on alpha Centauri

2015 (October 12, to be exact) marks the hundredth anniversary of the discovery of Proxima Centauri by the Scottish astronomer Robert Thorburn Ayton Innes, the director of the Union Observatory in Johannesburg, South Africa. Proxima was quickly recognized to be the smallest member of the alpha Centauri star system, and is now known to be the closest star to the Solar System. (The closest star to Earth is, and remains, the Sun… as various professors were apt to remind me over the years).

The alpha Centauri star system, famed of song, story, and videogames, is the closest star system to us (again, apart from the one we’re currently in). It’s 4.3 light years away, or 1.3 parsecs, or about 270,000 times farther from the Earth than the Sun is (AU)… which is still depressingly far away for anyone who would want to visit it with currently achievable tech: it’s taken New Horizons 9 years to travel 35 AU; so it would take about 69,000 years to get to Proxima Centauri at that speed (71,000 years to alpha Centauri A and B). alpha Centauri A has a spectral type of G2V, which means it’s a star very much like the Sun (it is, in fact, slightly more massive and slightly hotter). alpha Centauri B has a spectral type of K1V, which means it’s a few hundred degrees cooler and has a mass about 80% of the Sun. A and B are about the same distance apart as Uranus is from the Sun, and they orbit every roughly 80 years. Proxima Centauri (or alpha Centauri C) is an M5.5Ve star, which means it’s about 16% of the mass of the Sun, two thousand degrees Kelvin cooler than the Sun, and sits 15,000 AU (15,o00 times the distance from Earth to the Sun) from A and B. Its orbital period is… unknown, but if it’s circular and the total system mass is 2 solar masses, Proxima’s orbit is something like 1.3 million years long.  Proxima Centauri is so far from A and B that every so often astronomers, armed with new data, reconsider whether it’s actually gravitationally bound to the pair. (The answer always turns out to be “yes”) The alpha Centauri system is probably around 6 billion years old, and it’s so close to us that Proxima Centauri is a little over 2 degrees from A and B on the sky, or the width of four full moons. There are no planets in the system, though for a while it was thought that there was an Earth-mass planet in a 3.2 day orbit (so, a ball of molten lava) of alpha Centauri B. The alpha Centauri system is very, very well studied.

The alpha Centauri system was first suspected of being nearby because of their high proper motion, or apparent motion across the sky. Being stars, we’re not talking night-to-night movement like the ancients saw with the planets. Rather, this is motion best understood over a period of years, because the orbit of the star system around the Galaxy is slightly different from our own. In the case of alpha Centauri, they’re all so close to us that they appear to move (relatively) quickly through space. Seriously, here’s ten years of motion of Proxima Centauri (2000-2010):

The proper motion of Proxima Centauri (Rigil Kentaurus C) as seen on CTIOPI astrometric images from 2000-2010

An asteroid in the asteroid belt can move that far in the space of a few minutes, but Proxima moves pretty fast for a star.

And that brings us to a much more recent paper: A discovery of a new object near alpha Centauri. This hasn’t been getting all that much press, but plenty of astronomers have been talking about it on Twitter.  Basically, what this group of ALMA researchers have found is a source near alpha Centauri A and B that seems to be moving very fast; they suggest the object is alpha Centauri D.

There are many problems with this, which is why the paper ended up on the preprint server in the first place: A discovery this remarkable begs for an explanation, and at the time they posted it, the ALMA people had none.

First off: how was this not seen before? In ALMA’s favor, alpha Centauri is so incredibly bright (A is the third brightest star in the sky; B would be 21st on its own) that it’s hard to see anything close to them. But if this was a star that bright compared to A and B, it should be pretty bright itself (an M2 star, which would be brighter and MORE massive than Proxima Centauri), and shouldn’t have escaped the notice of absolutely everyone until now.

Second: this alpha Centauri D object appears (Figure 1 in the paper on page 2) to have moved significantly compared to alpha Cen A and B. In 2014, it was hovering between the two stars, and then in 2015, it was directly above alpha Cen A. That could be orbital motion, except that it’s a LOT of orbital motion for one year. A and B orbit each other every 80 years, and they barely seem to move between the two images. This mystery object is farther away than B, which means its orbital period should be even longer, and it shouldn’t have moved AT ALL. They basically admit in their own table 1 (columns 2 and 3) that the motion of this object is nothing like the known members of alpha Centauri.

That suggests that this alpha Cen D is actually something else entirely. If its motion isn’t the same as alpha Centauri, a few years (or decades, or a century ago) it should have been far enough from alpha Centauri that it could have been EASY to spot on its own. Innes should have seen it back in 1915.

So maybe it’s not a star. Maybe it’s a closer object in the outer reaches of our own solar system. In that case, it’s moving awfully slowly. There’s actually a second paper from ALMA that found ANOTHER source moving at 87 arcseconds per year (ten times faster than the fastest star, so more plausibly a member of the Solar System) that the discoverers think would have to be a distant planet in our Solar System.  Then again, noted Solar System astronomer Mike Brown (@plutokiller on twitter) did the math and figured out that if ALMA found a planet in the outer Solar System after such a short search, statistically there ought to be 200,000 Earth-sized planets in the outer solar system, which is absurd.

So… what IS it? Or, what are these?

There are a couple likely possibilities. It could be a detector flaw that nobody has noticed before; it could be the equivalent of a cosmic ray making a bright spot appear where there really isn’t anything. It’s probably two different sources ALMA usually can’t see, where one flared up in 2014 and one in 2015… two separate stars (or two distant quasars) that appeared out of nowhere because they finally became bright enough to be seen… This sort of thing actually happens all the time. Take a look at the .gif of Proxima Centauri above and you can see a few sources that are only visible in one or two of the frames due to the different exposure times and sky brightness. A string of mistaken identities could become measurements of a moving star. The reverse is also true; occasionally a high proper motion star (particularly a really high proper motion one like Proxima Centauri) will wander into a field of view and be mistaken for a new object.

There are other options astronomers are exploring, and that’s really the purpose of all of this. Science is allowed to be wrong, but put enough minds together and it’s self-correcting. For now, though, Proxima Centauri remains the closest star to Earth, and alpha Centauri the closest star system.


4 Comments Add yours

  1. It’s always intrigued me that, amidst the flurry of exoplanet discoveries (and much else), so little has been detected about our nearest star system. I know it’s a function of the observational methodology, instruments and opportunities – but still! I see the discovery paper leans towards the object being actually part of our own solar system, and far closer. All of it, in many ways, puts me in mind of Haldane’s quip about the universe being not only stranger than we imagine – but stranger than we CAN imagine.


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