Possible Super-Earth in the Habitable Zone at Alpha Centauri

Astronomers using a new technique may have not only found a super-Earth at a neighbouring star, but they may also have directly imaged it. And it could be nice and cozy in the habitable zone around Alpha Centauri.

It’s much easier to see giant planets than Earth-size planets. No matter which detection method is being used, larger planets are simply a larger needle in the cosmic haystack. But overall, astronomers are very interested in planets that are similar to Earth. And finding them is much more difficult.

We thought we’d have to wait for the ultra-powerful telescopes currently being built before we could directly image exoplanets. Facilities like the Giant Magellan Telescope and the European Extremely Large Telescope will bring enormous observing power to bear on the task of exoplanet imaging. But a team of researchers have developed a new technique that might do the job. They say they’ve imaged a possible sub-Neptune/super-Earth-sized planet orbiting one of our nearest neighbours, Alpha Centauri A.

The team presented their observations in an article in Nature Communications titled “Imaging low-mass planets within the habitable zone of ? Centauri.” The lead author is Kevin Wagner, an astronomer and Sagan Fellow at the University of Arizona.

“These results demonstrate the feasibility of imaging rocky habitable-zone exoplanets with current and upcoming telescopes.”

From “”Imaging low-mass planets within the habitable zone of ? Centauri.”

While astronomers have found low-mass exoplanets before, they’ve never sensed their light. They’ve watched as the planets revealed themselves by tugging on their stars. And they’ve watched as the light from the stars that host these planets dips when the planet passes in front of the star. But they’ve never directly imaged one. Until now, maybe.

This new detection method comes down to the infrared. One of the challenges in imaging Earth-sized exoplanets in infrared is to discern the light coming from an exoplanet when that light is washed out by all of the background infrared radiation from the star. Astronomers can search for exoplanets in wavelengths where the background infrared is diminished, but in those same wavelengths, temperate Earth-like planets are faint.

One method is to look in the near-infrared (NIR) part of the spectrum. In NIR, the thermal glow of the planet is not so washed out by the star. But the starlight is still blinding, and millions of times brighter than the planet. So just looking in the NIR is not a total solution.

Artist’s impression of a sunset seen from the surface of an Earth-like exoplanet. Credit: ESO/L. Calçada

The solution may be the NEAR (New Earths in the AlphaCen Region) instrument used in this research. NEAR is mounted on the ESO (European Southern Observatory’s) Very Large Telescope (VLT) in Chile. It works with the VISIR instrument, also on the VLT. The group behind NEAR is the Breakthrough Watch, part of Yuri Milner’s Breakthrough Initiatives.

The NEAR instrument not only observes in the desirable part of the infrared spectrum, but it also employs a coronagraph. The Breakthrough group thought that the NEAR instrument used on an 8-meter ground-based telescope would allow for better observations of the Alpha Centauri system and its planets. So they built the instrument in collaboration with the ESO and installed it on the Very Large Telescope.

ESO’s Very Large Telescope (VLT) has recently received an upgraded addition to its suite of advanced instruments. On 21 May 2019 the newly modified instrument VISIR (VLT Imager and Spectrometer for mid-Infrared) made its first observations since being modified to aid in the search for potentially habitable planets in the Alpha Centauri system, the closest star system to Earth. This stunning image of the VLT is painted with the colours of sunset and reflected in water on the platform. While inclement weather at Cerro Paranal is unfortunate for the astronomers using it, it lets us see ESO’s flagship telescope in a new light. Image Credit: ESO/VLT

This new finding came as a result of 100 hours of cumulative observations with NEAR and the VLT. “These results,” the authors write, “demonstrate the feasibility of imaging rocky habitable-zone exoplanets with current and upcoming telescopes.”

The 100-hour commissioning run was meant to demonstrate the power of the instrument. The team says that based on about 80% of the best images from that run, the NEAR instrument is an order of magnitude better than other methods for observing “…warm sub-Neptune-sized planets throughout much of the habitable zone of ? Centauri A.”

They also, possibly, found a planet. “We also discuss a possible exoplanet or exozodiacal disk detection around ? Centauri A,” they write. “However, an instrumental artifact of unknown origin cannot be ruled out.”

A figure from the study. Panel (a) on the left shows artifacts 1 and 2, which are called “detector persistence.” 3 is an “optical ghost” of Alpha Centauri A. Panel (b) on the right is a zoom-in on the inner regions of the (a) image. The candidate exoplanet is labelled as C1. Image Credit: Wagner et al, 2021.

This isn’t the first time astronomers have found exoplanets in the Alpha Centauri system. There are a couple of confirmed planets in the system, and there are also other candidates. But none of them have been directly imaged like this new potential planet, which has the placeholder name C1, and is the first potential detection around the M-dwarf in the system, Proxima Centauri.

Follow-up observations will have to confirm or cancel the discovery. The researchers say there’s a possibility that the signal could be an instrument artifact. “We also discuss a possible exoplanet or exozodiacal disk detection around ? Centauri A,” they write. “However, an instrumental artifact of unknown origin cannot be ruled out.”

It’s exciting to think that a warm-Neptune class exoplanet could be orbiting a Sun-like star in our nearest neighbouring star system. One of the Breakthrough Initiatives goals is to send lightsail spacecraft to the Alpha Centauri system and give us a closer look.

The foreground of this image shows ESO’s Very Large Telescope (VLT) at the Paranal Observatory in Chile. The rich stellar backdrop to the picture includes the bright star Alpha Centauri, the closest stellar system to Earth. In late 2016 ESO signed an agreement with the Breakthrough Initiatives to adapt the VLT instrumentation to conduct a search for planets in the Alpha Centauri system. Such planets could be the targets for an eventual launch of miniature space probes by the Breakthrough Starshot Initiative. Credit: ESO

But that prospect is out of reach for now. And in some ways, this discovery isn’t so much about the planet, but about the technology developed to detect it.

The large majority of discovered exoplanets are gigantic planets similar in mass to Jupiter, Saturn, and Neptune. They’re the easiest to find. But as humans from Earth, we’re predominantly interested in planets like our own. Earth-like planets in a star’s habitable zone get us excited about prospects for life on another planet. But they can also tell us a lot about our own Solar System, and how solar systems in general form and evolve.

If C1 does turn out to be a planet, then the Breakthrough group has succeeded in a vital endeavour. They’re the first to detect an Earth-like planet by direct imaging. Not only that, but they did it with an 8-meter, ground-based telescope and an instrument specifically designed and developed to detect these types of planets in the Alpha Centauri system.

The authors are confident that NEAR can perform well, even in comparison to much larger telescopes. The conclusion of the paper contains a description of the overall sensitivity of the instrument. Then they write that “This would in principle be sufficient to detect an Earth-analog planet around ? Centauri A (~20 µJy) in just a few hours, which is consistent with expectations for the ELTs.”

The E-ELT will have a 39-meter primary mirror. One of its capabilities and design goals is to image exoplanets, especially smaller, Earth-size ones, directly.

The ELT should see first light in 2024. This illustration shows the scale of the telescope, and also shows its segmented primary mirror, which is 39.3-metres in diameter (130-foot). Image Credit: ESO

Of course, the E-ELT will be an enormously powerful telescope that will undoubtedly fuel scientific discovery for a long time, not just in exoplanet imaging but in a variety of other ways. And other gigantic ground-based telescopes will change the exoplanet imaging game, too. What took hours for NEAR to see may take only minutes for the E-ELT, the Thirty Meter Telescope, or the Giant Magellan Telescope to see.

NEAR can’t compete with those telescopes and was never meant to.

But if these results are confirmed, then NEAR has succeeded where nobody else has, and for a fraction of the price of a new telescope. Either way, what NEAR has accomplished likely represents the future of exoplanet research. Rather than broad-based surveys like Kepler and TESS, scientists will soon be able to focus on individual planets.


Evan Gough

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