Alpha Centauri DOES have planets!

Ever since I wrote the two posts about the potential for planets at Alpha Centauri, those posts have been the most popular on this blog. With all the exoplanet action, this discovery was only a matter of time, because it seems that more than 90% of stars–maybe substantially more than that–have planets. I’ve written here about why it can be very hard to detect planets, even in stars that are “close”.

So it seems that Alpha Centauri B has a planet, which is now boringly named Alpha Centauri Bb. I think it needs a more interesting name.

The planet in question is about 13% more massive than Earth, which suggests that it is rocky, but orbits the Sun-like star far too closely to harbour life. The distance between it and the star is a mere 6 million kms (vs 150 million for Earth) and the surface temperature a mere 1200-odd degrees which makes it, as scientists wryly remark “unsuitable for life”. Or at least life as we know it. Science Fiction writers can go crazy here.

It is my prediction that if this planet exists this close to the star, others will be found. Already, some scientists say that the signal is too complex to suggest the planet’s existence with certaintly. A complex signal could well mean more planets. Trouble is, finding a Mars or Earth-sized planet possibly orbiting in a plane at an angle to the line of observation, as part of a triple-star system, and orbiting at a fair distance–as in 1 AU or more–is going to be like the proverbial needle in the proverbial haystack.

Meanwhile, get your space ships ready.

Once more on the search for exoplanets, and Alpha Centauri

Excuse me for the absence of regular science posts, but my brain has turned to mush from writing fantasy. Isn’t it easy when you’re allowed to make everything up and as long as it makes sense, no one cares about accuracy? But, yes, I will return to where I left my characters in the Jupiter system, or in a space station orbiting the fictional gas giant Sarasvati, in the not-too-distant future.

This morning I came across this very interesting article on the Centauri Dreams website. By the way, Centauri Dreams, the website of the Tau Zero Foundation, is a very rich source for writers of realistic SF, especially in relation to planetary exploration and interstellar travel.

The article summarises results and speculation arising from new planets discovered by the European Southern Observatory’s HARPS spectrograph, which provides the most accurate Radial Velocity measurements we currently have (see an earlier post on how planets are discovered). Because of its increased sensitivity, HARPS can detect smaller planets. The smallest planet found at this point in time is a mere 1.5 times Earth’s mass. One of the, perhaps expected, outcomes of the spectrograph’s bevy of newly found low-mass planets (super-Earths or near Earth-mass) is that there are many of these smaller planets, a lot more than there are very large planets, and that the previous bias was merely a product of larger planets being easier to detect. The galaxy is swarming with smallish rocky planets. It is quite likely that some will be found inside the habitable zone.

We may already have found some of these planets. Much was made last year of the ‘discovery’ of Gliese 581g, supposedly in the habitable zone of an M class star. However, further analysis has so far failed to confirm the existence of this planet. But the star has two other planets which orbit at the edge of the habitable zone, and out of these, Gliese581d looks the most promising. The width of the habitable zone is not absolute, but varies with the planet’s albedo (basically, how much light it reflects) and composition and (if any) atmosphere composition (see another post on that here). So a newly discovered planet, HD85512b, at 3.6 Earth masses, may also fit the bill. It is a little close to its K class parent star, but could harbour liquid water on its surface if certain conditions of composition and atmosphere are met (see original paper by Kaltenegger et al. here).

Using the HARPS spectrograph, another group of researchers report on the search for planets orbiting sun-like stars within 40 light years from our solar system. (original paper by Pepe et al here). This work has resulted in a the discovery of a number of planets, again, most in the smaller size category. One of the main targets for the hunt is Alpha Centauri B, but there are some problems, one of the main ones being that it is part of a triple star system, and that any model the describes the wobble of the star that is caused by an orbiting planet must take into account that there are two other stars in the system, and as you could understand that is tricky business.

Image depicting an exoplanet system snarfed from NASA JPL

So… does Alpha Centauri have any planets?

A short follow-up on this post I made a while ago (note the tantalising difference in wording of the title).

Just today, I stumbled across a scientific paper, thanks to a tweet by @b0yle, that outlines the priority of stars to which to send an as-yet-hypothetical interstellar mission (link here; the paper is not as jargon-laden as a lot of other scientific publications, so go read if this subject interests you).

Short answer: we still don’t know. And with our current techniques, we can’t be certain. We’re getting very close, though.

Some figures: there are 56 stars within 15 light years of us (give or take a few that may or may not be within this range and possibly very dim ones we haven’t yet discovered).

Most of those are M-class stars, ‘red dwarfs’. There are also 2 G-class stars, like the sun (Alpha Centauri A at 4.4 ly and Tau Ceti, at 11.9 ly). According to current evidence, up to 30% of all stars may have planets. At this point in time, most discovered planets have been very large.

The fact that we haven’t detected any in the Alpha Centauri system doesn’t mean that there are none.

Planet size is expressed in number of times the planet is heavier than Jupiter. It is still fairly hard to detect anything significantly smaller than that. Yet evidence suggests that smaller planets could be more common than larger ones.

Alpha Centauri is a star system of three stars (one K-class, one G-class–like the sun–and one M class). What is the statistical chance that none of them will have planets?