The search for extraterrestrial intelligence  needs all the help it can get. Depending on who you listen to, the chances of us spotting an intelligent technological society vary from an almost certainty to practically zero.

The trouble is the sheer size of the search. The Milky Way contains around 10^10 sun-like stars, any one of which may have a planet whose citizens are at this very moment pointing their beady eyes  or antennae in our direction.

But if we want to peer back, in which direction should we look?

Shmuel Nussinov at Tel Aviv University in Israel makes a thoroughly sensible suggestion of narrowing the search: why not look only towards stars that have a reasonable chance of having seen Earth?

We know of several ways to detect planets aroudn other stars but only one that might reveal an Earth-like body and that is to look for changes in brightness that are the signature of a transiting planet.

Earth passes in front of the sun for 13 hours once a year, dimming it by 77 parts per million. Venus transits for 11 hours every 7 months with even less dimming. Mars gives  three-fold weaker eclipse every 1.9 years and Mercury dimming is ten times weaker than Earth’s but occurs four times a year.

Only stars within a narrow angle of the ecliptic will be able to detect these transits. And so only civilisations on planets around these stars could possibly be aware of Earth might be broadcasting our way.

Common sense really.

Ref: arxiv.org/abs/0903.1628: Some Comments on Possible Preferred Directions for the SETI Search

Spotting Earth-like planets orbiting other stars is all the rage these days. But unless you have access to a space-based telescope it’s kinda tricky.

The problem is that the reflected light from a Jupiter-sized planet is roughly 10^4 fainter than the parent star. That’s hard to spot at any time but when this is coupled with the distortion that the Earth’s  atmosphere introduces, these planets become almost impossible to see.

One way around this is to use adaptive optics to smooth out distortions in the wavefront from the star system as it hits the telescope’s mirror. But these systems are expensive and the demands of higher resolution make them increasingly complex too.

Another option is to take exposures that are much shorter than the time over which the atmospheric distortion occurs. Then post processing of the image compared to the image of a reference star allows the distortion to be removed.

The trouble with this technique is that the wavefront distortion varies over the entire area of the telescope’s mirror. So this kind of processing ends up averaging the distortion rather than removing it.

Takayuki Kotani from the  Observatoire de Paris in France et amis are pioneering a technique called pupil remapping that could get around this.  Their idea is to place a bundle of optical fibres at the focal point of the telescope. These channel the light from each point in the mirror separately, which avoids the problem of averaging out the distortion and allows much better image processing.

They’ve tested the idea on an optical bench showing that it can achieve near theoretical performance. Installed in a decent telescope, it should allow a dynamic range of around 10^6. That will allow the detection of much fainter planets than is now possible and at a fraction of the cost of high resolution adaptive optics  or space-based telescopes.

Expect to see this technique producing good results in ground-based telescopes within months.

Ref:  arxiv.org/abs/0901.2165: Pupil Remapping for High Contrast Astronomy: Results From an Optical Testbed

“95% of the mass in galaxies and clusters of galaxies is in the form of an unknown type of dark matter,” say Katherine Freese at the University of Michigan, Ann Arbor, and buddies. What effect might this huge amount of stuff have on star formation?

The answer according to these guys is astounding. In the early universe, the first stars would have been powered by dark matter.

Here’s the thinking: the concentration of dark matter at that time would have been extremely high meaning that any ordinary stars would naturally contain large amounts of dark matter. Freese and co have calculated the effect of this stuff and say tit would have radically altered the evolution of these stars forming so-called “dark stars”.

Dark stars would have been driven by the annihilation of dark matter particles releasing heat but only in stars larger than 400 solar masses. That turns out to be quite feasible since stars containing smaller amounts of dark matter would naturally grow as they swept up dark matter from nearby space.

When the dark matter runs out, they simply collapse to form black holes

Interestingly, we should soon be able to to see these stars with forthcoming generations of telescopes. And if they are found, that would be further proof of the existence of dark matter.

Keep ’em peeled!

Ref: arxiv.org/abs/0812.4844: Dark Stars: the First Stars in the Universe May be Powered by Dark Matter Heating