A team of astronomers has found … well, something close to one of the Alpha Centauri stars that is consistent with the fact that it is a planet, possibly around the size of Neptune. However, to be very clear, it is not yet proven: it could be a cloud of dust or it could not even be real.
Call me “interested with caution.” But if it’s a planet, it would be a lot of fun.
Alpha Centauri is the closest stellar system to ours. It consists of three stars, a binary star formed by Alpha Centauri A and B, both orbited by Proxima Centauri, a faint red dwarf. Proxima is likely to have at least two planets, and there is evidence of a third.
The track is 4.37 light years away from us. Alpha Centauri A (or α Cen A), is slightly more massive, hotter, and brighter than the Sun, and the other (α Cen B) is less massive, cooler, and weaker than the Sun.
Because they are close to us, it is easier to look for planets in their habitable zones, the distance from any star where there may be liquid water on the surface of the planet. For distant stars, this area appears so close to a star that it is difficult to see due to glare. With closer stars, it appears farther away and easier to detect. This makes α Cen a tempting target for the direct image, literally taking a picture of the stars and looking for an exoplanet.
In general, this method is best used with infrared (or IR) light, where the stars are fainter and the planets are brighter, thus increasing the contrast. It has worked well for many exoplanets, but favors those that are far from their stars (where the star’s backlight is faint), massive (which makes them brighter), and young (they are still hot when there is a few million years, so they emit more IR light).
The direct image is not optimized to look for Earth-like planets in habitable areas, which are colder, smaller, and much closer to their stars. Still, new cameras and techniques may be able to see these planets that may not be bubbling with heat boilers.
New Earths in the α Centauri Region (or NEAR) is an experiment to use these new methods to search for planets around Alpha Centauri. It uses the very large 8.2 meter Monster Telescope in Chile with a camera called VISIR: VLT Imager and Spectrometer for medium infrared. Most IR cameras have shorter wavelengths, around 5 microns (the reddest light the human eye can see is about 0.8 microns). VISIR looks at 10 microns, where the coolest planets emit light, allowing you to see more Earth-like planets.
This is what astronomers used to observe at Alpha Centauri over 19 nights in May / June 2019. They obtained more than 70 hours of useful observations and used many techniques to reduce the brightness of the two stars, including the setting. behind a metal disk (called a coronograph) to block light from the inside of the star to reduce glare, subtracting one star image from another to reduce much light and more. Some of these methods are quite sophisticated and are needed to reduce the light of two of the brightest stars in the sky when viewed in one of the largest telescopes on Earth.
They found many artifacts (light spots due to processing techniques or reflections inside the telescope), making it difficult to know what is really and what is not. Still, they found some light in a relatively clean part of the image where there would be no known detector artifacts and in the right place to be a planet. It is seen in several images, which increases the confidence that it is real. Preliminary observations made a few years ago rule out ruling out whether it is a background star or a galaxy. The funny thing for me is that when all the images combine, it forms an elongated bubble, consistent with the orbital motion of a planet around α Cen A during the 19 nights of observations.
Still, and I stress it again, they can’t be sure it’s a planet, not even real. I will note that there was a brief rain of excitement at the discovery of a possible planet orbiting around α Cen a few years ago, but it later receded.
But if it is a planet, given its brightness, it fits with a planet between 3 and 11 times the diameter of the Earth orbiting the star at about 165 million kilometers, 1.1 times the Earth’s distance from the Sun. This is right in the habitable zone of the star (closer to the inner edge, but still). The above observations indicate that there should be no more than 7 times the width of the Earth there, so it is likely (again if it is real) to be a planet about the size of Neptune more or less.
They observe that it may not be a planet, but a cloud of dust. The Sun, for example, has a dust that surrounds it around comets. It is faint, but would look brighter in the middle of IR. They calculate that α Cen A would need about 60 times the amount of dust orbiting the Sun to explain the blob, which is a lot, but it has been seen that other similar stars have more, so that is not out of the question.
So, is this a planet orbiting around α Cen A? Mayyyyyybe. In my opinion, the authors are adequately cautious about this, without making huge claims or reaching any conclusions. It seems that what they found is 1) real, and b) could be a planet.
Clearly more observations are needed. I would love to see some of its prey at a time when, if it is a planet, it would be on the other side of the star as seen from Earth due to its orbital motion. If they found him there (and nothing where they had seen him before), he would be much more certain that he is real.
But as proof of concept, this new camera and the methods they employed are quite encouraging. We know that there can be planets around stars like the two in α Cen: our own Sun is proof of this, and we have found many planets orbiting stars in binaries, so there is no reason to think that stars they have no planets.
What we need are better evidence. But this is a very good start and I hope more observations come soon. I grew up in science fiction and Alpha Centauri aliens were so prevalent that it was almost a cliché to use them (despite Zefram Cochrane). Of all the stars in the sky, I would like to find planets.