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06/03/2009

I’ve decided to pool my blogging with a few other folks. Please visit We are all in the gutter

Where the L2?

05/24/2009

So the European Space Agency’s Herschel and Planck telescopes are currently on their way and science teams are running around preparing for the start of observations. You may have seen various reports referring to their final destination, L2. I thought it would be a good time to write a quick bit about where this is and why it’s important.

L2 is the second Lagrange point. Lagrange is not only a reasonable rhyme for orange but is also the name of the designer of a system of mechanics that formed the basis for the only uni course I failed (the story involves a headache and a monkey). When studying gravity, finding the orbit of one body around another is fairly easy. Add a third body and the problem becomes pretty difficult. Lagrange used his new way of formulating mechanics to find a number of solutions to this three body problem. Imagine the Earth orbiting round the Sun. If you take the gravity of both into account along with the orbit of the Earth you get five special points where an object can sit without being pulled out of place (see the picture). L1, 2 and 3 lie on a line that connects the Earth and the Sun and L4 and L5 lie in the Earth’s orbit 60 degrees in-front and behind the Earth respectively. All five points stay in the same place relative to the Earth and Sun and Lagrangian points exist in all two body systems, the Sun-Earth system, the Sun-Jupiter system, planet-moon systems and even in planetary systems around other stars. In the case of the L2 point (and L1 and L3), a spacecraft can orbit around here with occasional small corrections from propulsion systems. For a space telescope this a pretty good place to be, you can orbit around L2 without using too much fuel and annoying things that get in the way like the Sun or the Earth are always in the same direction so you just need to point your telescope away from both (several telescopes that look at the Sun sit at L1 so the Earth doesn’t get in their way).

However in the L4 and L5 points are even more interesting. Not only are these points where objects can sit but nearby things are drawn towards them. Hence stuff tends to collect in these points. Saturn’s fourth largest moon Dione has two smaller moons Helene and Polydeuces sitting in in it’s L4 and L5 points respectively. Tethys, another of Saturn’s moons, also has two smaller moons in its Langrange points. In the Sun-Jupiter system there are loads of objects collected around the L4 and L5 points. These are asteroids that just seem to have wandered in there and got stuck. Those that lie at the L4 point are named after Greek characters from the Trojan war and those that lie around the L5 point are named after Trojan characters. In total there are maybe a million of these Trojans (a slightly confusing name for both the Trojan and Greek ones) over 1km in size, although I don’t think there are a million names in the Illiad so I doubt they are all named after Trojan war characters. Interestingly I read this paper a while back which came up with a way of detecting Trojans in planetary systems around other stars.

Shredded Asteroids – bet you can’t eat three

04/30/2009

OK this may be a bit late but I thought I’d write about a story that got a bit of press last week. It’s based on this article by Jay Farihi and collaborators. Put simply most white dwarfs (the dead remains of stars, similar in mass to the Sun but the size of the Earth) should have atmospheres that are pure Hydrogen or pure Hellium. This is because the photosphere (the bit of the star the light is emitted from, the visible surface of the star if you like) isn’t very deep within the star. It’s thought that metals like Calcium will have sunk below this photosphere so there shouldn’t be any sign of these elements when these objects are studied. However some of these objects do show signatures of metals.

The question is, where did these come from? The authors look at two competing models, one where the white dwarfs get their metals from clouds of interstellar dust (clouds of dust that sits in the space between stars) and one where they get them from shredded asteroids. In the first model the white dwarfs move through space, collide with a dust cloud and gravity sucks some of the dust onto their surface. In the latter an asteroid in orbit around the white dwarf comes too close to it and is shredded by the strength of its gravity. Both cases should produce very different signatures when these white dwarfs are studied in mid-infrared light (about one tenth of the wavelength of visible light). Observations by the authors using the Spitzer space telescope combined with other observations suggest that some of these white dwarfs have signatures associated with disks of dust around them. This suggests an asteroid was shredded by the white dwarf’s gravity and formed a disk around it. All but a couple show signatures that are incompatible with sucking in interstellar dust. Hence it’s probable that these objects got their metals from sucking in dust that was made from shredded asteroids.

The final question is, how did the asteroids get close enough to be shredded. Surely they should just continue on their orbits like they do in the solar system. Well they authors speculate that this could be due to planets around the white dwarf. One of the these could give the asteroids a gravitational tug that could end up with them being flung towards the white dwarf, close enough to be shredded.

High proper motion stars mess with Google Sky

03/25/2009

I was alerted to this by a post on the Bad Science forum. There one of the posters had used the position of Barnard’s Star to estimate the date a Google Sky image had been taken. Barnard’s Star has the highest proper motion of any star known. This means it moves across the sky compared to background stars. Looking at the image I wondered why Barnard’s star appeared only once as a single, very blue image. These colour images are made by combining images of the sky taken in different colour filters. These images are often taken years apart so for an object such as Barnard’s star which moves very fast across the sky the positions in each filter will be different. Hence you would expect to see one blue, one green and one red image in three different positions, this doesn’t appear to happen.

I decided to check out a few other high proper motion stars to see what they looked like. Proxima Cent is a bit weird, I can’t seem to identify it on the image, perhaps it is the blue thing on top of a background star, there is certainly a bit of noise where the UKST I plate (Google Sky uses a combination of data from the UKST, POSS telescope and the Sloan Digital Sky Survey plus a few other sources of more detailed images) position from the 1970s is. A better example is Kapteyn’s Star is a better example. Notice the bright very blue object in the upper right, that is the blue image from 1975, while the noisy thing in the middle is around the position of the red image from 1998. You can see a better subtraction for Luyten’s star.

Frankly I’m not sure about the finer points of how Google Sky make their colour images. This is clearly an artifact of the way in which they combine the images. Anybody able to use this to work out why this is happening?

A previously seen planet?

03/14/2009

At the end of last year a couple of papers appeared with some very promising looking direct images of extrasolar planet candidates. Until now the bulk extrasolar planets (i.e. planets outside our solar system) have been found either by the radial velocity method where the motion of the parent star being pulled around by the planet is detected or the transit method where the planet obscures a portion of the parent star, blocks some of the light that would otherwise reach us here on Earth and makes the star appear a bit dimmer. One of the candidate direct images was of a planet around the nearby young star Beta Pictoris, the discovery paper by Lagrange and collaborators is here and the press release is here. These direct images are very difficult to acquire as the star is much, much brighter than the planet (in the case of Beta Pic about 1500 times brighter) and the atmosphere and telescope optics smear out the star’s light, covering the spot on the sky where the planet is. The group led by Lagrange used the Very Large Telescope in Chile along with the NaCo instrument (which both blocks out most of the light from the parent star and corrects for some of the atmospheric smearing). This has allowed them to image what looks like a planet near the star. Of course it could just be another, fainter, unrelated star behind Beta Pic, in these cases you need to come back a few years later to check the planet is moving through space along with the parent star to make sure. However the chance of this just being coincidence is pretty small.

So why am I writing about this now? Well this week a paper appeared that may indicate this planet was detected before, in 1981. Back then Beta Pic was seen to dim briefly, as if a planet passed in front of it. This of course begs the question “was the imaged planetary candidate responsible for the transit?” This is the question the authors try to answer. A planet will only transit if you are looking at the system edge-on and we have a clue that the Beta Pic system is very close to edge-on. Like many young stars Beta Pic has a disk of material around it that is thought to form planets. We know that the disk around Beta Pic is pretty close to edge on and you’d expect the planets in any system to orbit roughly in the plane of the disk. Hence it is possible the planetary candidate could transit in-front of the star. The authors then go on to try to work out (assuming the planetary candidate and the transiting body are the same thing) when a transit would happen again and what the planet’s orbit is. They find the most likely solution is a planet orbiting Beta Pic at a distance of eight times the Earth-Sun distance every 16-19 years. Both the direct detection and the transit suggest the planet is a gas giant.

The idea that the transit of a planet across its parent star could have been detected in 1981 sort of reminds me that astronomy is a passive science. In most research you design an experiment, have complete control over it, carry it out and note down the result. In astronomy you can’t grab two bottles of chemicals off the shelf and mix them, you can only look. If there is a planet around Beta Pic in a 16-19 year orbit then it was also there in 1981, it was also transiting at the end of the last century and in the mid-60s, just nobody was looking. Almost everything we can study, measure and analyse in astronomy is already out there, we just haven’t looked hard enough yet.

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