Bad astronomy | Third planet Proxima Centauri may have been discovered
At just over four light years away, Proxima Centauri is the closest star to the Sun in the entire Universe. We know it has one planet, called Proxima b, and there’s decent evidence for a second, called Proxima c.
A team of astronomers has announced that there could be a the third planet, now too! If confirmed, it will be called Proxima d and would be one of the smallest and most massive exoplanets ever discovered.
Yeah, maybe that’s a big deal.
Proxima, as the star is usually called, is a red dwarf about 1/7and the diameter of the Sun – not much larger than Jupiter, really – and about 1/8and its mass. It’s very dark, bright with only 1/20,000and of the Sun’s luminosity in visible light. Although it’s the closest star to our solar system, you need a decent telescope to see it. It wasn’t even discovered until 1915.
It orbits the binary star Alpha Centauri, composed of two stars closer to the Sun. Proxima’s orbit is huge, about a tenth of a light-year from binary, and it takes more than half a million years to circle once.
In 2016, the first planet was discovered orbiting Proxima. It is a little larger than Earth and orbits the red dwarf roughly every 11 days. Because Proxima is so faint, the planet receives almost as much energy from the star as Earth, despite being only about 7 million kilometers away from it – Earth is 150 million kilometers away. of the Sun, for comparison.
A second planet could orbit Proxima, announced in 2019. The evidence for this is good, but just a long way from being called confirmed, although there is independent evidence as well.
[Note this video talks about the third planet as if it’s confirmed, but as yet it is not.]
Looking at data from the second planet, there was evidence of a third, so astronomers followed up with more observations (link to article). The planet is far too faint to be seen against the star’s brightness, so it is not detected directly. Instead, they use what is called the radial Where reflex speed method – conveniently both give the abbreviation VR method. The star’s gravity causes the planet to orbit around it, but a planet has gravity, so they both orbit around their mutual center of mass, called the centroidthe planet making a wide path around it and the star a much smaller one.
As it orbits, half the time the star is heading towards us and half the time it is moving away. We then see a Doppler shift, a blue and red shift, respectively, as the star moves. We call the movement towards or away from us radialand the cause is one reflex of the movement of the planet, that’s where the name comes from. Also, the more massive the planet and the smaller its orbit, the greater the movement of the star and the easier it is to detect.
Astronomers observed Proxima with the Very Large Telescope using a camera called ESPRESSO – the SPectrograph Echelle for rocky exoplanets and stable spectroscopic observations – which is a very sophisticated and precise type of spectrograph, an instrument that breaks down the light from a star in thousands of extremely narrow individual colors to find the Doppler shift. As the features of the star’s spectrum move back and forth, the star’s velocity can be found.
In this case, this movement is extremely weak and extremely difficult to measure. Yet they found a signal indicating periodic motion that cycles every 5.12 days. The range of motion is incredibly small: Just 39 centimeters per second. That’s slower than the average walking speed! But if correct, that’s the orbital velocity of the star moving around the barycenter. This implies that the planet has a mass about ¼ that of Earth – about twice that of Mars – and has a period (that’s “the year”) of 5.12 Earth days. Only a handful of small planets orbiting other stars have been discovered so far. L98-59b is only 40% the mass of Earth, and Kepler-37b is only slightly larger than the Moon! These records will no doubt be broken over time.
Proxima d’s orbit would be quite circular, meaning it must be just over 4 million kilometers from the star. This is Close, less than a tenth of the distance from Mercury to the Sun. But Proxima is so faint that the planet wouldn’t be as hot as our innermost world, while remaining near the boiling point of water. Given the usual mix of materials that small planets are made of – rock and metals – it’s likely that Earth’s diameter is around 80%.
Now watch out, that’s all if the planet exists. Astronomers have used two different ways of analyzing the observations and I have to say they look pretty good – planet b shows up very well in their data – but Proxima is a weird star. It is highly variable, emits flares, and probably contains many star spots (like sunspots on another star). This can make measuring its speed very tricky. The team clearly analyzed the data carefully and the signal is good, but astronomers are very cautious. We will want to see independent data before declaring this planet to be real.
They discovered that there was a 2% chance that the planet could transit the star, passing directly in front of it. This makes for a mini-eclipse, and the light we see from the star would fade. Nothing has yet been seen like this despite years of observations, so it seems unlikely that the planet will transit. Pity; this would not only confirm the planet but also give us its size.
I will note that while exoplanet Proxima b has been confirmed and accepted into NASA’s official database of extraterrestrial worlds, Proxima c has not. It makes me wonder what would happen if this new one was confirmed first. Then this will be Proxima c, and the one that is not quite confirmed yet will have to wait and become Proxima D.
Confusing. But finding out at the cutting edge of what we can do is usually messy. It’s not like in the movies when an attractive person in a white coat leans back to look into the eyepiece and mutters, “I have to tell the others!” Findings are more like slight tremors in a graph with lots of noise and uncertainty, and have to wait for more or more sensitive observations before someone shouts “Eureka!”*
One of the most important qualities you need as a scientist is patience. And a sense of irony: we’ve discovered confirmed planets around stars tens, hundreds, and even thousands of light-years away. But right next to it?
*Note: No one is shouting that either.