Earth’s glow and the search for life elsewhere

In recent years there have been many discoveries of planets orbiting stars other than our Sun. Most of the 350-odd new planets were found by indirect means, such as a tiny orbital wobble, or a dip in the level of light from the parent star as the planet moves in front of it across the line of sight. In a few cases we’ve seen direct optical imaging of exosolar planets, but the bodies in question are Jupiter-like in size, and thus very different from Earth.

What most of us are interested in is distant planets that resemble Earth and may harbour life, intelligent or otherwise. To detect such bodies we need to look at biological processes and their effect on the local environment, and the most obvious remote signatures of life are to be found in atmospheric chemistry. Find exosolar planets whose atmospheres are similar to that of Earth, and we may be on to something.

How would an Earth-like planet’s light spectrum appear from afar? To answer that question we could start by looking at the Earth itself from a distance greater than the orbits of remote sensing and meteorological satellites. In tomorrow’s edition of the journal Nature, a group of astronomers led by Enric Pallé of the Instituto de Astrofisica de Canarias in Tenerife present observations of Earth’s glow during lunar eclipses. The researchers measured the spectrum of light emitted and reflected back from the Moon as it moved through the Earth’s shadow.

When it comes to the effect of known biological processes on planetary atmospheres, we are particularly interested in chemical species such as ozone, molecular oxygen, water, carbon dioxide and methane, none of which feature strongly in the commonly measured reflection spectrum. But Pallé and his colleagues found that the chemistry is revealed clearly in the transmission spectrum. The researchers also detected in the transmission spectrum signatures of Earth’s partially ionised upper atmosphere – the ionosphere – and of its major lower atmospheric constituent, molecular nitrogen, which is missing in the reflection spectrum.

The lunar transit technique could in principle be extended to transits of exosolar planets in front of their parent stars. However, as Pallé and his co-workers point out, the faint signal of a terrestrial planet’s atmosphere will be mixed with the bright light of its star. Even with missions such as the James Webb Space Telescope, due to be launched in 2014, it will be a challenge to resolve an exosolar planet’s transmission spectrum, and data from multiple transits will have to be combined in order to provide a detailed picture of the planet’s atmospheric chemistry. Still, the authors say that between 20 and 30 one-hour transits should give us a decent transmission spectrum of an Earth-like planet orbiting a low-mass star in the galactic neighbourhood.

Exciting times ahead for planetary astronomy.

Further reading: “Earth’s transmission spectrum from lunar eclipse observations”, Nature 459, 814 (2009).