Report - School of Physics

those with masses between about 0.5–10 M ⊕ or, equivalently assuming Earth density,
radii between 0.8–2.2 R ⊕ . Planets below this mass in the habitable zone are likely
to lose their life-supporting atmospheres because of their low gravity and lack of
plate tectonics, while more massive systems are unlikely to be habitable because
they can attract a H-He atmosphere and become gas giants. Habitability is also
likely to be governed by the range of stellar types for which life has enough time
to evolve, i.e. stars not more massive than spectral type A. However, even F stars
have narrower continuously habitable zones because they evolve more strongly (and
rapidly), while planets orbiting in the habitable zones of late K and M stars become
trapped in synchronous rotation due to tidal damping, which may preclude life apart
from close to the light-shadow line. Mid- to early-K and G stars may therefore be
optimal for the development of life.
Owen (1980) argued that large-scale biological activity on a telluric planet necessarily
produces a large quantity of O 2 . Photosynthesis builds organic molecules
from CO 2 and H 2 O, with the help of H + ions which can be provided from different
sources. In the case of oxygenic bacteria on Earth, H + ions are provided by the
photodissociation of H 2 O, in which case oxygen is produced as a by-product. However,
this is not the case for anoxygenic bacteria, and thus O 2 is to be considered
as a possible but not a necessary by-product of life (for this signature of biological
activity, as well as for any other, a key issue is that of false positives, i.e. cases where
the signature is detected but there is no actual life on the planet, while the case of
false negatives, when there is some life on the planet but the signature is absent,
is significantly less ‘serious’). Indeed, Earth’s atmosphere was O 2 -free until about
2 billion years ago, suppressed for more than 1.5 billion years after life originated.
Owen (1980) noted the possibility, quantified by Schneider (1994) based on transit
measurements, of using the 760-nm band of oxygen as a spectroscopic tracer of
life on another planet since, being highly reactive with reducing rocks and volcanic
gases, it would disappear in a short time in the absence of a continuous production
mechanism. Plate tectonics and volcanic activity provide a sink for free O 2 , and are
the result of internal planet heating by radioactive uranium and of silicate fluidity,
both of which are expected to be generic whenever the mass of the planet is sufficient
and when liquid water is present. For small enough planet masses, volcanic activity
disappears some time after planet formation, as do the associated oxygen sinks.
O 3 is itself a tracer of O 2 and, with a prominent spectral signature at 9.6 µm in
the infrared where the planet/star contrast is significantly stronger than in the
optical (1.4 × 10 −7 rather than 2 × 10 −10 for the Earth/Sun case), should be easier
to detect than the visible wavelength lines. These considerations are motivating
the development of infrared space interferometers for the study of bands such as
H 2 O at 6–8 µm, CH 4 at 7.7 µm, O 3 at 9.6 µm, CO 2 at 15 µm and H 2 O at 18 µm.
Higher resolution studies might reveal the presence of CH 4 , its presence on Earth
resulting from a balance between anaerobic decomposition of organic matter and its
interaction with atmospheric oxygen; its highly disequilibrium co-existence with O 2
could be strong evidence for the existence of life.
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