The Earth as a Benchmark

Research Paper:

Sterzik, M. F., Bagnulo, S. & Pallé, E. Biosignatures as revealed by spectropolarimetry of Earthshine. Nature 483, 64–66 (2012). [Supplementary Information]

News & Views:

Keller, C. & Stam, D. M. In search of biosignatures. Nature 483, 38–39 (2012).

Echoes & Waves: (social media: Altmetric)

The Earth in Time: One Month On the Moon (animation m4v 2m22s)

Frequenly Asked Questions:

1. What gave you the idea for this research? Did you start out thinking you were going to be able to get something that could be used to analyze exoplanets?
The final motivation is to establish a viable astronomical techniques to study and analyse the atmopsheres and surfaces of exoplanets, and in particular their biosignatures. However, we are convinced that the utilization of the Earth as still the only example of a life hosting planet is essential to be used as reference for earth-like exoplanets. Actually we had begun spectrapolarimetric Earthshine observations with the VLT already 6 years ago.

2. Why do you use (spectro-)polarimetry?
Most astronomical observations measure the brightness (or intensity) of the light coming from stars and other objects, often stretching the light into the familiar rainbow of colours that provides us with information on the nature of the emitting bodies, such as their temperatures and chemical makeup. For example, stars that appear predominantly white or blue will tend to be hotter than those that seem to be red, or yellow like our Sun. This new work exploits a different property of light, called polarisation, which tells us not only how bright an object appears, but also the direction of oscillation of the electromagnetic waves. This can sometimes reveal more about the emitting source and the materials through which it has passed on its journey to Earth.
There are many examples of polarisation all around us. Occasionally, we need to orientate a television aerial to receive a signal from a particular transmitter: by so doing, we are aligning the aerial so that it better picks up the horizontally or vertically polarised signal from that transmitter. Light reflected by certain surfaces such as a wet road, a lake, or a polished table, is polarised, and some people may have noticed that polarised sunglasses (polaroids) suppress part of the reflected light. Polarised light can tell us about the reflecting surface. In the case of the reflected light called Earthshine, it can tell us certain properties of the Earth’s atmosphere and surface. 

3. What’s the difference in the spectrum of light that’s been bounced back to Earth from the moon and light that is just bouncing off the Earth? Why does this matter for exoplanet studies?
We have used the moon as a giant mirror. This is the only way to see the Earth how it looks from space, but actually observing from the ground! Unfortunately, the Moon is not an ideal mirror and the signal to which we are interested in (the polarised reflected light) is modified when it bounces back from the Moon. The lunar surface damps the signal in which we are interested by a factor of 3, and this should be taken into account when our results are used as a benchmark for studies of extra-solar planets. Expected fractional polarization signals intrinsic to exoplanets are actually 3 times higher (but of course it would be strongly diluted by the hosting star)!

4. What are biosignatures?
We do not expect to see intelligent forms of life with our telescopes, but hope to detect characteristics associated with life, for instance gases such as oxygen, ozone, methane, and carbon dioxide. While these gases may also occur without the presence of life, their simultaneous presence with the abundances far from chemical equilibrium is only compatible with the existence of life. If life were suddenly to disappear, these gases would quickly react and combine with each other, and these characteristic “bio-signatures” would disappear too. 

5. How would a scientist use this technique to study an exoplanet? Does it need continuous observation? How detailed a picture do you think this could give us of an exoplanet’s atmosphere and potential biosphere?
A rough characterisation of the atmospheres of giant exoplanets is already in reach with present-day instrumentation and telescopes. For a more refined characterisation we need to wait for the next generation of extremely large telescopes.  Detection of spectroscopic features (such as O2, O3, H2O or an equivalent of the terrestrial vegetation red edge) that allow to infer biosignatures in Earth-like planets will be much more challenging.

6. Why is the spectro-polarimetric techniques  in particular well suited for ground-based studies of exoplanets?
While the precision of (normal) intensity spectra is affected by the Earths atmosphere when observing with ground-based telescopes (e.g. by spatially and temporally varying telluric lines), spectro-polarimetric signals are largely unaffected by atmospheric perturbations due to its intrinsically differential measurement character. Thus spectro-polarimetry with extremely large telescopes may become an interesting alternative to space-based missions for the characterization of exoplanets.

7. What follow-up work is planned?
We will continue to observe the Earth as a benchmark of a life hosting planet. We plan to obtain a better phase coverage (i.e., to observe the Earth under many different conditions) and in particular to follow the polarized “glint” of the suns reflex on the ocean. Our immediate objective is to compare the observed spectra with theoretical models of the Earth’s atmosphere and surface, as to improve on theoretical models, and eventually to apply them to observations of exo-solar planets.  We will also analyse circular spectropolarimetric data, that may contain “chiral” signatures of the Earth.

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