A team of astronomers has found something near one of Alpha Centauri’s stars that corresponds to it being a planet possibly about the size of Neptune. To be very clear, it has not yet been proven – it may be a cloud of dust or it may not be real at all.
Call me “cautiously interested.” But if it’s a planet, that would be really cool indeed.
Alpha Centauri is the closest galaxy to ours. It consists of three stars, a binary consisting of Alpha Centauri A and B, both orbiting Proxima Centauri, a faint red dwarf. Proxima probably has at least two planets, and there is evidence of a third.
The binary number is 4.37 light years 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 fainter than the Sun.
Because they are close to us, it is easier to look for planets in their habitable zones, the distance to both stars where liquid water could exist on a planet’s surface. To distant stars, that zone appears so close to a star that it is difficult to see because of its brilliance. With stars that are closer, it appears further away and easier to spot. That makes α Cen a tempting target for direct imaging, which literally takes a picture of the stars and searches for an extrasolar planet.
In general, this method is best with infrared (or IR) light, where stars are fainter and planets brighter, increasing contrast. It worked well for quite a few exoplanets, but it prefers the ones that are far from their stars (where the star’s background light is fainter), massive (making them brighter), and young (they’re still hot as a few million years old, so they give off more IR light).
Direct imaging is not optimized to find Earth-like planets in habitable zones that are cooler, smaller, and much closer to their stars. However, new cameras and techniques may be able to see these planets that may not be bubbling cauldrons of heat.
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 telescope in Chile with a camera called VISIR: VLT Imager and Spectrometer for mid-InfraRed. Most IR cameras look at shorter wavelengths, about 5 microns (the redest light the human eye can see is about 0.8 microns). VISIR looks at 10 microns, where cooler planets emit light, making planets more like Earth.
This is what the astronomers used to look at Alpha Centauri over the course of 19 nights in May / June 2019. They received more than 70 hours of useful observations and used many techniques to reduce the brightness of the two stars, including them behind. a metal disk (called a coronagraph) to block light from the inner part of the star to reduce glare, 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 from two of the brightest stars in the sky, seen through one of the largest telescopes on Earth.
They found many artifacts – spots of light due to processing techniques or reflections in the telescope – that made it difficult to know what is really real and what is not. Still, they found a flash of light in a relatively clean area of the image where there should be no known detector artifacts, and in the right place to be a planet. It can be seen in multiple images, instilling confidence that it is real. Previous observations from a few years ago have shown that it is a background star or galaxy. Most intriguing to me, when all the images are combined, it forms an elongated blob, consistent with the orbital motion of a planet around α Cen A during the 19 nights of observations!
Still, and again, I emphasize this, they cannot be sure that it is a planet, or even real. I will note that there was a short wave of excitement over the discovery of a possible planet orbiting α C a few years ago, but it was later retracted.
But if it is a planet, given its brightness, it fits a planet between 3-11 times the diameter of the Earth orbiting the star about 165 million kilometers, 1.1 times the distance from the Earth to the sun. That’s right in the star’s habitable zone (closer to the inner rim, but still). Previous observations indicate that no planet should exist there that would be more than about 7 times the width of the Earth, so it is likely (again if it is real) to be a planet about that size as Neptune.
They note that it may not be a planet, but a cloud of dust. For example, the sun has thrown dust orbiting it off comets. It’s dim, but would appear brighter in the center of the IR. They calculate that α C and A takes about 60 times the amount of dust orbiting the Sun to explain the blob, which is a lot, but other similar stars have been seen that have more, so it’s not ruled out.
So is this a planet orbiting α C and A? Mayyyyyybe. The authors are, in my opinion, quite observant about this, making no big claims or hasty conclusions. It seems that what they found is 1) real, and b) could be a planet.
Obviously, more observations are needed. I would like to see some taken at a time when, if it is a planet, it would be on the other side of the star, as seen from Earth, because of its orbital motion. If they found it there (and nothing where they’ve seen it before) I’d be a lot more confident that it’s real.
But as a proof of concept, this new camera and the methods they used are quite encouraging. We know that planets can exist around stars like the two in α Cen – our own sun is proof of that, and we have found many planets orbiting stars in binaries – so there is no reason to think that the stars don’t have planets.
What we need is better evidence! But this is a really good start, and I hope more observations are coming soon. I grew up sci-fi, and Alpha Centauri’s aliens were so widespread it was almost cliché to use them (despite Zefram Cochrane). Of all the stars in the sky, I would be the most excited to find planets there.