I wonder, for a planet so (relatively) close to our own star system, what kind of- How big would a telescope have to be in order to detect an earth-sized planet by direct observation in the Alpha Centauri system?
NGTM-1R is pretty much correct. In terms of angular resolution it is within our ability, but by far the biggest problem is the enormous contrast between the star(s) and planet(s).
Back of the envelope calculation:
This planet is 0.04AU from its star, and the star is 1.34pc away. The separation angle is thus tan
-1(1.94x10
-7pc/1.34pc) or 0.03 arcseconds.
The resolving power of a circular aperture is given by sin(Θ) = 1.22λ/D where λ is wavelength and D is diameter of aperture. For .03 arcseconds, and viewing in visible light (say 530nm for green), that requires a diameter of about 4.5 meters. There are numerous optical telescopes larger than this, the largest I believe is LBT at about 12 meters.
Better yet is if you consider a hypothetical planet around Alpha Centauri with an orbital distance of 1AU (that's not necessarily the best place for habitability in this case, but whatever). The point is this would be separable from the star with a telescope only 7 inches across! I could achieve this with my 8" scope in my backyard! Right? Well, actually no, because we have to consider how bright is the planet relative to the star. Stars are pretty damn bright and trying to pick out the planets next to them is like trying to spot the feeble glow of a firefly right next to a bigass searchlight.
Let's consider again a hypothetical planet orbiting 1AU from Alpha Centauri B. The contrast between the planet and its star is just the ratio of their luminosities. In visible light the luminosity of the planet is the solar flux at the planet times the planet's cross sectional area. We can write this as
where r
p is planet radius and a is its orbital distance.
Crunch the numbers and the contrast for an earth-size planet at 1AU from its star is about 4.5x10
-10. 45 billion times fainter!
ed: Also that's a best-case scenario where the planet is seen in full phase and has 100% reflectivity. In reality it'd be even worse. The contrast problem can be overcome by a number of methods though, such as observing in infrared wavelengths, or observing hotter and larger planets, and using an occulting disk to block out the star's glare.
I'm just thinking. This is awesome and all, but that planet is obviously roasty-toasty in a way that Venus could only dream about. That close to its primary, I wonder if it would even have an atmosphere at all, or if would have been boiled away by a) heat and b) blasted off by the solar wind?
With earth's mass and 1500°C it probably cannot retain an atmosphere with nitrogen, water vapor, or other such volatiles. I'm not completely sure, but I think that might even be hot enough to form a fabled 'silicate atmosphere', where the atmosphere is literally vaporized rock.
This is an awesome find and I do not want to diminish that in any way, but the technique used to detect this planet was pushed damn close to the limit. If there were an earth-sized planet in the habitable zone, would there be a way to detect it other than direct observation?
Yes, but it would be much easier in the case of star systems with transiting planets, as the gravitational perturbations from undiscovered planets will cause tiny variations in the transit times of the planets we already found. This is the same principle that allowed us to accurately predict the existence and location of Neptune.
Also wondering if having Alpha Centauri A so nearby could serve the same function as Jupiter does in our own system for clearing out potential meteors, or is the separation too great?
Nah, it's too far away. Also if it could clear out miscellaneous asteroids then it would disturb the orbits of the outer planets as well. At
best it could affect the orbits of stuff waaay out in the Oort Cloud; I think I recall a paper discussing the possibility of comets getting transferred between star systems in this manner.
