Microlensing

The methods described up to this point often depend on a periodic signal of some sort, usually based on the orbital period of an exoplanet. Microlensing is different given that it is an event which typically only occurs once.

‘Lensing’ is a phenomenon which occurs due to general relativity, where the path of light (with respect to an observer) can be bent due to passing close to a massive object. A nearby star passes through the sky more quickly that a distant star, but if they align the perceived brightness of the background star would increase as it was lensed by the foreground star. If we now consider the foreground star has a planet orbiting it, given that the planet also has a mass (though smaller) it will also lens the background star (Figure1.15). Therefore, monitoring the sky for such lensing events, and inspecting them more closely for smaller, similar increases in brightness close to the main incidence can indicate the presence of a planet. The first discovery from a microlensing survery was Bond et al.(2004) who detected the signature of a 2-body system for the star OGLE 2003-BLG-235, where two peaks were observed in the light curve of this star. However it was clear that the only possible solution was two bodies with a mass ratio of∼0.004. By assuming a the primary object was a main-sequence star, the inferred mass of the companion was∼1.5 Jupiter masses, with an orbital radius of roughly 3 AU.

Figure 1.15: Illustration showing the concept of microlensing. As a background star passes behind a foreground star, the observed brightness increases as the foreground star lenses the background star’s light. During the lensing, if there is a planet around the foreground star, there will be a sharp increase in brightness.

1.7 ‘Space – The Final Frontier’

As mentioned previously, there have been various attempts to detect exoplanets from both ground-based observatories and space telescopes. With over 4000^g currently confirmed exo-planets, the detection rate is increasing almost exponentially (Figure1.16).

However, many things can be extrapolated about this group of planets which boil down to how and where they were found. For example, each method typically finds planets within specific regions. As can be seen in Figure1.17, Imaging tends to find exoplanets in distant orbits with higher masses; microlensing also finds planets with long-period orbits; whereas exoplanets discovered via RVs or transits have a much larger range of orbits and masses. However, it is clearer to see the distribution of periods and method of discovery in Figure1.18a; transits dominate significantly, but the discoveries usually have slightly shorter periods than other methods. Additionally, of the transit discoveries, the majority were found with space-based telescopes. In fact, over 2680 came from theKeplerandK2missions alone.

The mission goal ofKeplerwas to find Earth-like exoplanets around solar-type stars. The importance of space- vs ground-based telescopes is significant for this goal, as Earth’s atmo-sphere and day-night observing restrictions have serious effects on ground-based photometry.

Going into space, these no longer matter and therefore near continuous viewing of targets is possible at high precision. In fact, a significant proportion of the Earth-like exoplanets

dis-gAs of 1st August 2019, from theNASA Exoplanet Archive

Figure 1.16: Number of confirmed discoveries per year for the five methods described in Section1.6:

RVs, transits, direct imaging, astrometry and microlensing. There is a near-exponential increase in the number each year.

Figure 1.17:Distribution of the mass and period of confirmed discoveries, split by the discovery method (RVs, transits, direct imaging, astrometry and microlensing). Each method typically finds planets within different regions.

Figure 1.18: Distribution of the confirmed exoplanet periods, split by discovery method (a) and locale (b). A significant proportion of confirmed discoveries come from space-based telescopes targeting transiting exoplanets. The majority of those found from the ground were part of radial velocity searches, apart from a small peak between 1 and 10 days which came from ground-based transiting surveys .

covered so far have come from telescopes in space (Figure1.19), usually in combination with ground-based follow-up efforts. The larger, Jupiter-like planets have been discovered by both space- and ground-based telescopes.

The most recent addition to the series of space-based telescopes isTESS, Transiting Exo-planet Survey Satellite, which will cover an area 400x larger than Kepler (85% of the sky).

Launched April 18th, 2018, it has a 2-year primary mission; it is expected to find thousands of exoplanets (Stassun et al. 2018;Barclay et al. 2018). First light ofTESSwas August 7th, 2018 (Figure1.20). Previous ground-based telescope surveys mainly found Jupiter-like exoplanets,

Figure 1.19: Distribution of the confirmed exoplanet radii, split by discovery locale. Nearly all Earth-like planets have been found by space-based telescopes, with ground-based telescopes primarily finding Jupiter-like, gas giant planets.

butTESSwill find a large number of small planets around the nearest and brightest stars in the sky (which are the best for follow-up observations).

1.8 Thesis Outline

This thesis is organised in two main halves: one dedicated to the discovery of new transiting exoplanets; the other to the analysis of starspots.

Chapter2describes the key methods and tools used to discover and characterise exoplanets, which is generalised to no specific mission. This is followed by Chapters 3 and 4, which demonstrate how the toolkit from Chapter2was used in two different cases (specifically how the general methods from Chapter2were adapted).

In Chapter5, details of howKeplerstars were analysed for the effect of starspots on their light curves can be found. Additionally in Chapter5, there is also an attempt to analyse Solar data in the same way. The methodology detailed in Chapter5is then discussed with respect to its relevance for exoplanet discovery in Chapter6, where there are two cases where the stellar activity work has been utilised in determining masses of exoplanets.

The final chapter, Chapter7, concludes this thesis with a summary of the work done and thoughts for future work.

There are two appendices. AppendixA gives the full list of detected exoplanets by the

Figure 1.20: TESS(Transiting Exoplanet Survey Satellite) will cover an area 400x larger thanKepler and was launched April 18th, 2018. During its 2-year primary mission, it is expected to find thousands of exoplanets. This image is from a 30-minute cadence from August 7th, 2018. Key features seen include the Large and Small Magellanic Clouds and globular cluster NGC 104; and the brightest stars in the image are Beta Gruis and R Doradus. (Image credit: NASA/MIT/TESS)

method described in Chapters2-4(split into two tables, one for stellar properties and the other for planetary). In AppendixB, all publications to which I contributed, but were not directly connected to the work described in this thesis, are listed.

Chapter 2

Methodology & Toolkit

‘I was taught that the way of progress was neither swift nor easy.’

Dr Marie Skłodowska Curie