Alpha Centauri Has an Earth-Sized Planet

The Bad Astronomer writes "Astronomers have announced that the nearest star system in the sky — Alpha Centauri — has an Earth-sized planet orbiting one of its stars. Alpha Cen is technically a three-star system: a binary composed of two stars very much like the Sun, orbited by a third, a red dwarf, much farther out. Using the Doppler technique (looking for very small changes in the velocities of the stars) astronomers detected a planet orbiting the smaller of the two stars in the binary, Alpha Centauri B. The planet has a mass only 1.13 times that of the Earth, making it one of the smallest yet detected.However, it orbits the star only 6 million kilometers out, so it's far too hot to be habitable. The signal from the planet is extremely weak but solidly detected (PDF), giving astronomers even greater hope of being able to find an Earth-like planet orbiting a star in its habitable zone."

Comments

Re:Dear /S/cientists

[Anonymous Coward]

Any system of bodies is going to have a center of gravity. My guess (not having read TFA) is that this planet is many times further away from the binary stars than they are to each other.

From the PDF, it seems to be the opposite:

With a separation to its parent star of only 0.04 AU, the planet is orbiting very close to Alpha Centauri B compared to the location of the habitable zone.

Re:Dear /S/cientists

[harperska]

Having RTFA (I know), this planet is very close in to one of the stars, in this case Alpha Centauri B. There are two possibilities for planets in a binary system, either orbiting close in to one of the stars, or far away from both. I think I remember reading once that Alpha Centauri A and B are far enough apart from each other that there is a good chance that planets in either star's habitable zone would have stable orbits.

Temperature = 1500K

[kf6auf]

That sounds really cool. Or hot since, unfortunately, the close proximity to its star means that it probably has a surface temperature of 1500 K.

I guess I'd be more interested in a different-sized planet a bit further away from its star.

Re:Dear /S/cientists

[bjorniac]

It's rather the same way the moon orbits the earth. If you have a binary system, a planet can quite happily orbit very close to one of the two stars so long as the distance between the planet and the star it orbits is smaller than the distance between stars. The pair of stars will orbit their mutual center of mass, and the planet will orbit a single star.

Of course, the three body problem is an open question in physics, but if you make the assumption that one of the masses is much smaller than the other two it (which is the case for planets orbiting stars) it becomes quite solvable, especially if you're happy with numerical simulations of orbits.

A similar situation is possible if the planet is a long way from the pair of stars, and would then orbit their center of mass. That isn't the case here, but is certainly a feasible solution to the problem. You only really get orbits that are highly erratic when the planets orbital radius is over a quarter of the distance between the stars.

Throughout this I've assumed equal mass stars. Feel free to put a factor of M1/M2 in front of every distance I gave for non-equal mass stars.

Re:Dear /S/cientists

[Anonymous Coward]

The stars are actually very wide spaced compared to the planet-star system itself. As a result, the planet is well within the gravity well of B. At a minimum AB separation is 11AU - well over 200 times the B-planet separation.

Orbit centred on A: https://upload.wikimedia.org/wikipedia/commons/e/ee/Orbit_Alpha_Centauri_AB_arcsec.png

Simulation plus table of info: http://www.solstation.com/orbits/ac-absys.htm

Re:Dear /S/cientists

[MyLongNickName]

Not an erratic orbit at all. Picture Jupiter. If it suddenly increased its mass by a factor of 20, it might have enough mass to become a star, but would have virtually no impact on the orbit of Mercury, and very little on Earth or Venus. Just because a body becomes a star does not require planets to orbit both stars. In actuality, all planets orbit the center of mass of the solar system. In our solar system's case that resides inside the sphere of our sun.

Re:Unfortunately

[murdocj]

Not unfortunate, just a recognition of reality. At this moment in time, the science return for sending unmanned probes / orbiters / rovers vastly exceeds the return on sending humans. We'll continue to develop space capability and at some point it may make sense to send humans to Mars ... or maybe not.

And please do NOT invoke the whole "omg we have to get off this rock" argument. If an asteroid impact blew most of Earth's atmosphere and water into space and annihilated 99.999% of the species, Earth would STILL be easier to live on than Mars.

Re:Dear /S/cientists

[Vekseid]

It'll get ejected - that configuration isn't stable.

For Alpha Centauri A and B, the 'stable zone' is out to roughly Jupiter's orbit from each star - plenty of room for both to have habitable worlds.

Re:Dear /S/cientists

[MyLongNickName]

For those who care

Jupiter is about 0.0009 solar masses. Current models of nuclear fusion predict that if an object has mass of about 0.07 solar masses it will begin a fusion reaction. So Jupiter would need to swell to 80 times its current mass.