With over 5000 Exoplanets already identified, it is possible to see patterns in the data and to begin
to speculate on whether our solar system is unique. Dr Gillen’s talk was grounded in his work over
many years to develop ways of identifying planets orbiting distant stars. He reminisced about
drinking champagne with Didier Queloz in 2019. He and Michel Mayor had just been awarded the
Nobel Prize for their work in making the first ever discovery of an exoplanet. This was a big gas giant
with a breathtakingly quick four day orbit round 51 Pegasi. The movement of spectral lines from the
spectrograph of the star had demonstrated that a large planet was swinging round the star. Its
detection had already threatened to upend theories of how planets form.
The majority of later discoveries arose from slight dimming of stars as planets moved in front of
them. Dr Gillen’s current work was to adapt algorithms to tease out signals pinpointing the tell-tale
regular motion of planets from the “noise” of stellar activity. Already this had shown that there
were many earth-sized planets (with an estimated 17% of stars hosting such).
However, plotting the mass of planets against orbital diameter had thrown up several curious
features. One was that the majority of planets so far seen were orbiting at a distance from their
suns of less than that of Mercury. Another was that around 40% of planets were either “super-
earths” or “mini-Neptunes” – neither type orbit our sun.
Of course, longer period planets were not susceptible to discovery in reasonable timescales by
current methods, and most systems so far observed were over a billion years old. Hence there was
a need to survey younger stars. New space missions, including the James Webb Telescope and the
TESS, the Transiting Exoplanet Survey Satellite, should assist.
Dr Gillen went on to talk about newer theories of how life could form on exoplanets. While there
was plentiful evidence of the availability of organic molecules that can react together to eventually
form the building blocks of life, it was clear that intense radiation near to a star could snuff out life
and strip promising planets of the atmospheres they required to sustain life. Hence, a balance is
required where bursts of radiation in the form of stellar flares could spark initiation of lifeforms on
planets in the “habitable zone” as long as this flare radiation is not too intense to erode the planet
ary atmosphere and/or sterilise the planet surface.
– Andrew Ramsay