, we can now move below the detection limits for the first time,” he said.

To increase the sensitivity of the image setup, the team used a so-called adaptive secondary telescope mirror that can correct the distortion of light through the Earth’s atmosphere. In addition, the researchers used a starlight-blocking mask that optimized them for the middle-infrared light spectrum to block the light of one of the stars at a time. In order to observe the habitable zones of both stars simultaneously, they also pioneered a new technique to switch back and forth rapidly and rapidly between the observations of Alpha Centauri A and Alpha Centauri B.

“We move one star and one star from the cliff paragraph every tenth of one second,” Wagner said. “It allows us to observe each star half the time, and, more importantly, it also allows us to subtract one frame from the next frame, which is all essentially just noise, removed from the camera and the telescope. “

Using this approach, the unwanted starlight and “noise” – unwanted signal from within the telescope and camera – become essentially random background noises, which can be further reduced by stacking images and subtracting the noise using specialized software.

Similar to the effect on noise-canceling headphones, which emit soft music over a steady stream of unwanted jet engine noise, the technique has enabled the team to remove as much of the unwanted noise as possible and the much fainter signals emitted by potential planetary candidates within the habitable zone.

The team observed the Alpha Centauri system for nearly 100 hours over the course of a month in 2019, collecting more than 5 million images. They collected approximately 7 terabytes of data, which they made publicly available at http://archive.eso.org.

“This is one of the first dedicated multi-night exoplanet image campaigns, in which we stacked and used all the data we had collected for almost a month to reach our final sensitivity,” Wagner said.

After the removal of so-called artifacts – false signals removed from the coronary shaft by the instrumentation and residual light – the final image revealed a light source designated ‘C1’ which could possibly indicate the presence of a candidate for an exoplanet within the habitable zone.

“There is one point source that looks like what we would expect a planet to look like, which we cannot explain with the systematic error correction,” Wagner said. “We are not at the level of confidence to say that we have discovered a planet around Alpha Centauri, but there is a signal that could be with a subsequent verification.”

Simulations of what planets within the data look like suggest that ‘C1’ may be a planet from Neptune to Saturn at a distance from Alpha Centauri A that is similar to the distance between Earth and the sun, Wagner said. However, the authors clearly state that the possibility that C1 is due to the unknown artifact caused by the instrument itself cannot yet be ruled out without subsequent verification.

Finding a potentially habitable planet within Alpha Centauri was the goal of the Breakthrough Watch / NEAR initiative, which stands for New Earths in the Alpha Centauri region. Breakthrough Watch is a global astronomical program looking for Earth-like planets around stars in the area.

“We are very grateful for the breakthrough initiatives and ESO for their support in reaching another milestone for the imagery of Earth-like planets around our neighboring stars,” said Markus Kasper, lead scientist of the NEAR project, and a co-author of the article said.

The team plans to launch another imaging campaign within a few years in an effort to capture this potential exoplanet in the Alpha Centauri system elsewhere, and to see if it matches what is expected is based on the modeling of the expected orbit. Further clues may come from follow-up observations using different methods.

The next generation of extremely large telescopes, such as the Extremely Large Telescope of the European Southern Observatory, and the Giant Magellan Telescope, for which the University of Arizona manufactures the primary mirrors, are expected to increase direct observations of nearby stars. which will likely house planets in their habitable zones by a factor of 10, Wagner explained. Candidates to look at include Sirius, the brightest star in the night sky, and Tau Ceti, which presents an indirectly perceived planetary system that Wagner and his colleagues will try to portray directly.

‘To make the ability shown here a routine observation mode – to pick up heat signatures of planets orbiting in the habitable zones of nearby stars, will be a game changer for exploring new worlds and for the search for life in the universe, “said co-author Daniel Apai, an associate professor of astronomy and planetary science at UArizona, who leads the NASA-funded Earths in Other Solar Systems program that partially supported the study.