Friday, October 19, 2012

The Southern Wild

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This isn't a news blog. I'm not even sure what this blog is for sometimes. So it shouldn't come as a surprise that there was spectacular news from the European Southern Observatory on Tuesday night that I haven't mentioned until now. Alpha Centauri, the nearest star system to our solar system, has at least one planet. Greg Laughlin posted an excellent overview of the technical and cultural implications of this discovery here.

Alpha Centauri has planets! And not just any planets, the one discovered is almost a twin of Earth in size, one of the smallest ever discovered. This is wonderful news. Alpha Centauri is a stellar archipelago anchored by three main sequence stars, and the larger two (A and B) are close in size and age to our Sun. Multiple star systems are more common than singletons like the Sun, and finding at least one planet in the Alpha Cen system underlines the message that's been slowly emerging from the search for extrasolar planets. Where there are stars, there are usually planets, and where there are planets, there are possible homes for life.

No exoplanets will be discovered closer to home. This is the primary reason the ESO's discovery is so exciting. The new planet's proximity will make it more practical to observe and travel to than any other planetary system yet known. Note that that doesn't mean observation and travel will be easy. Alpha Centauri is still a frighteningly long distance away at the speeds human-built spacecraft have yet achieved. Even at the speed of light, it's more than a four year journey across the interstellar void. The most power-hungry engines we can imagine building today will only be able to manage a few percent of that speed. The most capable engines we can build today can only manage a fraction of a percent of that lower bar. Still, the divide between Sun and Alpha Cen is less than that to any other destination beyond the solar system, so this discovery is a boon for learning more of our planetary companions abroad.

I want to be sure that I'm not overhyping this discovery. The planet discovered is a strangely hellish place. She orbits so closely to Alpha Centauri B that a year there lasts about three and a quarter Earth days, and the planet's surface is so hot that the surface is probably more lava than rock. There are no oceans of water there, but there could be lakes of copper. Alpha Cen B's companion is not a home for starship pioneers, and would be a very challenging place even for robotic exploration. The main reason the announcement is so exciting is not so much for the freakish world it unveiled, but because it shows that more clement places may await discovery nearby.

Laughlin's description of the data collection and analysis that led to the announcement illustrates what a demanding challenge the ESO team has answered. The new planet is much too dim to see against the white-hot glare of Alpha Centauri A and B. Instead, astronomers saw the signature of her presence in the doppler shift of Alpha Cen B swinging back and forth against the planet's gravitational tug. A and B orbit each other in a lazy double-elliptical path that pulls them through a cycle once every 80 years, but even at that rate Alpha Centauri B is sprinting away from the Sun and double the fastest speed astronauts have ever achieved. The wobble due to the planet discovered perturbs this path by the speed of a falling leaf. The light leaving Alpha Cen B carrying the signal of the planet's presence left with a sticky residue of noise from starspot activity, convection, and stellar rotation. After travelling for four years across the vacuum of space, it was garbled further by turbulence in the last few milliseconds on the way through Earth's atmosphere to the ESO's telescopes in the Chilean desert. Like CERN's discovery of the Higgs boson earlier this year, this discovery is as much a triumph of statistical analysis as it is of instrument design and perseverance. While only one planet is currently known, the signal of an Earth-sized planet the right distance from B to have watery oceans will be harder to tease out of the data than that of the planet announced on Tuesday. An American team has been collecting high-cadence doppler observation data on Alpha Cen B since 2008. In a few years, they or the Europeans may have even more startling discoveries to announce.

Alpha Centauri B is the second-brightest star in its system, and the planet ESO announced earlier this week is the first planet discovered around B, so it will officially be known as Alpha Centauri B b for the time being. That's an awkward name, one that makes the planet sound more like bank account than a world. She'll be given a more appropriate name when astronomers and engineers are much further along in her exploration, as the Curiosity team has done with Mount Sharp on Mars.

Due to an awkward and unfortunate coincidence of Earth's orbit, attitude, and plate tectonics, most people won't be able to see the Alpha Centauri system tonight. Viewed from Earth, it's far south of the equator, and only appears directly overhead in the wild waters of the Southern Ocean. Alpha Centauri never appears for observers north of Houston, and is only occasionally visible between the equator and the Tropic of Cancer.
A few summers ago I was in Campinas, Brazil during July and August, just north of the Tropic of Capricorn. The familiar constellations of the north, the Big Dipper, Cassiopeia,  Polaris, where nowhere to be found on the long clear nights of the southern tropical winter. Each cloudless night, the Southern Cross glowed like a string of lighthouses pointing the way to the Antarctic, and the constellation Centaurus shimmered to its east. The brightest star, the Alpha of Centauri, formed one of the Centaur's forward hooves, and shined clear, twinkling, and colorless, like a diamond in a sea of rubies, sapphires, and pebbles. Now I know there was a world on fire that I was seeing in that point of light. Alpha Centauri is nothing to those in north, but in the southern skies it's brighter than Arcturus, beckoning those who look up from Sao Paulo, Sydney, and Cape Town. As people once ventured to those places, so we'll one day venture further south. South of the Cape of Good Hope, south of Amundsen's camp at the pole, south of orbits of all the planets, south until the Sun is just a white fleck in the northern sky, and home is among the worlds of Centauri

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