I
Roger DymockThe quest for ‘ET’ (Exoplanet Transits, not
aliens!)
f you thought that detecting exoplanets is the purview of professional astronomers only, then
think again. e dips in brightness caused by some exoplanets as they transit their star can be
detected by amateurs using medium- or large-aperture telescopes, astronomical CCD cameras and
some suitable processing software.With this in mind, the Exoplanets Division of the British Astronomical Association (BAA) is
collaborating with both amateur and professional astronomers to provide ground-based observations
of exoplanet transits in support of the ARIEL (Atmospheric Remote-sensing Infrared Exoplanet
Large survey) mission, which was formally given the green light by the European Space Agency in
November and will launch in 2029. So why not try your hand at observing an exoplanet transit, and
participate in a new pro-am project to support ARIEL, called ‘ExoClock’.Measuring exoplanetary transits isn’t as difficult as it sounds. An exoplanet transit is similar to a
transit of Mercury or Venus in that the exoplanet passes between its host star and Earth. As it does
so there is a very small drop in the brightness of the host star as the planet blocks some of the star’s
light – we’re talking thousandths of a magnitude, or millimagnitudes (mmag). is ratio of the size
of the transiting planet to that of its star is indicated by the depth of the transit on a light-curve
(which measures how the brightness of an object changes with time). Observers who are familiar
with measuring the magnitudes of variable stars, asteroids and comets should nd measuring these
small magnitude changes quite a similar process, but if you are new to this aspect of astronomy, then
don’t be put off – with a little practice you too can get involved and thus make a signi cant
contribution to the project.ExoClock
e ARIEL mission, which is being led by Giovanna Tinetti of University College London, will
observe the transits of approximately 1,000 exoplanets, mostly hot gas giants, to determine the
make-up of their atmospheres, which could give us clues about how they may have formed and how
they are changing. e late Sir Stephen Hawking stated “ ere is no bigger question in science than
the search for extraterrestrial life”. Homing in on exoplanet atmospheres, even those of inhospitable
gas giants, is part of the growing research programme to characterise exoplanets in order to
determine how they form and evolve, and what is required for any of them to support life.As a planet transits its star, the upper reaches of its atmosphere are not entirely opaque to the star’s
light, which can pass through it. However, certain molecules within the atmosphere absorb some of
that light at speci c wavelengths. ARIEL will conduct transit spectroscopy, which allows
astronomers to distinguish the absorption lines in a star’s spectrum produced by those atmospheric
molecules.Some of the exoplanets that ARIEL will be targeting will not have been observed for several years,
given the limited number of telescopes and amount of telescope time available to professional
astronomers, and therefore predicted transit times could be in error as a result of a phenomenon
known as transit-timing variations. In this case, these variations arise from the original ephemeris –
or example, the orbital period – being in error, especially if the transits have not been observed in
some time. If this is the case for a particular planetary system, and scientists don’t know about it,
then the transits could be missed by ARIEL. e ExoClock project is an attempt to rectify this, by
employing amateur astronomers to ll in the missing transit-timing data.To get started, visit the ‘ExoWorlds Spies’ website (exoworldsspies.com) and download the HOPS
software, which you can use to process your images to obtain transit light-curves. ere are datasets
already listed on the website that you can practice with. e website also describes how you can
image targets using your own telescope and process the light-curves so obtained. You don’t even have
to own your own telescope to participate – you can use, for free, the MicroObservatory Robotic
Telescope, which is managed by the Harvard-Smithsonian Center for Astrophysics and can be
accessed online (see Find out more, left). You may nd it useful to compare your results with light-
curves on the ExoClock database at http://www.exoclock.space, where you can also register to participate in
the ExoClock project and upload your data. Should you run into problems, you will nd that the
project team is very helpful.To assist you in getting started, we list six transit targets visible in the rst third of 2021, with their
light-curves taken from the ExoClock project database. A selection of transit times for UK observers
are given, but observers can input their own speci c requirements at exoworldsspies.com/en/
scheduler/to get a full listing.Roger Dymock is the Assistant Director of the Exoplanets Division of the BAA’s Asteroid and Remote Planets Section.A SELECTION OF TRANSIT TIMES FOR UK OBSERVERS
Observers should input their own speci c requirements at exoworldsspies.com/en/scheduler/ to get a
full listing.HAT-P-32b
is extraordinary planet, HAT-P-32b, which orbits close to an F-type star 950 light years away in
the constellation of Perseus, is a hot jupiter – a gaseous giant just 5.1 million kilometres from its star,
transiting every 2.15 days. Being so close to its star, it is heated to 975 degrees Celsius. is heat
results in the planet becoming swollen, so although it has a mass 0.86 times the mass of Jupiter, it is
bloated to such a degree that its diameter is 250, 000 kilometres, which is 1.79 times greater than
Jupiter’s diameter. e planet was discovered by the HATNet (Hungarian Automated Telescope
Network) in 2004.HAT-P-20b
Another hot jupiter planet, HAT-P-20b is a true heavyweight, weighing in at 7.59 times the mass of
Jupiter, yet with a diameter of just 0.95 times that of Jupiter, meaning it is a far denser world. It
orbits an eleventh-magnitude K-type star located 232 light years away in the constellation of
Gemini. ere’s some evidence that the star experiences plagues of starspots, which could cause
variations in its brightness that shouldn’t be confused with a transit. With an orbital distance of 5.49
million kilometres, HAT-P-20b transits its star every 2.88 days.XO-2Nb
Discovered by the XO Telescope, which consists of a pair of 200mm telephoto lens cameras, this hot
Jupiter, XO-2Nb, orbits an eleventh-magnitude star located about 500 light years away in the
constellation of Lynx. e planet has a little over half the mass of Jupiter, but its diameter is almost
the same as Jupiter’s – another case of bloating from the heat of its nearby star, which raises the
planet’s temperature to 925 degrees Celsius. With an orbital distance of 5.52 million kilometres, it
transits every 2.61 days.HAT-P-3b
Orbiting a K-type magnitude +11.5 star 450 light years away in Ursa Major, this gas giant, HAT-P-
3b, transits its star every 2.89 days at a distance of 5.83 million kilometres. It has a mass 0.61 times
that of Jupiter and a diameter of 124, 400 kilometres, or 89 per cent as large as Jupiter. Judging by its
density, it has a large rocky core containing the equivalent of 75 Earth masses. Observations by the
Spitzer Space Telescope suggest that HAT-P-3b’s atmosphere is very dark, re ecting very little light.HAT-P-12b
Some 468 light years from Earth, this remarkable gas giant, HAT-P-12b, orbits a thirteenth-
magnitude K-type star in Canes Venatici. It stands out from other gas giants because of its low mass- just a fth that of Jupiter – though its diameter is 95 per cent that of Jupiter. Recent observations
with the Large Binocular Telescope in Arizona suggest that HAT-P-12b has a cloudy atmosphere,
possibly looking somewhat like Jupiter’s atmosphere, with haze settled above the clouds, while joint
observations by the Hubble and Spitzer space telescopes identi ed water in those clouds. e planet
transits once every 3.21 days, orbiting its star at a distance of 5.74 million kilometres.
TrES-1b
An exoplanet more suited to observation in spring and summer is TrES-1b, located 523 light years
away in the constellation of Lyra. e planet orbits magnitude +11.8 K-type star GSC 02652- 01324
every 3.03 days at a distance of 5.98 million kilometres. It has a diameter fractionally larger and a
mass two-thirds that of Jupiter, while its atmosphere is heated to 725 degrees Celsius. It was
discovered by the Trans-Atlantic Exoplanet Survey (hence the acronym) performed by telescopes at
Palomar Observatory in California, Lowell Observatory in Arizona, and Teide Observatory in
Tenerife.Find out more
You can read up on the ARIEL mission at https://arielmission.space
Join the ExoClock website at http://www.exoclock.space
Download data sets and the HOPS software to turn your images into light-curves at exoworldsspies.com
An overview of the ExoClock project can be viewed on the website of the British Astronomical Association at
britastro.org/node/20102
Join the MicroObservatory Robotic Telescope Network at https://mo-www.cfa.harvard.edu/MicroObservatory/
Information about the MicroObservatory Robotic Telescope Network can be found at
britastro.org/sites/default/files/Microobs_0.pdf
Visit the BAA’s Exoplanet Division at britastro.org/section_front/15474
Amateur astronomers are being asked to help track the transits of hot jupiters – gas giant exoplanets orbiting close to their stars.A new project allowing amateur astronomers to participate in the study of exoplanets and
support a forthcoming European Space Agency mission is offering observers a chance to
broaden their horizons, writes the BAA’s Roger Dymock.
The depth of a transit depends on the relative size of the planet doing the transiting, and the diameter of the star it is orbiting.Some of the hot jupiters orbiting close to their stars are heated to almost a thousand degrees Celsius, or more, which
causes their atmosphere to bloat and sometimes stream away on the stellar wind, like a comet’s tail.
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January 2021
Astronomy Now