A team of astronomers has found … well, something close to one of the stars of Alpha Centauri that corresponds to the fact that it is a planet, possibly as large as Neptune. To be very clear, however, this has not yet been proven – it could be a cloud of dust or even not really at all.
Call me “cautiously interested.” But if it’s a planet, it’s gonna be really cool.
Alpha Centauri is the closest galaxy to ours. It consists of three stars, a binary star consisting of Alpha Centauri A and B, both orbiting Proxima Centauri, a dull red dwarf. Proxima is likely to have at least two planets, and there is evidence for a third.
The binary is 4.37 light years from us. Alpha Centauri A (or α Cen A), is slightly more massive, warmer and brighter than the sun, and the other (α Cen B) is less massive, cooler and duller than the sun.
Because they are close to us, it is easier to search for planets in their habitable zones, the distance from any star where liquid water can exist on the planet’s surface. For stars in the distance, the zone looks so close to a star that it is difficult to see because of its brightness. With stronger stars, it seems farther and easier to spot. This makes α Cen an attractive target for direct imaging, literally taking an image of the stars and searching for an exoplanet in it.
In general, this method uses the best infrared (or IR) light, where stars are dimmer and planets are brighter, which increases the contrast. It worked well for quite a few exoplanets, but it benefits stellar people who are far from their stars (where the star’s backlight is weaker), massive (which makes them brighter) and young (they are still hot than a few million years old, so they emit more IR light).
Direct imaging is not suitable for searching for earthy planets in habitable zones, which are cooler, smaller and much closer to their stars. However, new cameras and techniques can see these planets that may not be bubbling heat cabinets.
New Earth in the α Centauri region (or NEAR) is an experiment using these new methods to search for planets around Alpha Centauri. It uses the sample 8.2 meter Very Large Telescope in Chile with a camera called VISIR: VLT Imager and Spectrometer for middle infrared. Most IR cameras look at shorter wavelengths, about 5 microns (the reddest light the human eye can see is about 0.8 microns). VISIR looks at 10 microns, where cooler planets emit light, making it possible to see planets more like the earth.
This is what the stargazers watched over the course of 19 nights in May / June 2019 at Alpha Centauri. They obtained more than 70 hours of useful observations and used many techniques to diminish the brightness of the two stars, behind a metal disk (called a corona grave) to block the light from the inner part of the star to shine. to reduce, and subtract one constellation from another to reduce much of the light, and more. Some of these methods are quite sophisticated and are necessary to reduce the light of two of the brightest stars in the sky when viewed through one of the largest telescopes on earth.
They have found many artifacts – light spots due to processing techniques or reflections in the telescope – that make it difficult to know what is exactly and what is not. Yet they found a ray of light in a relatively clean part of the image where there should be no known detector artifacts, and in the right place to be a planet. It is seen in various images, which gives rise to the confidence that it is genuine. Previous observations made a few years ago exclude that it is a background star or galaxy. The most interesting thing to me is that when all the images are combined, it forms an elongated spot that corresponds to the orbital motion of a planet around α Cen A during the 19 nights of observations!
Yet, and again I emphasize this, they can not be sure that it is a planet, or even a real planet. I will note that there was a brief surge of excitement about the discovery of a possible planet orbiting α Cen a few years back, but it was later withdrawn.
But if it is a planet, it fits, given its brightness, to a planet between 3–11 times the diameter of the earth orbiting the star about 165 million kilometers, 1.1 times the earth’s distance from the sun . It’s right in the habitable zone of the star (closer to the inside, but still). Previous observations indicate that no planet may exist more than about 7 times the width of the Earth, so it is likely (again if true) to be a planet about the size of Neptune.
They note that it may not be a planet, but a cloud of dust. The sun, for example, has dust orbiting comets. It is faint, but looks brighter in the middle of the IR. They calculate that α Cen A requires about 60 times the amount of dust that the sun orbits to explain the spot, 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 it a planet orbiting α Cen A? Mayyyyyybe. The authors, in my opinion, are pretty careful about it, and do not make big claims or draw any conclusions. It seems that what they have found 1) is real, and b) may be a planet.
It is clear that more observations are needed. I would like to see some taken at some point, if it were a planet, it would be on the other side of the star, seen from Earth due to its orbital motion. If they would find it there (and nothing where they had seen it before), I would have much more confidence that it really is.
But as proof of the concept, this new camera and the methods they use are quite encouraging. We know that planets around stars like the two in α Cen can exist – our own Sun is proof of that, and we’ve found many planets orbiting stars in binaries – so there’s no reason to think that the stars does not have planets.
What we need is better evidence! But this is a very good start, and I hope there will be more observations soon. I grew up on sci-fi, and aliens from Alpha Centauri were so common that it was almost cliché to use them (despite Zefram Cochrane). Of all the stars in the sky, I would be very excited to find planets there.