How Cavendish Laboratory at the University of Cambridge is hunting exoplanets with SPECULOOS project
Deep in the Atacama desert of northern Chile, another chapter is being written in the story of humankind’s search for life on other planets.
Aided by the Cavendish Laboratory at the University of Cambridge, SPECULOOS – the Search for habitable Planets EClipsing ULtra-cOOl Stars – is beginning scientific operations this month.
Its first observations have been made at the European Southern Observatory’s Paranal Observatory, following the installation of an array of four telescopes equipped with one-metre primary mirrors.
They will focus on detecting Earth-sized planets orbiting ‘nearby’ ultra-cool stars and brown dwarfs.
Laetitia Delrez, of the Cavendish Laboratory, a co-investigator in the SPECULOOS team, told the Cambridge Independent: “The Cavendish is funding one of the four telescopes. The team in Cambridge is also involved in the data processing. We are developing the data reduction programme that will be used to process all the data coming from this facility.
“The telescopes are kitted out with cameras that are highly sensitive in the near-infrared. This radiation is a little beyond what human eyes can detect, and is the primary emission from the dim stars SPECULOOS will be targeting.”
It is a daunting task. The planetary systems of ultra-cool stars and brown dwarfs are mostly unexplored.
Although researchers have now detected thousands of exoplanets, only a few of them have been found orbiting such bodies, and even fewer lying within their parent star’s habitable zone.
This is because these dim stars are hard to observe. But there are lots of them. In fact, they make up 15 per cent of the stars in the nearby universe.
SPECULOOS is designed to explore 1,000 of them, including the nearest, brightest and smallest.
Ultra-cool stars – the name is relative – are those with an effective temperature under 2,430 °C (4,400F). It is thought that their low mass and the relatively small size of the proto-planetary disk of gas that surrounds them when newly born could lead to the formation of relatively large numbers of Earth-like planets.
This theory was borne out by the discovery of TRAPPIST-1, the famous star now known to have seven temperate terrestrial planets orbiting it – the largest number detected in any other planetary system.
“We found TRAPPIST-1 from a sample of 50 stars. With Speculoos, we will be targeting about 1,000 of them, so we are hopeful we will find more systems like TRAPPIST-1, which would be really exciting,” said Dr Delrez.
Brown dwarfs, meanwhile, are curious ‘substellar’ or star-like objects – sometimes known as failed stars. Their mass falls between the heaviest gas giant planets and the lightest stars, making them too small to sustain the normal nuclear fusion of hydrogen to helium, meaning they fade and cool. For most of their lives, they might emit a mild magenta hue.
But if that sounds pleasant enough, look out for the huge turbulent storms that are thought to be common on brown dwarfs, with clouds made of molten iron and sand.
Planets have been found to orbit around these curious objects, however, including one discovered by ESO’s flagship Very Large Telescope (VLT), which also lies at the Paranal Observatory.
The site – lying at an altitude of 2,635m (8,645ft) and some 24 miles from the lights of the nearest village –features near-perfect observing conditions, with dark skies and a stable, arid climate.
The telescopes installed there for SPECULOOS have been named Io, Europa, Ganymede and Callisto after the four Galilean moons of Jupiter.
Together they comprise the SPECULOOS Southern Observatory (SSO), and they have achieved ‘first light’ – that is, recorded their first engineering and calibration images.
“I’ve been there a few times to help with the installation and commissioning,” said Dr Delrez. “It is quite a lot of work to install and calibrate them, and to be sure we can operate them remotely because they are robotic.
“We are very happy with the quality of the data so far. We did some guide measurements with the TRAPPIST-1 system and the light curves we are getting are stunning. We are definitely able to detect small planets around the stars we are targeting. We could even detect a planet as small as Mars.”
Michaël Gillon, of the University of Liège in Belgium, is principal investigator of the project and also led the team that discovered TRAPPIST-1.
He said: “SPECULOOS gives us an unprecedented ability to detect terrestrial planets eclipsing some of our smallest and coolest neighbouring stars. This is a unique opportunity to uncover the details of these nearby worlds.”
A number of tactics can be used to detect exoplanets, including the radial velocity method, which measures the ‘wobble’ caused in a star by the gravitational pull of exoplanets. But it cannot be used to characterise an exoplanet’s atmosphere.
“If we want to probe the atmosphere of an exoplanet, there is the direct imaging method or the transit method,” said Dr Delrez. “The technology for the direct imaging method is not yet able to observe the atmosphere of potentially habitable planets. There is a problem of contrast and separation from the star. With it, we are only able to observe very massive hot planets that are quite far from their star.”
SPECULOOS, then, will perform its planet-hunting using the ‘transit’ method, which monitors the subtle dip in a star’s light as a planet passes in front of it, like a mini partial eclipse. If a host star is small, then the amount of light blocked by an exoplanet is more substantial and therefore easier to detect.
“We are using those stars because they are very small stars, the size of Jupiter usually.
“Thanks to this, the transit light curve is deeper than if we were looking at a bigger star like the Sun, for example, so the transits are easier to detect,” explained Dr Delrez.
The transit method has been used since 2011 by the project’s prototype TRAPPIST-South telescope at ESO’s La Silla Observatory, which detected the TRAPPIST-1 planetary system.
Only a small proportion of the exoplanets discovered by this method have been Earth-sized or smaller.
But the research team hope that the small size of the stars being targeted by SPECULOOS and the high sensitivity of its telescopes will make the detection of Earth-sized transiting planets in the habitable zone possible.
What is not known is if planets around brown dwarfs could ever host life.
“This is a debated topic in the field. We don’t know yet. We want to start there and see what we find,” said Dr Delrez.
Once discovered, follow-up observations on these planets will use large ground facilities or space telescopes, potentially including the forthcoming James Webb Space Telescope, which the University of Cambridge has also played a part in building.
The two TRAPPIST 60cm telescopes at ESO’s La Silla Observatory and in Morocco will provide support for the project, which will also grow to include the SPECULOOS Northern Observatory under construction in Tenerife, and SAINT-Ex being built in San Pedro Mártir, Mexico.
The project team may also be able to collaborate with the Extremely Large Telescope (ELT), ESO’s future flagship telescope, being constructed on Cerro Armazones, also in Chile.
The ELT, with its enormous 39-metre mirror, promises to be able to observe planets detected by SPECULOOS in unprecedented detail and hopefully provide analysis of their atmospheres.
The effort is a truly international affair. In addition to Cambridge and Liege, the project, which has received funding from the European Research Council, also involves the King Abdulaziz University in Saudi Arabia, and scientists from the universities of Bern, Birmingham, and Warwick.
The telescopes and their brightly coloured mounts were built by German company ASTELCO and are protected by domes made by Italian manufacturer Gambato.
The name SPECULOOS, meanwhile, reflects its Belgian roots: it is a type of spiced biscuit traditionally baked in Belgium and other countries for Saint Nicholas’s day on December 6.
The TRAPPIST project, similarly, was named after Trappist beers, mostly brewed in Belgium.
“These new telescopes will allow us to investigate nearby Earth-like worlds in the universe in greater detail than we could have imagined only 10 years ago,” said Dr Gillon. “These are tremendously exciting times for exoplanet science.”
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