Proxima Centauri

bilston

http://www.bbc.co.uk/news/science-environment-37167390

According to this, scientists believe there is an Earth sized planet orbiting our nearest neighbouring star. What's more they believe it may support liquid water.

I ask this as a layman, but would this qualify as the most significant Exo Planet discovery to date?

Gwynston

Sounds pretty significant it to me!
I don't think much of ESO's star size comparison pic though.
Why do the stars have 'shadows'? :

KingBrian2

Were all going on a trip to the stars, next stop Proxima B.

Myrddin

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?

ps200306

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.

Myrddin

^^ Fascinating stuff (as always), cheers

Maximus Alexander

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

jfSDAS

Great post ps200306.

Sky & Telescope add in their article at http://www.skyandtelescope.com/astronomy-news/exoplanet-found-around-proxima-centauri-2408201623/ that the planet may have the same side permanently turned towards it's parent star. Upcoming planet finder missions and ground-based instruments have a good chance of imaging the planet directly

ps200306

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.

Myrddin

Likely a silly question, but is Proxima Centauri the size it is because it's slowly coming down from being a Red Giant, transitioning into a White Dwarf...or was it always* that size?

*as in it never had an inflationary period going from a yellow dwarf into a red giant & back down again

Maximus Alexander

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.

Myrddin

Maximus Alexander wrote: »

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.

I see, cheers. Only thing that confuses me is you liken our sun as a main sequence star in the above text, but isn't ours scheduled to inflate as a red giant, ultimately becoming a white dwarf in about 4 billion years? (plus or minus a day or so)

Maximus Alexander

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.

ps200306

Maximus Alexander is exactly right. A red giant like Betelgeuse is the same spectral type M as Proxima Centauri, but the two stars are on a completely different evolutionary track. The Hertzsprung Russell diagram is a piece of genius, but it originally just graphed spectral type (which is a good proxy for colour and temperature) against luminosity. This more modern one from Wikipedia fills in a whole lot of other detail inferred later from astrophysics:



All of the main sequence stars (the main diagonal on the diagram) are in the hydrogen burning phase. The only difference between them, by and large, is their initial mass which determines all their other properties. An individual star does not move (very much) on the main sequence during its lifetime. The red giants are out on their own, which begs the question why we don't see a continuous evolution off the main sequence into red giantdom on the HR diagram if main sequence stars turn into red giants. The short answer is that the evolutionary changes that occur when a star runs out of hydrogen fuel occur relatively rapidly so we don't often catch a star in the act of transition.

Nevertheless there is some structure to the red giant branch which you can see in another version of the HR diagram on that same Wikipedia page. This tells us about various transitions that occur in the later life of some stars. These changes are all mass dependent though, so the HR diagram should not be misconstrued as a picture of what happens to every star in its lifetime. And there is still the so-called "Hertzsprung gap" between the main sequence and the base of the red giant branch due to stars rapidly transitioning from the subgiant stage when they have consumed about 10% of their total hydrogen fuel. It is a surprising fact that a star that has burned hydrogen stably for many billions of years will undergo some later transitions in mere millennia. (Indeed, some of the more catastrophic changes to higher mass stars occur literally in seconds).

The red giant phase occurs when the star transitions from hydrogen to helium burning, and subsequent phases involve the fusion of successively heavier elements. Stars between about 0.5 and 8 solar masses will eventually end up with mainly carbon and oxygen in their cores and late in life will puff off most of their envelope (which will always have remained mostly consisting of hydrogen) leaving a so-called CO (Carbon-Oxygen) white dwarf. Stars below 0.5 solar mass such as Proxima Centauri never have sufficient mass to generate the temperatures to initiate helium burning and -- after trillions of years -- will eventually transition directly to He (Helium) white dwarfs.

It should be pointed out, of course, that a trillion years is orders of magnitude longer than the current age of the universe, so any He white dwarfs that exist must have gotten there by an unusual route. Funny things happen in binary star systems where one star may cannibalise another by gravitational attraction. A compact star such as a white dwarf may find itself next to an expanding red giant companion which overflows its Roche lobe -- meaning that material pours off the giant onto the white dwarf. This can cause nova explosions as material is transferred, or may even rejuvenate the white dwarf, returning it to the main sequence. Meanwhile a larger companion can be stripped of its hydrogen envelope leaving a prematurely decrepit He white dwarf.

Myrddin

ps200306, you need to post every day...I could read that stuff all day & lap it up

ps200306

You're gonna embarrass me now.

Proxima Centauri may have another planet. A 'super-Earth' of 6x mass of our Earth with an orbital period of about 5.2 years. Way outside the habitable zone though. Surface temp of -233 C.

https://www.space.com/proxima-centuri-candidate-alien-planet-proxima-c.html

https://www.universetoday.com/144618/a-second-planet-may-have-been-found-orbiting-proxima-centauri-and-its-a-super-earth/

https://www.nationalgeographic.com/science/2019/04/proxima-c-new-super-earth-may-orbit-star-next-door-proxima-centauri/

Whatever about the planets being discovered, I think the stars themselves too are fascinating. They are far from being the 'boring plodders' of the universe when compared alongside the massive stars and their evolution to supernovae, neutron stars & black holes etc. The sheer longevity of them is absolutely mindboggling. Wolf 359 (9% solar mass) might stay in the main sequence for 8 trillion years, with Proxima Centauri (12.2% solar mass) itself hanging in there for 4 trillion. They remind me of the tortoise and the hare fable.

ps200306

Deleted User wrote: »

Whatever about the planets being discovered, I think the stars themselves too are fascinating. They are far from being the 'boring plodders' of the universe when compared alongside the massive stars and their evolution to supernovae, neutron stars & black holes etc. The sheer longevity of them is absolutely mindboggling.

+1. The universe will be a very different place when today's red dwarfs are old. Already they comprise three quarters of all stars, including most of our nearest neighbours. Bigger stars burn out in mere billions of years (or even millions). In four billion years we will collide with the Andromeda galaxy and, after dancing around it a few times, merge with it.

By that time, the streams of gas that the Milky Way is sucking in from the Magellanic Clouds will be exhausted. New star formation will end, except for existing molecular clouds disturbed by the merger with Andromeda. We'll have lost our spiral structure and become an elliptical galaxy with stars orbiting in random directions. After a few more billion years, the galaxy will be dimmer and redder, as young luminous stars will be rare. Yet today's red dwarfs will still be youngsters.

Roll on more tens of billions of years. We will probably have merged with the other galaxies in our local group. Meanwhile, the far away galaxies that we can see today will have started disappearing over our cosmological horizon, carried away by the expansion of the universe. Yet we'll still only be a small fraction of the way through our first trillion years. (When I say "we", I'm assuming that we ourselves have found our way to a lifeboat planet around a red dwarf, as our own star will be a distant memory).

A trillion years from now, even the galaxies in our local supercluster will have disappeared from view*. Our entire universe will consist of our galaxy. The night sky will contain a sprinkling of stars, though not very many as the vast majority will be too dim to see, just like Proxima is today. We'll still have a diffuse light from the direction of the galactic centre though it won't have the dust lanes of today as all that gunk will have been blown away. There won't be much sign of any structure, either within the galaxy or outside it.

And after all this, the red dwarfs that are around today will still be mere teenagers, with trillions of years still ahead of them.




(* assuming dark energy, which may be in doubt).