Archive for the ‘Stars in their eyes’ Category

How to narrow the search for ET

Wednesday, March 11th, 2009

ecliptic

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

The puzzle of planet formation

Wednesday, February 18th, 2009

planet-formation

“The formation of planets is one of the major unsolved problems in modern astrophysics.” That’s how Rafael Millan-Gabet at Caltech and John Monnier from the University of Michigan begin their account of how our understanding of planet formation is about to undergo a revolution.

Driving this change will be a new generation of telelscopes and techniques capable of measuring and in some cases imaging planet formation in action.

It’s worth pointing out the poverty of our current understanding. At the heart of the problem is the fascinating question: why are all the planets different?

The ones in our solar system ought to have formed out of the same stuff at more or less the same time and yet no two are alike. And now the extrasolar planets seem to be demonstrating a similar variety.

The  trouble is that astronomers have only the vaguest understanding of what goes on inside  the circumstellar discs where planets are supposed to form. They have little idea of the circumstances in which accretion dominates over gravitational instability, whether “dead zones” exist in circumstellar discs where planets cannot form or what mechanisms are at work in transporting angular momentum within early solar systems.  They don’t even know when planets form.

The new measurements that will be possible in the coming years should hep to answer at least of these puzzles. And that makes this an exciting field to be in. Watch this space for developments

Ref: arxiv.org/abs/0902.2576: How and When do Planets Form? The Inner Regions of Planet Forming Disks at High Spatial and Spectral Resolution

Spotting alien Earths on the cheap

Monday, January 19th, 2009

alien-earth-spotting

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

First stars “powered by dark matter”

Friday, January 2nd, 2009

dark-stars

“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

Astronomers find hottest and fastest exoplanet

Friday, December 19th, 2008

wasp-12b-data

As astronomers discover greater numbers of planets orbiting other stars, they are able to revise their theorie sof planet formation accordingly. Exotic planets are particularly prized because they push the boundaries of theoretical understanding to its limits.

Today,  Leslie Hebb  at the University of St Andrews in Scotland and colleagues announce the discovery of one of the most exotic planets yet seen. WASP-12b orbits a star about 3 times as bright as the sun. Its orbit has a radius about 1.8 times that of Jupiter  but it is only about a quarter as dense.

But get this. WASP-12b has an orbital period of only  1.09 days, the shortest ever seen, and a surface temperature of 2516 K because it is the most highly radiated planet ever discovered. That’s too records for one planet.

How these kinds of planets can form, nobody is certain. But one thing’s for sure: WASP-12b must be on one helluva roller coaster ride.

Ref: arxiv.org/abs/0812.3240: WASP-12b: the Hottest Transiting Extra-Solar Planet Yet Discovered

Why astronomical units need to be redefined

Thursday, December 18th, 2008

In 1983, the International Bureau of Weights and Measures defined the metre as the distance travelled by light in a vacuum in 1⁄299,792,458 of a second. That makes a metre a fixed unit of length.

For astronomers, however, distance is rather more malleable. In astronomy, distance is measured in astronomical units. Astronomers think of an au as the distance between the Earth and the Sun.
But since the Earth’s orbit is elliptical, this distance isn’t constant. At one time an au was defined as the length of the semi-major axis of the Earth’s orbit. But that isn’t constant either.

So in 1976, the Bureau International des Poids et Mesures in France defined the au to be the distance from the centre of the Sun at which a particle of negligible mass would orbit in 365.2568983 days.

The trouble with this definition is that it depends both on the mass of the Sun and the gravitational constant G.

That’s not good say, Nicole Capitaine and Bernard Guinot at the Observatoire de Paris. The gravitational constant and the mass of the sun can only be measured with limited accuracy. And who’s to say their value isn’t changing anyway? It makes no sense to define a fundamental unit of length in these terms.

They say astronomers desperately need a new definition and point out there is an obvious choice: define an au as some suitable multiple of a metre.

That would bring astronomy into line with SI units and making various calculations much more straightforward.

So what are they waiting for?

Ref: arxiv.org/abs/0812.2970: The Astronomical Units

The exoplanet photo gallery is bigger than you think

Monday, November 17th, 2008

fomalhaut

Astronomers tend to get excited by pinpricks of light. And perhaps today they have more reason than usual to celebrate the pixels that Paul Kalas at the University of California, Berkeley, and pals have found in one of the Hubble Space Telescope’s images.

These pixels, they say, represent the first optical image of a planet orbiting another star. The star in question is Fomalhaut in the southern constellation of Piscis Austrinus and one of the brightest in the sky.

Kalas and co say the planet is about three times the mass of Jupiter orbiting at a rather distant 119 AU. By comparison, Neptune orbits at around 30 AU so this is going to be one cold body.

That’s impressive work that has had significant press coverage but let’s put it in perspective.

Last year, the infrared Spitzer Space Telescope photographed HD 189733b, a Jupiter-sized gaseous planet orbiting a yellow dwarf in the constellation of Vulpecula. It even produced a heat map of the surface showing, unsurprisingly, that the planet is warmer at the equator than at the poles. But the map of HD 189733b got almost no coverage. And images of various “hot Jupiters” have been around for perhaps a decade or so.
I guess Hubble just has a better PR team.

Ref: arxiv.org/abs/0811.1994: Optical Images of an Exosolar Planet 25 Light Years from Earth∗

The cosmic ray revolution

Wednesday, November 12th, 2008

Cosmic rays, the high energy protons and helium nuclei that constantly bombard the Earth, have puzzled astronomers for the best part of one hundred years. Where do they come from and how are they accelerated to energies in excess of 10^20 eV—that’s about the energy that Roger Federer gives a tennis ball during a serve? (By contrast, the  Large Hadron Collider will be able to accelerate protons to a mere  10^12 eV.)

To tackle these questions, astronomers have built a giant cosmic ray telescope about the size of Rhode Island in Argentina. It’s called the Pierre Auger telescope and in the short time it has been operating, it is already challenging astronomers’ views about the origin of cosmic rays. In particular, it’s beginning to look as if the highest energy comsic rays come from active galactic nuclei.

Serguei Vorobiov from University of Nova Gorica in Slovenia summarises the highlights. Worth a read if you want to get up to speed on a new generation of astronomy.

Ref: arxiv.org/abs/0811.0752: The Pierre Auger Observatory — a New Stage in the Study of the Ultra-High Energy Cosmic Rays

Saturn’s anomalous orbit flummoxes astronomers

Friday, November 7th, 2008

saturn

One of the first tests of Einstein’s theory of general relativity was to explain the precession of the perihelion of Mercury, which had long bamboozled astronomers. Newton’s law of gravity simply cannot account for it. But relativity does.

Now it’s Saturn’s turn to flummox astrophysicists. The Russian astronomer Elean Pitjeva, who heads the Laboratory of Ephemeris Astronomy at the Institute of Applied Astronomy in St Petersburg, has analysed a huge data set of planetary observations dating back to 1913, including 3D observations of the Cassini spacecraft now orbiting Saturn.

She says that the precession of Saturn’s perihileon, as predicted by general relativity, needs to be corrected to fit the data. The correction is tiny: -0.006 arcseconds per century.

That’s an astonishing claim but perhaps not surprising given the growing body of evidence that some kind of correction to gravity is needed to explain various puzzling phenomena such as the Pioneer and Flyby anomalies.

Obviously Pitjeva’s work needs to be independently verified but already the astronomy-mill is hard at work guessing what might cause the deviation from Einsteinian physics.

It’s possible that known physics will do the trick: for example, our knowledge of trans-neptunian objects may have enough uncertainty to allow for this kind of correction.

Lorenzo Iorio at the National Institute of Nuclear Physics in Pisa Italy, outlines various explanations of known physics:

Our knowledge of trans-neptunian objects may have enough uncertainty to allow for this kind of correction but this turns out to generate a prograde precession no the retrograde precession found by Pitjeva

The Lense-Thirring effect generates a force that is four orders of magnitude too small to account for the difference

Mutual cancellations among unmodelled or mismodelled effects may have conspired to cause the effect but Iorio says this looks exceedingly unlikely

Neither do various exotic modifications of gravity or the DGP braneworld model explain the figures, says Iorio

So what’s left? A magnificent conundrum for astronomers to puzzle over until they get better data and/or a new theory of gravity that explains all.

Ref: arxiv.org/abs/0811.0756: On the Recently Determined Anomalous Perihelion Precession of Saturn

Martian methane and the smoking gun of life

Tuesday, October 28th, 2008

martian-clathrates.jpg

The presence of methane in the Martian atmosphere is a puzzle. Methane is broken down rapidly by sunlight and cannot last long in any atmosphere. A few simple calculations show that the lifetime of a CH4 molecule in martian climes is around 500 years. So methane ought not to exist in the Martian atmosphere at all unless it is being replaced on a regular basis.

So where could it have come from? Astrobiologists find this exciting because the methane in Earth’s atmosphere comes largely from cow farts (or more precisely the bacteria that live in their guts). The absence of  cowpats on Mars rules out the presence of ruminants on the red planet but leaves open the possibility that another primary source could be responsible, such as bacteria .

But before they can consider the possibility of life on Mars, astrobiologists must rule out every other possibility. One of these is that clathrates near the Martian surface are constantly releasing small amounts of methane as temperatures and pressure near the surface change.

Now Caroline Thomas et amis at the Universite de Franche-Comte in France have worked out how likely that is and say there are two possibilities.

First, they say that clathrates could only exist near the surface of Mars if the atmosphere had once been methane rich (otherwise the clathrates could never have formed). Perhaps the atmosphere was once temporarily enriched by a comet impact.

Second, there has to be some other source of methane, perhaps biological.

So how to distinguish between these scenarios. The discovery of gray crystalline hematite deposits on the surface could be a proof of an early methane-rich martian atmosphere.

Otherwise a biological source is a real option. Let’s get those rovers a-huntin’.

Ref: arxiv.org/abs/0810.4359: Variability of the Methane Trapping in Martian Subsurface Clathrate Hydrates