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A team of astronomers have found… well, something near one of the stars of Alpha Centauri that is consistent with the fact that it is a planet, possibly the size of Neptune. . However, to be very clear, this is not yet proven – it could be a cloud of dust or it might not even be real at all.
Call me “cautiously interested”. But if it’s a planet, that would be really cool.
Alpha Centauri is the closest star system to ours. It consists of three stars, a binary star made up of Alpha Centauri A and B, both orbited by Proxima Centauri, a dark 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 away 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 darker than the Sun.
Because they are close to us, it is easier to search for planets in their habitable zones, the distance from either star where liquid water might exist on a planet’s surface. To distant stars, this area looks so close to a star that it is difficult to see due to the glare. With stars closer together, it appears farther away and easier to spot. This makes α Cen a tempting target for direct imaging, literally taking an image of stars and searching for an exoplanet there.
In general, this method is best to use infrared (or IR) light, where the stars are paler and the planets brighter, thus increasing the contrast. This has worked well for quite a few exoplanets, but it favors those that are far from their stars (where the star’s background light is weaker), massive (making them brighter) and young (they are still hot. when a few million years old, so they emit more IR light).
Direct imagery is not optimized to look for Earth-like planets in habitable areas, which are cooler, smaller, and much closer to their stars. However, new cameras and techniques might 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 monstrous 8.2-meter telescope in Chile with a camera called VISIR: VLT Imager and Spectrometer for Mid-InfraRed. Most infrared cameras look at shorter wavelengths, around 5 microns (the reddest light the human eye can see is around 0.8 microns). VISIR looks at 10 microns, where cooler planets emit light, making it possible to see planets more similar to Earth.
This is what astronomers used to watch Alpha Centauri over 19 nights in May / June 2019. They obtained over 70 hours of useful observations and used many techniques to reduce the brightness of both stars, including including putting them behind a metal disc (called a coronograph) to block light from the inner part of the star to reduce glare, subtracting one star image from another to reduce a lot of 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 seen by one of the largest telescopes on Earth.
They found numerous artefacts – specks of light from processing techniques or reflections inside the telescope – which make it difficult to know exactly what is real and what is not. Still, they found a spot of light in a relatively clean part of the image where no known detector artifacts should be, and in the right place to be a planet. It is seen in several pictures, which increases the confidence that it is real. Previous observations taken a few years ago rule out that it is a star or a background galaxy. Most intriguing to me, when all the images are combined, it forms an elongated drop, consistent with the orbital motion of a planet around α Cen A over the 19 nights of observations!
Yet, and I stress it once again, they cannot be sure that it is a planet, or even real. I’ll note that there was a brief wave of excitement about the discovery of a possible planet orbiting α Cen a few years ago, but it was later retracted.
But if it is a planet, given its brightness, it corresponds to a planet between 3 and 11 times the diameter of the Earth orbiting the star at about 165 million kilometers, 1.1 times the distance of the Earth in relation to the Sun. It’s right in the star’s living area (closer to the inner edge, but still). Previous observations indicate that no planet more than 7 times the width of Earth should exist there, so it is likely (again if real) a planet the size of Neptune or larger.
They note that it may not be a planet, but rather a cloud of dust. The Sun, for example, has dust orbiting comets. It is weak, but would appear brighter in the middle of the IR. They calculate that α Cen A would need about 60 times the amount of dust orbiting the Sun to account for the drop, which is a lot, but other similar stars have been seen to have more, so it’s not not out of the question.
So is it a planet orbiting α Cen A? Mayyyyyybe. In my opinion, the authors are very circumspect about this, not making huge claims or drawing any conclusions. Looks like what they found is 1) real, and b) could be a planet.
Obviously, more observations are needed. I would love to see some taken at a time when, if it was a planet, it would be on the other side of the star as seen from Earth due to its orbital motion. If they found it there (and nothing where they’ve seen it before) I’d be much more convinced it’s real.
But as a proof of concept, this new camera and the methods used are rather encouraging. We know that planets can exist 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 are no reason to think that the stars do not have planets.
What we need is better evidence! But this is a great start, and I hope more observations will be available soon. I grew up in science fiction and Alpha Centauri’s aliens were so prevalent it was almost a cliché to use them (despite Zefram Cochrane). Of all the stars in the sky, I would be the most excited to find planets there.
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