Report - School of Physics

3.3 ESA Themes: 2015–2025
In mid-2004, ESA solicited a call for ideas for its scientific programme beyond 2015,
resulting in several outline proposals in the area of exo-planets:
A Large UV Telescope (Lecavelier): the proposal considers a large UV telescope
as a promising way to conduct a deep search for bio-markers in Earth-mass
exo-planets, through the detection of relevant atmospheric signatures (such as ozone,
water, CO, and CO 2 ) in a very large number of exo-planets, and planetary satellites,
up to several hundred parsec distant. It notes that the detection of atmospheric signatures
in HD 209458b has been made through space UV observations, and stresses
that large numbers of prime targets for these observations will be detected by Gaia.
It underlines the possibility of time-resolved spectroscopy (over the 10-min typical
transit time of an Earth-mass planet) providing spatial information of atmospheric
constituents along the planet surface (poles versus equator, presence of continents).
A JWST-size telescope is proposed, and some quantitative expectations are given.
Search for Planets and Life in the Universe (Léger): the proposal makes the
general case for the continued development of an ESA strategy for the improved
detection and characterisation of exo-planets, primarily through the techniques of
transits and direct detection. This proposal was submitted on behalf of more than
200 individuals and institutions.
Astrometric detection of Earth-Mass Planets (Perryman): the proposal
points out that, beyond the micro-arcsec astrometric accuracies of ESA’s Gaia mission,
10 nano-arcsec accuracies would permit the detection of astrometric perturbations
due to Earth-mass planets around Sun-like stars at 100 pc. If a survey-type
mission were feasible, the concept could lead to the systematic survey of many hundreds
of thousands of stars for Earth-mass planets – important for the generation
of target objects if the fraction of Earth-mass planets turns out to be very small.
Earth-mass perturbations around a solar-mass star are 300 nano-arcsec at 10 pc,
or 30 nano-arcsec at 100 pc, the latter requiring an instantaneous measurement accuracy
a factor 3 better, i.e. 10 nano-arcsec at, say, 12 mag. This is a factor of
some 1000 improvement with respect to Gaia. Keeping all other mission parameters
(efficiency, transverse field of view, mission duration, total observing time per star,
and image pixel sampling, etc.) unchanged, we can consider reaching this accuracy
simply through a scaling up of the primary mirror size. The Gaia primary mirror
has an along-scan dimension D = 1.4 m and a transverse dimension H = 0.5 m; the
final accuracy scales as σ ∝ D −3/2 H −1/2 . These desired accuracies would therefore
require a primary mirror size of order 50×12 m 2 , and a focal length (scaling with D)
of about 1600 m, similar to the scale of the optics derived for the mini-version of
Life Finder. Accuracy levels of ∼ 10 nano-arcsec are still above the noise floors
due to interplanetary and interstellar scintillation in the optical, or stochastic gravitational
wave noise. Astrometric accuracy limits due to surface granular structure
and star spots were discussed in Section 1.3. To reach the astrometric precision of
300 nano-arcsec at 10 pc means that the photocentre of a star must be determined to
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