Lauren Fuge
A University of Sydney-led project is set to build a space telescope to look for rocky planets in the “Goldilocks” zone around Alpha Centauri – where the conditions are just right for liquid water to exist.
Alpha Centauri is the closest star system to the Sun, at just 4.3 light-years (a mere 40 trillion kilometres) away. It’s a triple-star system, consisting of Sun-like stars Centauri A and B, and Proxima Centauri, a faint red dwarf.
This system is ripe for planetary exploration from the new telescope project, named TOLIMAN (Telescope for Orbit Locus Interferometric Monitoring of our Astronomical Neighbourhood). In 2016, an Earth-sized exoplanet was found in the habitable (or Goldilocks) zone around Proxima Centauri, and another in 2019. It’s also possible that Centauri A has a Neptune-sized planet in the habitable zone, though this is not yet confirmed.
“Our nearest stellar neighbours – the Alpha Centauri and Proxima Centauri systems – are turning out to be extraordinarily interesting,” says Pete Worden, executive director of the Breakthrough Initiatives, which is partnering with the University of Sydney, Saber Astronautics and NASA’s Jet Propulsion Laboratory on the project.
“The TOLIMAN mission will be a huge step towards finding out if planets capable of supporting life exist there.”
The project is being led by astronomer Peter Tuthill from the University of Sydney, who says that although we have discovered thousands of exoplanets around distant stars, we know little about the ones on our own doorstep.
“Getting to know our planetary neighbours is hugely important,” Tuthill says. “These next-door planets are the ones where we have the best prospects for finding and analysing atmospheres, surface chemistry and possibly even the fingerprints of a biosphere – the tentative signals of life.”
Other telescopes, like NASA’s Transiting Exoplanet Survey Satellite (TESS) mission, are also aiming to find terrestrial planets in the habitable zone – so what makes TOLIMAN different?
It all comes down to precision.
“Our TOLIMAN mission will launch a custom-designed space telescope that makes extremely fine measurements of the position of the star in the sky,” explains Eduardo Bendek, team member from NASA’s Jet Propulsion Laboratory.
“If there is a planet orbiting the star, it will tug on the star betraying a tiny, but measurable, wobble.”
This detection method is called astrometry. It works by measuring the position of the star on the sky. A star’s proper motion is normally in a straight line, but if there’s an unseen planet around it, this will cause the star to wobble in relation to the background stars.
Only a handful of exoplanets have been found using the astrometric method so far, but it will likely be a big player in the future of exoplanet hunting. In particular, it’s sensitive to planets far from their stars, and it’s also great for looking for exoplanets in nearby star systems.
This is in contrast to the transit method – the most successful exoplanet-hunting technique to date – which watches for a small dip in a star’s brightness as a planet transits in front of it. The method works better for planets with tighter orbits, and it also relies on a lucky alignment of planets in front of their stars – more difficult to capture when stars are near to Earth.
The power of astrometry is currently being demonstrated by ESA’s Gaia satellite, which is in orbit making precise measurements of hundreds of thousands of stars. The mission’s main aim to create a 3D stellar map of the Milky Way. But astronomers will also be able to hunt through the dataset of stellar movements to search for telltale signs of exoplanets.
The TOLIMAN telescope will be a dedicated exoplanet mission, focused on finding rocky planets in the habitable zone through the astrometric method. It will use a diffractive pupil lens: a mirror that spreads incoming starlight into a complex, flower-like pattern, making it easier for astronomers to spot the movements of stars that betray the influence of planets.
TOLIMAN is projected to deliver results by the mid-2020s.
Pete Klupar from the Breakthrough Initiatives says that it’s important to further study the Alpha Centauri system because it may be the first destination we hit up on an interstellar journey.
“These nearby planets are where humanity will take our first steps into interstellar space using high-speed, futuristic, robotic probes,” he says.
“If we consider the nearest few dozen stars, we expect a handful of rocky planets like Earth orbiting at the right distance for liquid surface water to be possible.”