Proxima Centauri

Myrddin wrote: »

Being so close to a red dwarf, does that mean it's very likely this planet has been scorched at some point in the past?

An M type star has quite a cool surface, under 3500 K, which is why they think this planet could be in the Goldilocks zone for liquid water even though it is close to its parent. The problem with M type dwarfs is that they produce a lot of X-ray flares that could zap any potential planetary life.

We see flares on our own star. They are magnetic phenomena. Charged material is funneled along magnetic field lines, which twist and stretch, then snap like an elastic band. In the process, the material is superheated and lofted into space along with bursts of X-rays. As a result, in our own sun the temperature increases from 5,700 K at the surface to two million degrees in the corona. This is counter-intuitive because we expect the temperature to be falling as we travel further from the Sun's surface. But the additional heating is provided by conversion of magnetic energy by a process called magnetic reconnection.

Where does the magnetic energy come from in the first place? It is generated by the rotation of the star itself, which consists of a highly charged plasma. All other things being equal, the magnetic field should be organised into regular field lines which emerge from the axial poles of the star (below left). But turbulent convection within the star's outer layers traps the field lines and winds them up, causing the magnetic field to distort and burst out through the surface (below right).



So then where does the convection come from? A star is heated by the fusion reactions going on in its core, themselves initiated by the temperatures and pressures caused by the ongoing gravitational collapse of the star. The more massive the star, the higher the internal temperature and pressure. The temperature determines the type of nuclear fusion reactions that occur. The temperature sensitivity of these reactions can determine the temperature gradient within the inner core. The rate of some reactions can be proportional to the seventeenth power of the temperature! This tends to make the reaction rate and the temperature drop off extremely rapidly with distance from the inner core.

In G type dwarf stars like our Sun, and even smaller K and M type dwarfs like Proxima Centauri, we only have to worry about the set of fusion reactions called the proton-proton chain which fuse hydrogen to helium. Core temperatures are on the order of fifteen million degrees. Within the core, energy is carried outward by radiation only. Photons from fusion reactions scatter off ions and slowly work their way outward through the bulk of the star. At higher distances from the core the density and viscosity of the plasma drop off because less of the star's weight is above you, and if there is a sufficient temperature gradient we get the onset of convection, like in a pot of boiling water or in this lava lamp:



Convective heat transport is chaotic, and produces charge separation between negative electrons and positively charged hydrogen ions. This convective zone is where the magnetic field gets distorted.

It follows immediately that if we have a core temperature of fifteen million degrees and a surface temperature of approximately zero (relatively speaking), that the highest temperature gradients will occur in the smallest stars. Thus, in our Sun the convective layer only extends 200,000 km deep from the surface, about 30% of its radius. But in an M type dwarf with its higher temperature gradient, the convective layer extends all the way down to the outer core. This traps the magnetic field much more effectively and results in higher magnetic energy released in flares at the surface, and a higher X-ray flux. That's why cooler stars don't necessarily have a more benign environment for planets in their Goldilocks zone.

Yep, a fascinating answer to a question that I never even though to ask; just accepted that red dwarves are crazy. Thanks

Thanks jfSDAS, that S&T article fills in a few numbers I hadn't seen. Proxima Centauri b is at 0.05 AU from its parent, which -- if Proxima produced the same X-ray flux as the Sun -- would mean the planet received 400 times higher X-Ray exposure, by the inverse square luminosity distance relation (0.05² = 1/400). That seems to be the original estimate, which they then revised down to 100x although they don't say why. That would mean Proxima only produces a quarter of the X-ray flux as the Sun. However, flux is a measure per unit area, and since Proxima is only 14% of the Sun's radius and thus only 2% of its surface area, the X-ray intensity at its surface is about a dozen times that of the Sun by my reckoning.

It was always that size. In simple terms (that I'm sure ps200306 might be able to elaborate on if he wishes) a red dwarf is a main sequence star like the sun, but smaller. It simply formed out of less material and is a smaller star, so it's cooler and redder. Red giants and white dwarves are different, they are phases that some stars enter towards the end of their lives after they've burned up the hydrogen in their core.

Yep, that's what will happen to the sun but at that stage it will have left the main sequence and will be 'dying'. Because of its lower mass, this will never happen to a red dwarf like Proxima Centauri. It will continue to burn on for potentially trillions of years and when it's finally finished, as far as I know, will just turn directly into a white dwarf.