A New Approach to the Search for Habitable Planets

A new mid-infrared exoplanet imaging system, combined with a long observation time, enables ground-based telescopes to directly capture images of planets three times the size of Earth within the habitable zones of nearby stars.

Thanks to advances reported in the journal Nature Communications by an international team of astronomers, it is now possible to capture images of planets that could potentially sustain life around nearby stars.

Efforts to directly image exoplanets – planets outside our solar system – have been hampered by technological limitations, resulting in a bias toward the detection of larger-than-Jupiter planets that are located around very young stars and far outside the habitable zone – the “sweet spot” in which a planet can sustain liquid water. Astronomers must look elsewhere for alien life if they are to find it.

“If we want to find planets with conditions suitable for life as we know it, we have to look for rocky planets roughly the size of Earth, inside the habitable zones around older, sun-like stars,” said the paper’s first author, Kevin Wagner, a Sagan Fellow in NASA’s Hubble Fellowship Program at the University of Arizona’s Steward Observatory.

Wagner claims that the method described in the paper is a tenfold improvement over existing capabilities for directly observing exoplanets. According to Wagner, most studies on exoplanet imaging have looked at infrared wavelengths of less than 10 microns, falling just short of the wavelength range where such planets shine the brightest.

“There’s a good reason for that,” Wagner explained, “because the Earth itself is shining at you at those wavelengths.” “Your signal is being drowned out by infrared emissions from the sky, the camera, and the telescope itself. The good reason to concentrate on these wavelengths is that they are where an Earth-like planet in the habitable zone of a sun-like star will shine brightest.”

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

The team used the European Southern Observatory’s Very Large Telescope (VLT) in Chile to observe our nearest neighbor star system, Alpha Centauri, which is only 4.4 light-years away. Alpha Centauri is a triple star system composed of two stars, Alpha Centauri A and B, that are similar in size and age to the sun and orbit each other in a binary system. Alpha Centauri C, also known as Proxima Centauri, is a much smaller red dwarf that orbits its two siblings at a great distance.

A planet about twice the size of Earth that orbits in the habitable zone around Proxima Centauri has already been detected indirectly via observations of the star’s radial velocity variation, or the tiny wobble a star exhibits under the tug of the unseen planet. According to the study’s authors, Alpha Centauri A and B may have similar planets, but indirect detection methods are not yet sensitive enough to detect rocky planets in their more widely separated habitable zones, as Wagner explained.

“With direct imaging, we can now push beneath those detection limits for the first time,” he said.

The team used an adaptive secondary telescope mirror to increase the sensitivity of the imaging setup, which can correct for light distortion caused by the Earth’s atmosphere. Furthermore, to block the light from one of the stars at a time, the researchers used a starlight-blocking mask optimized for the mid-infrared light spectrum. They also pioneered a new technique for rapidly switching between observing Alpha Centauri A and Alpha Centauri B in order to observe both stars’ habitable zones at the same time.

A new way to look for life-sustaining planets

“Every tenth of a second, we move one star on and one star off the coronagraph,” Wagner explained. “This allows us to observe each star for half the time, and, more importantly, it allows us to subtract one frame from the subsequent frame, removing everything that is essentially just noise from the camera and telescope.”

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

The technique allowed the team to remove as much of the unwanted noise as possible and detect the much fainter signals created by potential planet candidates inside the habitable zone, similar to the effect of noise-canceling headphones, which allow soft music to be heard over a steady stream of unwanted jet engine noise.

Over the course of a month in 2019, the team observed the Alpha Centauri system for nearly 100 hours, collecting over 5 million images. “This is one of the first dedicated multi-night exoplanet imaging campaigns,” Wagner explained. “We stacked all of the data we accumulated over nearly a month and used that to achieve our final sensitivity.”

After removing so-called artifacts, which are false signals produced by the instrumentation and residual light from the coronagraph, the final image revealed a light source labeled “C1,” which could potentially indicate the presence of an exoplanet candidate within the habitable zone.

“There is one point source that looks like what we would expect a planet to look like, and we can’t explain it with any of the systematic error corrections,” Wagner explained. “We are not confident enough to say we discovered a planet around Alpha Centauri, but there is a signal there that could be that with further verification.”

According to Wagner, simulations of what planets in the data might look like suggesting that “C1” could be a Neptune- to Saturn-sized planet at a distance from Alpha Centauri A comparable to the distance between the Earth and the sun. However, the authors state unequivocally that without further verification, the possibility that C1 is due to some unknown artifact caused by the instrument itself cannot be ruled out.

The goal of the Breakthrough Watch/NEAR initiative, which stands for New Earths in the Alpha Centauri Region, has been to find a potentially habitable planet within Alpha Centauri. Breakthrough Watch is a global astronomical program that searches for Earth-like planets in the vicinity of nearby stars.

“We are very grateful to the Breakthrough Initiatives and ESO for their support in achieving another steppingstone toward the imaging of Earthlike planets around our neighbor stars,” said Markus Kasper, the NEAR project’s lead scientist and co-author on the paper.

In a few years, the team plans to launch another imaging campaign in an attempt to catch this potential exoplanet in the Alpha Centauri system in a different location and see if it is consistent with what would be expected based on modeling its expected orbit. Follow-up observations using various methods may yield additional clues.

The next generation of extremely large telescopes, such as the European Southern Observatory’s Extremely Large Telescope and the Giant Magellan Telescope, for which the University of Arizona produces primary mirrors, are expected to increase direct observations of nearby stars that may harbor planets in their habitable zones by a factor of ten, according to Wagner. Sirius, the brightest star in the night sky, and Tau Ceti, which hosts an indirectly observed planetary system that Wagner and his colleagues will attempt to directly image, are among the candidates to be studied.

“Making the capability demonstrated here a routine observing mode — picking up heat signatures of planets orbiting within the habitable zones of nearby stars — will be a game-changer for the exploration of new worlds and the search for life in the universe,” said study co-author Daniel Apai, an Arizona associate professor of astronomy and planetary science who leads the NASA-funded project.

NEAR was primarily funded by the Breakthrough Watch program and the European Southern Observatory (ESO). Breakthrough Watch is run by Breakthrough Initiatives, which is funded by the Breakthrough Foundation. Breakthrough Watch provided the instrument upgrades that allowed the observations to be made, and ESO provided telescope time.

Please see the research paper “Imaging low-mass planets within Alpha Centauri’s habitable zone” for a complete list of authors and institutions, as well as funding information.