Report - School of Physics

4.3 Statistics of Exo-Planets: Implications for Darwin/OWL
As noted under Appendix A, the McKee-Taylor Decadal Survey Committee (McKee
& Taylor, 2000) qualified its endorsement of the TPF mission with the condition
that the abundance of Earth-size planets be determined prior to the start of the
TPF mission. A similar strategy will be desirable for Darwin.
The various missions and experiments discussed throughout this report will contribute
to detailed statistical information of planetary distributions over different
mass ranges, but cannot provide complete information on the occurrence of Earthmass
planets in the Solar neighbourhood (say, to 15–20 pc) in advance of the planned
Darwin launch: transit measurements are highly incomplete, the Gaia lower mass
limit is above 10 M ⊕ , and the SIM survey will presumably also be incomplete down
to the levels of 1–2 M ⊕ . Gaia and SIM may assist in identifying nearby stars accompanied
by Jupiter-type planets (i.e., Jupiter mass in a Jupiter orbit), which may be
a representative pre-requisite for the development of life. Nano-arcsec astrometry
could provide the necessary information in principle, but practical implementation
lies some years in the future.
Nevertheless the SIM and COROT/Kepler/Eddington results should clarify whether
Earth-mass planets are common or not. If they are not detected, and therefore not
common, Darwin’s task in identifying Earth-mass planets within 20–30 pc will be
even more challenging. Then, either additional identification strategies are required
(for example, the development of an Antarctic facility dedicated to the purpose, such
as a coronographic ELT in the visible/near infrared, or a large interferometer in the
thermal infrared), or it is accepted that a significant fraction of Darwin observations
is devoted to a search for its own spectroscopic candidates. Thus Darwin could be
launched relying on a statistical estimate of the number, and size, of the terrestrial
planets that are expected around the stellar target list, combined with the GENIE
results on levels of exo-zodiacal emission. If statistics indicate that the number of
Earth-mass planets accessible to Darwin is of the order of several tens, this strategy
would be acceptable. Conversely, it could also be argued that a null result from a
200-star survey would be a valuable scientific result in its own right.
Current understanding suggests that metallicity is a crucial factor influencing the
formation of planets. Higher metallicity therefore seems to be the most promising
stellar characteristic that would indicate whether a star is likely to harbour planets.
The recently published Geneva-Copenhagen survey (Nordström et al., 2004), mostly
aimed at stellar kinematics, has also provided valuable information on metallicities
for over 14 000 F and G type dwarf stars using Stromgren photometry. A more detailed
measurement of stellar parameters, including metallicities, requires mediumto
high-resolution spectra over a large wavelength range. For extra-solar planet research
an accurate knowledge of the properties of the host star is essential, even
more so when looking for the weak signatures of the planetary atmosphere. A full
spectral characterisation of stars in the solar neighbourhood would be a valuable
step towards a target list for major projects such as TPF, Darwin or OWL.
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