Report - School of Physics

4.6 Open Areas: Survey Mission Beyond Kepler/Eddington
The absence of specific survey missions beyond 2015 noted in Section 4.1 can only be
justified once all interesting targets in the sky to be studied in the future have been
discovered. For small terrestrial planets, the Kepler mission is the only approved
discovery mission. Even with Eddington, statistics of the occurrence of Earth-mass
planets will still be poorly known, and the need for a larger, more performant, space
transit survey seems already rather compelling. A case could also be made for an
all-sky transit survey from space, in order to find the nearest transiting terrestrial
planets, facilitating follow-up with astrometry and spectroscopy. The arguments
presented in Section 3.3, ‘Understanding the Planetary Population of our Galaxy’,
also draw attention to a lack of further search programmes for terrestrial planets
beyond Kepler.
4.7 Other Considerations
4.7.1 Fundamental Physical Data
It is not evident that present knowledge of basic atomic data is adequate to understand
the high-resolution spectra of normal host stars over the broad wavelength
region discussed in Section 4.4.1, and the spectral signatures of planets and their
atmospheres. Preliminary investigations suggest that there is a need for improved
atomic and molecular lines particularly in the near infrared. To illustrate the point
and the potential risk of inadequate atomic data we give two examples: (i) Li in low
mass post-AGB stars; expanded wavelength tables for Ce have shown that the line at
670.8 nm previously identified with (slightly redshifted) Li is coincident with a Ce II
line at 670.8099 nm. Abundance analysis subsequently showed that for all practical
purposes no Li is present in these stellar photospheres (Reyniers et al., 2002). This
is an important finding for stellar evolution and nucleosynthesis since an elaborate
mechanism had to be evoked in order to explain the presence of Li in these evolved
stars; (ii) variability of the fine structure constant: evidence from the absorption
features in distant quasars suggested a variability of the fine structure constant over
time. Originally, these studies had been hampered by limited accuracy of laboratory
wavelengths for the relevant species (Pickering et al., 2000). Most recently a further
complication introduced by the sensitivity to the abundance of heavy isotopes has
been described for Mg (Ashenfelter et al., 2004), where uncertainties in the isotope
ratios might imitate a variation of the fine structure constant at the observed level.
Nearer to the issues of exo-planets, the debate about the nature of Jupiter’s interior
centres on the (unknown) equation of state of H/He mixtures at high pressures.
Regardless of the exact details these examples illustrate that the lack of accurate
and reliable atomic data can lead to serious misinterpretation of astrophysical data.
When dealing with fundamental issues such as the properties of extra-solar planets
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