Search for the Avatar world

 

Imagine this. In the not too distant future, planetary explorers discover another Earth-like world, a perfect home away from home. And – by an astounding stroke of luck – this world, christened Pandora, resides within the closest star-system to us and so can be reached within just a few years of starship voyage.

Unlike Earth, Pandora is not a planet but an Earth-sized moon orbiting a bloated, Jupiter-like gas giant. And its star, Alpha Centauri B, is not solitary like our Sun, but part of a system of triplets. Pandora is a fertile world, though, lush with dense green vegetation that emits a purple glow at night; the Pandoran air is rich with oxygen, but contains trace amounts of poisonous gases, so humans need breathing apparatus. There’s a rich variety of alien animal life inhabiting the Pandoran jungles, including a species of intelligent, 3m-tall humanoids.

Of course, this is all science fiction. It’s the premise of James Cameron’s new film Avatar, to be precise. But there’s a great deal of real science behind this film that’s worth a closer look. Take the Alpha Centauri star system, a fascinating group of three fiery bodies. Its brightest star, Alpha Centauri A, is much like our own Sun but has a binary companion called Alpha Centauri B, which is slightly smaller and more orange-coloured. Orbiting both of these on a much wider path is Proxima Centauri, a cooler, dim red star.

As a habitable moon in orbit around a gas giant, itself orbiting Alpha Centauri B, Pandora’s situation is not unlike Jupiter and its moons, except that Jupiter’s largest moon, Ganymede, is 40 times smaller than the Earth. But it is possible that a gas giant a few times heftier than Jupiter could form an Earth-sized moon. In fact, astronomers are already searching for massive ‘exomoons’ orbiting some of the extra-solar gas giant planets we’ve discovered so far. Such giant exomoons would be likely to have lots of water, and feel powerful tides from their giant parent. There is nothing in principle that would prevent a world like Pandora from developing complex life.

The star system itself should also be capable of supporting life. Planetary scientists have run computer simulations of the Alpha Centauri system. These suggest that any planet within the habitable zone of either star – where it would receive just the right amount of warmth for liquid water and oceans – would be safe. A planet orbiting either of Alpha Centauri’s primary stars would certainly enjoy some spectacular sunsets, though the gravitational interplay within a binary system like Alpha Centauri could prove dangerous to the orbital stability of any habitable worlds.


Of plants and planets

That said, the jury is still out as to whether planets could actually form here in the first place. Planets form by building up from a swirling disc of rocky debris over millions of years. In a system like Alpha Centauri, the gravitational interference from the other binary star would be likely to wreck any forming planets except those very close to their parent, right on the inner edge of the habitable zone. Hence, a gas giant planet probably couldn’t form in this star system. So perhaps the exact scenario in Avatar, with Pandora being a giant moon of a Jupiter-like planet, isn’t actually very likely.

Pandora’s green/purple foliage is a more viable proposition. Vegetation on Earth looks green because plants have pigments that soak up sunlight to create the energy they need. The pigments absorb red and blue light, but reflect green. Astronomers have analysed the spectrum of light from Alpha Centauri B and found that it is similar to our Sun, so plants on Pandora would evolve under similar conditions. Many bacteria, fungi and coral here on Earth are bioluminescent, like the plants on Pandora, emitting a purple or blue light at night.


Search for Pandora

Kepler is a space telescope launched last year by NASA, specifically to hunt for Earth-like worlds. Kepler will spend over three years monitoring 100,000 stars simultaneously (though not Alpha Centauri, not least because it’s in the wrong direction), and is expected to find dozens of habitable Earth-like planets.

However, discovery of another Earth-like world is only the first step. The next stage is to get to know what the environment of the planet is like and search for possible signs of life. So far, the Hubble and Spitzer space telescopes have been doing a great job studying gas giant exoplanets. And when it is launched in 2014, the James Webb Space Telescope (JWST), with its much larger mirror and more sensitive instruments, will allow us to analyse the light coming from terrestrial exoplanets. Catching the spectrum of light that has passed through the planet’s atmosphere will enable us to read its chemistry and look for the ‘biosignatures’ of alien life.

The problem, though, is that any habitable planets discovered by Kepler are likely to be too far away for JWST to observe effectively. But Kepler will at least give us an idea of how many Earth-like planets are in our Galaxy; the hope is that other planet-hunting missions will then find some close enough for us to follow up with the JWST.

Credit: NASA/ESA/P. Kalas/J. Graham/E. Chiang/E. Kite (University of California, Berkeley), M. Clampin (NASA Goddard Space Flight Center) M. Fitzgerald (Lawrence Livermore National Laboratory) and K. Stapelfeldt and J. Krist (NASA Jet Propulsion Laboratory)

Hubble has found many gas giant-type exoplanets, such as Fomalhaut b (bottom right), orbiting its star (centre) within a ring of dust.


Could we find Pandora?

The Kepler and James Webb space telescopes offer a lot of promise for exoplanet research (see right) but neither, unfortunately, could be used to look for planets in the Alpha Centauri system, and other planet searches have so far turned up a blank for this region. Some astronomers argue that since these Sun-like stars are nearby we should dedicate a telescope observing campaign to checking for planets. They’ve calculated that any terrestrial worlds in the habitable zone of Alpha Centauri B could be picked up within five years of observations using a 1m ground-based telescope.

And if astronomers do detect an Earth-like planet (or perhaps a giant moon like Pandora) in our local stellar neighbourhood, we could even scan it for signs of life without ever leaving home. One of the effects of a biosphere is that it actually changes the chemistry of the planet itself. The most obvious sign of life on Earth is the very high level of oxygen in the atmosphere, pumped out by photosynthetic bacteria and plants. Different molecules absorb different wavelengths of light, and so by carefully analysing the light passing through a planet’s atmosphere we could read its chemistry, and hopefully spot the tell-tale fingerprint of oxygen in the air. Suddenly, the incredible worlds of science fiction and astronomy seem that little bit closer.


This article first appeared in the January 2010 issue of Sky at Night magazine

 
 
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