Alpha Centauri, the nearest star system to our Sun, is like a treasure trove with many scientific discoveries just waiting to be found. Part of what makes it so compelling is that our efforts to detect extrasolar planets there have failed to yield any concrete results to date. While the study of exoplanets has progressed exponentially in recent years, with 4,575 confirmed planets in 3,392 systems in the Milky Way (and even neighboring galaxies), astronomers are still having difficulty determining if anyone is next door.
In the coming decades, Breakthrough Initiatives plans to send a mission there known as Starshot, a lightsail craft that could make the journey in 20 years. On Nov. 16th, Breakthrough Initiatives announced another project for detecting exoplanets next door. It’s called the Telescope for Orbit Locus Interferometric Monitoring of our Astronomical Neighbourhood (TOLIMAN), a space telescope dedicated to finding rocky planets orbiting in Alpha Centauri’s circumsolar habitable zone (aka. “Goldilocks Zone”).
The project, which comes from the Arabic name for Alpha Centauri, is a collaborative effort between Breakthrough Initiatives and NASA’s Jet Propulsion Laboratory (NASA JPL) in California, and the University of Sydney and Saber Astronautics in Australia. Work began on the mission in April of 2021 with a $788,000 grant awarded to Saber Astronautics by the Australian Government’s International Space Investment: Expand Capability.
Artist’s impression of the planet around Alpha Centauri B. Credit: ESO
Like Starshot, Project TOLIMAN is inspired in part by what we’ve learned about our closest stellar neighbor in recent years. In 2016, astronomers with the Pale Red Dot campaign confirmed the presence of a rocky planet orbiting within Alpha Centauri C’s (Proxima Centauri) habitable zone (Proxima b). In 2020, a second exoplanet was announced (Proxima c), a possible Super-Earth or mini-Neptune that resides well beyond Proxima b’s orbit.
There was also the mysterious radio signal detected in 2020 that appeared to be coming from Proxima Centauri. While this was recently revealed as Earth-bound interference, it still stoked our fascination with our closest stellar neighbor. However, there are still many unanswered questions about Alpha Centauri A+B, not the least of which is if this binary system could have any potentially-habitable planets of its own.
All attempts to confirm the existence of exoplanets around Alpha Centauri A+B to date have failed to yield anything definitive. In 2012, astronomers announced the possible detection of an exoplanet around Alpha Centauri B, but this was later shown to be the result of an artifact in the data analysis. But earlier this year, an exoplanet candidate (Candidate 1, or C1) was detected in Alpha Centauri A’s habitable zone, possibly a super-Earth or mini-Neptune.
Part of the problem with Alpha Centauri A+B is that these tightly-orbiting binaries throw off the most effective exoplanet-hunting methods. Since the two stars transit (pass in front of) each other regularly, there are regular dips in luminosity from one star blocking the other. This makes it very hard to detect dips in luminosity that might be caused by an exoplanet transiting in front of them (aka. the Transit Method).
This artist’s impression shows the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System. Credit: ESO/M. Kornmesser
Similarly, the gravitational influence A and B exert on each other makes it extremely difficult to determine if any “orbital wobble” is caused by the gravitational force of planets. This is known as the Radial Velocity Method (or Doppler Spectroscopy), which is the same method astronomers used to detect Proxima b and Proxima c orbiting their more loosely-bound companion Proxima Centauri.
To date, the largest share of confirmed planets beyond our Solar System were discovered by space telescopes that relied on the Transit Method – such as NASA’s Kepler and the Transiting Exoplanet Survey Satellite (TESS). Ground-based observatories with powerful spectrometers – like the Very Large Telescope (VLT), the La Silla Observatory, the W.M. Keck Observatory, and many more – discovered the second-largest share using the Radial Velocity Method.
However, finding exoplanets closer to home requires more finely tuned instruments, which is where the TOLIMAN mission comes in. As Pete Klupar, the Chief Engineer of Breakthrough Watch said in a recent Breakthrough Initiatives press release:
“These nearby planets are where humanity will take our first steps into interstellar space using high-speed, futuristic, robotic probes. 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. The signal we are looking for requires a real leap in precision measurement.”
The TOLIMAN concept was first proposed by Professor Peter Tuthill of the Sydney Institute for Astronomy (at the University of Sydney) during the 2018 SPIE Astronomical Telescopes+Instrumentation Conference in Austin, Texas. As they stated in their proposal paper, the telescope was intended to constrain the likelihood of rocky planets in particular star systems, which special attention dedicated to stars within 10 Parsecs (32.6 light-years).
Artist’s impression of the TOLIMAN space telescope. Credit: Tuthill et al. (2018)
As they explained, the key to this telescope was a diffractive pupil mirror pattern that does not concentrate light into a tightly focused beam (which is what is done with conventional telescopes). Instead, TOLIMAN’s mirrors spread starlight into a complex flower pattern that makes it easier to register the fine details required to detect the small “wobbles” in a star’s motion caused by the gravitational influence of one or more planets.
You might say that TOLIMAN offers a refined approach to the Radial Velocity Method by making it easier to tease out faint signals when making spectroscopic observations of a star. At the time of the 2018 SPIE Conference, the telescope’s potential had already been evaluated through a Foundational Mission Study, jointly funded by the Breakthrough Prize Foundation and the University of Sydney. As Dr. Eduardo Bendek, a member of the team from NASA’s Jet Propulsion Laboratory, said:
“Even for the very nearest bright stars in the night sky, finding planets is a huge technological challenge. Our TOLIMAN mission will launch a custom-designed space telescope that makes extremely fine measurements of the position of the star in the sky. If there is a planet orbiting the star, it will tug on the star betraying a tiny, but measurable, wobble.”
Possibilities for the Future
The ability to discern the presence of exoplanets around our nearest stellar neighbor will be a game-changer, for sure. At a distance of 4.24 to 4.37 light-years from Earth, the stars of the Alpha Centauri trinary will be the first place we send interstellar probes to, like Starshot. The discovery of Proxima b and c has already inspired support for interstellar missions that could study these planets up close.
Project Starshot, an initiative sponsored by the Breakthrough Foundation, is intended to be humanity’s first interstellar voyage. Credit: breakthroughinitiatives.org
Since its discovery, Proxima b has been the source of no shortage of scholarly debate. While it fits many of the qualifications for “habitability,” there is the issue of it being tidally locked with an active star that is prone to flaring. Nevertheless, much of the theoretical research and climate modeling conducted since its discovery contains the caveat that habitability depends on the state of the planet’s atmosphere and the presence of surface water.
In all cases, the researchers responsible indicated that any resolutions regarding potential habitability are dependent on future discovery. Since Alpha Centauri A is a Sun-like star less active than Proxima Centauri, it would be good to know if any rocky planets are orbiting in its habitable zone. Said Project leader Professor Peter Tuthill from the Sydney Institute for Astronomy at the University of Sydney, this mission will open a new window on the Universe:
“Astronomers have access to amazing technologies that allow us to find thousands of planets circling stars across vast reaches of the galaxy. Yet we hardly know anything about our own celestial backyard. It is a modern problem to have; we are like net-savvy urbanites whose social media connections are global, but we don’t know anyone living on our own block.
“Getting to know our planetary neighbors is hugely important. These next-door planets are the ones where we have the best prospects for finding and analyzing atmospheres, surface chemistry and possibly even the fingerprints of a biosphere – the tentative signals of life.”
“Our nearest stellar neighbors – the Alpha Centauri and Proxima Centauri systems – are turning out to be extraordinarily interesting.” added Dr. Pete Worden, Executive Director of Breakthrough Initiatives.” The TOLIMAN mission will be a huge step toward finding out if planets capable of supporting life exist there.”