steorra: Jupiter's moon Europa (europa)
[personal profile] steorra posting in [community profile] astronomy
A piece of big astronomy news came out yesterday: An earth-sized planet has been discovered orbiting Alpha Centauri B, a relatively sun-like star (a bit smaller and fainter than the Sun) in the nearest star system to the Sun. The planet is not particularly earth-like, though, since it orbits the star in only about 3.2 days, and thus would be extremely hot.

I started to write a long entry talking about Alpha Centauri system the system and the discovery, but I think instead I'm going to link a few articles - there are many more that could easily be found - and talk a bit about some things I noticed in reading the original paper.

The Exoplanet Next Door - a news story from Nature, which also published the paper reporting the discovery.
Planet Found in Nearest Star System to Earth - press release from the observatory through which the discovery was made.
Rocky planet orbiting one of our stellar neighbours - a good story by Keith Cooper at Astronomy Now.
Alpha Centauri and the New Astronomy - Emily Lakdawalla's recommended reading on the topic, a story by Lee Billings at Centauri Dreams.

And, as background, also recommended by Emily Lakdawalla, an earlier, pre-discovery, in-depth story by Lee Billings about the search for planets in the Alpha Centauri system.

The original paper is available, with supplementary material, here from Nature if you have access; if you don't, the paper (without supplementary material) is also available here from the website of the observatory through which it was discovered.

I gave the paper a quick reading, and while some of it was too technical for me to understand, I was able to follow the basics of it. And I was once again amazed by the amounts of information that astronomers are able to squeeze out of apparently tiny amounts of data - in this case, the spectrum of a star observed repeatedly over time.

The planet was discovered by doppler spectroscopy, one of the main methods of discovering exoplanets. The basics of this are fairly easy to understand - as the planet and star mutually orbit their common centre of gravity, the planet moves lots, but the star also moves a little. And as the star moves, we see its light undergoing doppler shift - blueshifted as it moves towards us, and then redshifted as it moves away. The doppler shifts involved are pretty small, especially for a planet of only Earth-like mass, so they're difficult to detect. It involves careful picking out the signal created by the planet from noise created by other sources.

What amazed me was the complexity of modelling the noise created by other sources, and what that modelling implied we could deduce about Alpha Centauri B. For instance, one of the noise sources they had to account for was the fact that in a sun-like star like Alpha Centauri B, the outer layers of the star undergo convection, and the up-and-down motion of convection produces doppler shifts.

More interesting to me, they also had to take the star's rotation into account. As it rotates, one side turns towards us, producing blueshifted light, while the other side turns away from us, producing redshifted light. If the star was equally bright everywhere, the blueshift and redshift would cancel each other out. But if the star has sunspots, it disrupts that; as a sunspot appears on the edge approaching us, less of the blueshifting side of the star is bright, producing an overall redshift in the light coming from the star. When the sunspot crosses the face of the star to the side turning away from us, the redshifting side of the star is less bright, producing an overall blueshift. From such effects, they could determine that Alpha Centauri B has a rotational period of about 38 days (for comparison, the Sun's rotation period is about 24 days).

Another layer of complexity is added by the fact that different latitudes of the Sun, and apparently also of Alpha Centauri B, rotate at different rates - faster at the equator and towards the poles. And for the Sun, the solar activity cycle tends to produce sunspots at different latitudes at different times in the cycle - at the start of the solar activity cycle, spots tend to occur around 30 degrees north and south of the equator, and over the course of the cycle move towards the equator. Measuring the rotational period of the Sun based on sunspot averages would then make the Sun appear to rotate slower at the beginning of the cycle and faster as the cycle progresses (as the sunspots approach the equator). And something similar seems to happen on Alpha Centauri B - the the rotational period appeared to be 39.76 days in 2009, 37.80 days in 2010, and 36.71 days in 2011.

There were more noise effects that they had to account for too, but I thought the rotational period and activity cycle effects were particularly neat.

Here's an article that talks about some of the noise sources in non-academic terms.
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