Over the course of this thesis the most important advance in research into exoplanets and their stellar hosts has been the development of new methodologies and skills. Astronomers have coded better tools, formed larger collaborations and developed better and better instruments.
K2received its final "goodnight" commands;TESS was launched and started operations; and ESPRESSOsuccessfully saw first light.
7.1.1 Know Thy Star
The work discussed in Chapters5and6addresses a crucial aspect of both stellar physics and exoplanet discovery which needs careful attention as more and more precise instruments come online. Treating the stellar effect in RVs and light curves simply as a ‘noise’ term will become less and less effective as smaller planets are discovered in more distant orbits. It will become essential to understand the behaviour of the star and the way this behaviour affects the light observed.
107
In Chapter5, the first large-scale survey of starspots, Giles et al.(2017), showed that the lifetime of starspots can be measured and that it has a strong dependence on the spectral type and size of spots. This was done by generating autocorrelation functions (ACFs) of over 2000 stars and measuring how the side lobes decayed away from the central peak.
Giles et al.(2017) provides statistical evidence that:
1) larger starspots live longer;
2) starspots of a given size on a cooler star live longer than starspots of the same size on a hotter star;
3) the Sun is not unusually quiet for its spectral type.
These are conceptually reasonable expectations, and perhaps could have been established in individual cases: this large-scale survey is the first confirmation that they are true across the board.
As well as the large survey ofKepler stars, the method was also adapted and applied to a light curve generated from archival data of the spot coverage of the Sun. However, due to additional signals present in the ACF (such as the long-term activity cycle and a signal which appeared to be a seasonal), it was not possible to apply the same quantitative tools. Whilst it was not possible to quantitatively measure the lifetimes of the sunspots, it was possible to establish qualitatively that the lifetimes would be of the order of a solar rotation, which matches previous measurements. This does however raise the question as to why it was not possible to perform the same test – does the fact that the Sun is observed in significantly more detail than a distant star is make a difference? Or is there something about the Sun which is intrinsically different from other stars?
As discussed in Chapter6, one current method for modelling the RVs of planets involves GPs which have a term that behaves like the starspot lifetime (López-Morales et al. 2016;
Haywood et al. 2018). Measuring starspot lifetimes from one source of data can significantly benefit a different source and improve our abilities to find exoplanets. This means that for transiting exoplanets from surveys which contain a long baseline (such as Kepler) then the measured starspot lifetime can be determined and utilised for performing RV follow-up of that candidate; especially if the star is particularly active and therefore needs a significant amount of analysis to extract the planetary signal.
Additionally, the results ofGiles et al.(2017) could be used photometrically. As part of the planning for theCHEOPSmission, software to simulate the type of light curves expected from the mission was created. For this to be as exact as possible, it was necessary to include a stellar activity aspect. To do so, early results ofGiles et al.(2017) were used to determine the typical spot sizes and lifetimes of different stellar types, which were decided when generating light curves for specific uses.
Furthermore, it was possible to demonstrate initial estimates of the effect of unocculted starspots on the observed transit depth of an exoplanet. As starspots decrease the overall stellar brightness, this also in effect reduces the perceived stellar radius. By underestimating the stellar radius, this would overestimate a planet’s radius. As part ofGiles et al.(2017), the starspot size proxy can also be considered as a typical starspot coverage – and this can be split with regards the stellar effective temperature and also starspot lifetime type. Ultimately, the effect was quite small, with the largest overestimation of a planet radius being of the order of 1% (typical uncertainties on a planet radius from multiple transits will be larger than this). But knowing that the effect would be minimal is valuable information, and whilst the 1% overestimation is currently a ‘small’ value with future instruments and advances it may become a point of concern in the future.
7.1.2 Know Thy Planet
The techniques described in Chapter2were successfully applied to the second half of theK2 mission, C11-18 (Chapter3and4). This included downloading, ‘cleaning’ and processing of the publicly available light curves. Given the population of stars observed by each campaign (see Table3.1) and the final candidates eligible for RV follow-up with CORALIE, the following is determined:
• for C11-18,185,508 stellar light curves were downloaded for analysis;
• 37,578 were deemed to have transit-like signals (∼20.26%);
• from those 37,578, only 18,654 (∼49.64%) met magnitude requirements for CORALIE, and the BLS criteria for the ranking;
• of the remaining candidates from the BLS, only 149 (∼ 0.80%) were deemed of good enough quality to be passed as potential candidates for CORALIE follow-up after being inspected by-eye.
Whilst there was a large number of candidates outputted from the BLS, they were only approximately 20% of the total number of stars observed, where only half of those met the criteria that were necessary for follow-up with CORALIE. More rigorous criteria could have been used, but given the risk of missing good candidates due to the ever evolving behaviour of K2 on the light curves, it was safer to allow ‘poorer’ candidates to be put forward and allow the human eye to save the less robust, but still strong candidates from being lost (e.g. the long-period monotransit candidate discussed in Chapter4).
As for the 149 final candidates eligible for CORALIE follow-up (see TableA.2 for more details), 32 were either already discovered and confirmed or were confirmed by other teams as follow-up was ongoing or pending. In fact, of the 149 final candidates, 47 candidates have
received RV observations (either as just initial points or intensive follow-up), and 11 were since confirmed (or part of a previous follow-up effort e.g. WASP-151 b). Of the remaining 36 candidates, 15 were found to be spectral binaries or other stellar contaminant flags which ruled them to be false-positives. Currently, 21 candidates are receiving RV follow-up. This includes the continued follow-up of K2-311b (Giles et al. 2018b), in the hope to characterise its mass.
From this two planets were found and confirmed. K2-140b (Giles et al. 2018a): a slightly inflated, Jupiter-sized planet in a 6.57-day orbit around a V=12.5 star. This planet was a proof of concept confirmation for the new pipeline in place in Geneva, from taking the light curves from the publicly available data, performing analysis and conducting follow-up with the Swiss telescope and other instruments.
K2-311b (Giles et al. 2018b) is the second discovery from the new Geneva-based pipeline.
It is a monotransit candidate which, after extensive analysis of its transit, RV follow-up and stellar properties, was deemed to be a Jupiter-like planet with an estimated orbital period of
∼10 years, orbiting an evolved sub-Giant star. Once finally confirmed, this would be the longest known transiting exoplanet.