Meet Cambridge's planet-hunter
Dr Nicholas Walton, of the Institute of Astronomy, is helping the European Space Agency's CHEOPS mission to study exoplanets
Are we alone in this universe?
Humankind’s ability to answer this fundamental question is growing as advances in technology help us probe deeper into the secrets of the cosmos.
Within months, the ExoMars Trace Gas Orbiter (TGO) – launched in March by European and Russian space agencies – will give us another clue as it measures tiny levels of atmospheric gases around Mars.
Depending on what else it sniffs out, it will help to answer whether the methane previously detected is geological in origin, or the natural waste product of Martian microbes on or below the surface.
The ExoMars mission will continue when a six-wheeled autonomous rover, being built at Airbus in Stevenage, is sent in 2020 to trundle over the red planet’s surface and drill two metres down into the soil in search of signs of life, either current or extinct.
But Mars is just one planet.
And while once it was far from clear that others existed beyond our solar system, we now know that ‘exoplanets’, as they are known, are common – and probably number in the trillions.
“If you go back 50 years, we thought our system was almost a freak occurrence and the formation mechanism for planets had a really, really low probability,” says Dr Nicholas Walton, of the Institute of Astronomy in Cambridge.
“Now we know that exoplanets are a fairly common part of star formation. We now have better theoretical models of how planets are made when a star forms. There are a lot of stars and galaxies out there… even if it’s only a third of all stars that have planets, there are probably more than 100 billion exoplanets in our Milky Way.”
As of May 1, we have confirmed the existence of 3,767 planets in 2,816 systems, 628 of which have more than one planet.
CHEOPS – the CHaracterising ExOPlanet Satellite – will help us learn more about some of them when it is launched this winter.
“It is the first of the European Space Agency’s small missions. Its larger missions are normally 10-plus years from proposal to launch.
“The idea here is to get it selected and launched in five years and to keep it within a smaller budget. Instead of 500 million to one billion euros for a typical larger mission, it’s around 100 million euros – half from ESA and half from partners,” says Nic, who is a CHEOPS board member and lead for the Institute of Astronomy’s participation in the mission.
“The idea is to fill a gap in our exploration of exoplanets. It will look for and characterise exoplanets in star systems where we’ve got a pretty good idea that they exist.
“Most of the targets are stars where exoplanets have been discovered by radial velocity studies, which tend to give us the mass but not the radius of the exoplanet.”
Observing the passage, or ‘transit’, of an exoplanet across a star will enable CHEOPS to make very precise measurements to changes in the light coming from the star –known as photometric dimming. This will tell scientists more about that planet, including its radius.
CHEOPS will focus on those for which the mass has already been estimated, by ground-based spectroscopic studies, to be in a range from super-Earths – up to several times our own planet’s – to that of Neptune, which has a mass 17 times that of Earth.
“Previous transit detection missions have been designed to look at lots of stars by monitoring large fields on the sky, and try to detect new exoplanets as they transit their host star. Whether or not a particular star had an exoplanet wasn’t known beforehand,” says Nic.
“CHEOPS only has a small field of view but will point at a star only when we know that an exoplanet will be transiting. Then it will move to the next interesting star, so it can always be observing transits.
“The idea is to look at bright stars – because you want to look at ones that are good for follow-up by other telescopes and facilities, to characterise the atmosphere of the planet, for instance. And we want to be sensitive to Earth-type planets – so planets up to a few Earth radii, around solar-type stars and with orbits more typical of Earth.
“CHEOPS is going to look for those types of objects with very high precision photometry. The dip in light you might see from an ‘Earth-like’ planet transiting a solar type star is only about one hundred parts per million, so a change in brightness of only 0.01 per cent. The system is designed to have the precision of 20 parts per million meaning that it will be sensitive to characterising Earth-like exoplanets.”
Armed with a telescope and a CCD – like the sensor found in a camera – the 250kg CHEOPS will be ‘baffled’ to prevent stray light interfering with its measurements.
“The problem is scattered light, primarily from sunlight reflected off our own Earth, and other light sources, which means you need a clever design so the optics of this are optimised to keep the ‘noise’ down,” says Nic.
And it could help discover new exoplanets.
“We can look for transit timing variations. If you’ve got a single planet going around a star, when it goes in front of the star it will always be at the same time.
“But if you’ve got a more massive planet outside the orbit – like Jupiter outside Earth – then the time that the inner planet does its transit will change depending on where the outer orbit planet is. It can either give you an acceleration or deceleration.”
Detecting this gravitational effect can help the Swiss-led international research team infer whether other planets may be present.
Changes in the observed light curves of the exoplanet transits can also help the team understand something about the atmosphere of an exoplanet.
“If you’ve got large ‘clouds’ in a planet’s atmosphere, perhaps analogous to the Great Red Spot on Jupiter, then there will be subtle changes in the light curve of the host star every time its exoplanet transits it, depending on where the ‘cloud’ is on the exoplanet,” says Nic.
“Potentially it will be possible to infer whether the exoplanet has very little atmosphere, or is more Earth-like with an atmosphere and structure indicative of containing ‘weather’ systems. You can then start focusing infrared spectroscopic missions, such as the soon-to-launch James Webb Space Telescope, on the more ‘interesting’ objects to learn more about the composition of the exoplanet atmospheres.
“Another thing with the light curves is that you can infer the planet’s shape. Spherical exoplanets have a more regular light curve than those that have a squashed shape. Earth is not quite spherical because of tidal effects.”
He adds: “It’s a neat little mission, largely because it’s always going to be observing, in detail, exoplanets as they transit in front of their host star.”
Work on the science instrument was completed last month in Bern, Switzerland, and CHEOPS was transferred to Madrid, Spain, for testing. With a launch window of December 2018 to March 2019, CHEOPS will hitch a ride into space with another payload aboard a Soyuz rocket operated by Arianespace from Europe’s spaceport in Kourou. Its mission should last at least 3.5 years, although it is hoped it may have enough fuel for five.
Meanwhile, the hunt for more exoplanets goes on.
NASA’s Kepler space telescope has been responsible for more than 2,000 of our exoplanetary finds to date, including the discovery last December of an eighth planet circling Kepler-90, a Sun-like star some 2,545 light-years from Earth. The discovery, made with the aid of Google machine learning of Kepler data, means our solar system is now tied for the most planets around a single star.
Kepler is expected to run out of fuel within months. But last month NASA launched its successor – The Transiting Exoplanet Survey Satellite (TESS) – which will conduct a two-year search for exoplanets, monitoring more than 200,000 stars for temporary drops in brightness caused by transits.
Then comes the James Webb Telescope, in which the Institute of Astronomy in Cambridge has also been involved. The largest space telescope ever built will be launched in 2020 and will help examine some of the best exoplanet candidates for habitable life from those studied by TESS and CHEOPS.
Nic and the Institute of Astronomy are also involved in the work of the Gaia spacecraft, from which data was recently released on the position of nearly 1.7 billion stars. It too can use photometry to detect exoplanets.
But its data can also be used to see the tiny ‘wobble’ in a star’s position caused by the presence of an exoplanet. This astrometric technique has only been used so far to detect a handful of exoplanets – using ground-based measurements – but Gaia could change that to tens of thousands.
Further missions are sure to follow all these, as humankind’s search for life on other planets gathers pace.
“I can’t believe there aren’t other planetary systems with life,” says Nic. “On Earth, there doesn’t seem to be any environment too hostile for some sort of life. It wouldn’t surprise me if we find life anywhere if we look hard enough.
“What that life might be is another question. You only have to look at Earth to know there is a massive diversity in living organisms, from single-cell bacteria to humans. There is a lot of discovery out there.”
With CHEOPS, some more of that exciting discovery is about to get under way.
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