Alpha Centauri, a quick biography

So you’ve probably heard the exciting news, that a small, earth-mass planet has been identified around one of the components of Alpha Centauri. It’s the nearest neighbouring system to our Sun. Despite that it isn’t the brightest star in the sky (that’s Sirius), also it’s a southern star so is not very familiar to those of us who live in the Northern hemisphere. In-fact I didn’t see it until I was 28 and had moved to Hawai`i. Here’s a picture I took of it rising above Kilauea.

Alpha Centauri over Kilauea

Alpha and Beta Centauri rising over Kilauea. Alpha Centauri is itself a double star with a planet around one of them and a wide third star in the system called Proxima Centauri.

Because it was so far south, Alpha Cent. wasn’t well studied by early European astronomers. In the 1830s Thomas Henderson, the first Astronomer Royal for Scotland measured its distance using trigonometric parallax. This was the first distance measurement to a star other than the Sun. However he hesitated in publishing the result and Bessell scooped him to the first published distance to a star (61 Cygni).

The system itself actually consists of three stars. The two brightest components appear as a single source to the naked eye. The two stars orbit each other at a distance about 17-18 times the Earth-Sun distance. There’s third star in the system, Proxima Centauri. It’s 100 times too faint to see with the naked eye and while the two brighter components are of similar mass to the Sun, Proxima is less than an eighth of that. That said for people like me who study low mass stars, it’s still not that low mass. It was discovered by another Scot, Robert Innes who worked as a wine merchant in Australia before taking up astronomy fulltime. Simply, he found a star with a similar motion across the sky to Alpha Centauri and given their close positions (yet it’s still 17,000 times the Earth-Sun distance from Alpha Centauri AB), deduced they were a pair. This is what I do a lot in my research, but I use large catalogues produced by data pipelines, he used many painstaking measurements by hand. Sometimes I feel like modern astronomy is cheating. One interesting side-note, this double star with a wide companion set-up seems to be more common than a single star with a wide low mass companion. A rather nice  recent paper by Peter Allen and others quantified this and indicated that this may say something about how these systems form.

A quick note about the planet. It appears to be too close to the star to sustain liquid water and hence life. This system has a fairly complicated Habitable Zone. The planet is orbiting the smaller of the two stars but it will also be heated by Alpha Centauri A. Duncan Forgan wrote a paper about this earlier this year saying the difference will be small but will induce oscillations of a few degrees. Also sometimes on the surface of the planet Alpha Centauri B will have set but the planet will still be lit by Alpha Centauri A. This will of course change as the two stars orbit each other. Duncan has a nice blog post about the paper where he also talks about sleeping rhythms on a hypothetical planet around Alpha Cent B. When Alpha Centauri A is on the other side of Alpha Centauri A from the planet I guess sunset will look a bit like that on Tatooine. Although with a different brightness ratio between the two stars.

So it’s Alpha Centauri has a planet. It isn’t able to support life, but maybe there’s another one in the system that can. What this discovery makes me think of is Civilisation. No, not the noted BBC TV series, but the classic Sid Meier strategy game. One of the victory conditions was to send a spacecraft to Alpha Centauri. I never got that far, on more difficult levels my civilisation would die in the Bronze Age and in the harder levels I’d get bored of nuking phalanxes in about 1900 and give up. However if I hadn’t given up perhaps I could have built a ship to head for what the Nature press release calls a “scorched barren rock”.

It’s raining rocks, get a concrete umbrella

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There’s paper out today, along with a press release about Corot-7b. This is a hot super-earth (4-5.6 Earth masses) in a 20 hour orbit around it’s parent star. It is so close to it’s stellar host that it is tidally locked, meaning one side of it constantly faces the star, the other is in perpetual darkness (like the way we on Earth always see the same face of the Moon). Hence the hot side is about 2300°C, the cold side 220 below frezzing. It was discovered using the transit method by the COROT satellite, an instrument half for planet finding, half for Astroseismology. Previous studies have determined that it is probably a rocky planet like the Earth with a wierd environment, so a team of astronomers based in Washington University, St Louis set about trying to model the atmosphere. The planet’s day side is so hot that rock can be vapourised, so using code developed to study at high temperature vulcanism on Io they simulated how this exotic system would behave. Firstly, the rocky materials would rise, reach a point higher up in the atmosphere where it is cooler, condense and then rain out in the form of pebbles, those minerals with the lowest condensing point falling from the highest height. Secondly the atmosphere would consist of oxygen from the boiled off rocks but also metal vapours like sodium or potassium. Actually sodium has been detected in the atmospheres of other extrasolar planets.

Come to think about it a concrete umbrella would melt, any suggestions for a more effective parasol welcome.

Idea nicked from my former collegue John’s facebook status update.

A previously seen planet?

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At the end of last year a couple of papers appeared with some very promising looking direct images of extrasolar planet candidates. Until now the bulk extrasolar planets (i.e. planets outside our solar system) have been found either by the radial velocity method where the motion of the parent star being pulled around by the planet is detected or the transit method where the planet obscures a portion of the parent star, blocks some of the light that would otherwise reach us here on Earth and makes the star appear a bit dimmer. One of the candidate direct images was of a planet around the nearby young star Beta Pictoris, the discovery paper by Lagrange and collaborators is here and here and the press release is here. These direct images are very difficult to acquire as the star is much, much brighter than the planet (in the case of Beta Pic about 1500 times brighter) and the atmosphere and telescope optics smear out the star’s light, covering the spot on the sky where the planet is. The group led by Lagrange used the Very Large Telescope in Chile along with the NaCo instrument (which both blocks out most of the light from the parent star and corrects for some of the atmospheric smearing). This has allowed them to image what looks like a planet near the star. Of course it could just be another, fainter, unrelated star behind Beta Pic, in these cases you need to come back a few years later to check the planet is moving through space along with the parent star to make sure. However the chance of this just being coincidence is pretty small.

So why am I writing about this now? Well a paper has appeared that may indicate this planet was detected before, in 1981. Back then Beta Pic was seen to dim briefly, as if a planet passed in front of it. This of course begs the question “was the imaged planetary candidate responsible for the transit?” This is the question the authors try to answer. A planet will only transit if you are looking at the system edge-on and we have a clue that the Beta Pic system is very close to edge-on. Like many young stars Beta Pic has a disk of material around it that is thought to form planets. We know that the disk around Beta Pic is pretty close to edge on and you’d expect the planets in any system to orbit roughly in the plane of the disk. Hence it is possible the planetary candidate could transit in-front of the star. The authors then go on to try to work out (assuming the planetary candidate and the transiting body are the same thing) when a transit would happen again and what the planet’s orbit is. They find the most likely solution is a planet orbiting Beta Pic at a distance of eight times the Earth-Sun distance every 16-19 years. Both the direct detection and the transit suggest the planet is a gas giant.

The idea that the transit of a planet across its parent star could have been detected in 1981 sort of shows that astronomy is a passive science. In most research you design an experiment, have complete control over it, carry it out and note down the result. In astronomy you can’t grab two bottles of chemicals off the shelf and mix them, you can only look. If there is a planet around Beta Pic in a 16-19 year orbit then it was also there in 1981, it was also transiting at the end of the last century and in the mid-60s, just nobody was looking. Almost everything we can study, measure and analyse in astronomy is already out there, we just haven’t looked hard enough yet.