Are you out there? Joe Haythornthwaite & Tom Ruen/CC BY-SA 4.0

The fate of an entire world is at stake. Astronomers are enlisting every telescope and space probe they can think of in the hunt for the solar system’s potential ninth planet, and some unlikely sources may be key to tracking it down.

Last month, Konstantin Batygin and Mike Brown at the California Institute of Technology in Pasadena announced they had found indirect evidence for “Planet Nine“. Following up on previous hints, they analysed the wonky orbits of small bodies beyond Neptune and determined they may have been caused by a planet 10 times the mass of Earth, with an orbital axis 700 times longer than the distance from Earth to the sun.

Now the race is on to spot the planet directly. We can’t just point the Hubble Space Telescope out into the void and track it down – the planet’s potential orbit is so large that we don’t know where to look, so a thorough search would waste Hubble’s precious time. Not only that, but Planet Nine’s suggested location is so far from the sun that it would barely reflect enough light for us to see.


So astronomers are getting crafty. Instead of visible light, they are looking for other unlikely signals that could help narrow the search.

Nicolas Cowan of McGill University in Montreal, Canada, and his colleagues have calculated that it should emit its own kind of signal we can pick up – radio waves.

The proposed planet is large enough to have retained a small amount of heat from its formation. Using Uranus and Neptune as a model, the team calculated this would be just tens of degrees above absolute zero – which means it would faintly radiate millimetre-length radio waves.

Motion sensing

It just so happens we have a bunch of telescopes searching the skies at these wavelengths, though planet-hunting astronomers don’t normally use them. Instead, they are used to look for the cosmic microwave background (CMB), the remnant of the first light left over from the big bang, which is at the same wavelength.

Cosmologists use telescopes like BICEP2 and Planck to map this radiation and learn more about the universe. They don’t normally concern themselves with mere planets.

“Cosmologists never look for moving targets,” says Cowan. But his cosmologist colleague Gil Holder, who works in a neighbouring office, heard the news of Planet Nine last month, and asked Cowan whether it would show up in CMB telescopes. “Apparently Neptune is so bright that they use it as a calibration source,” says Cowan.

Seeing a single bright spot at these wavelengths isn’t enough to detect a planet, as it could just be part of the background radiation. But a planet’s motion should help it stand out from the background.

Working with Nathan Kaib of the University of Oklahoma, the team calculated that Planet Nine’s speed across the sky should be distinct from the thousands of asteroids that are similarly bright, making it easy to spot with just a few months’ observations.

Many CMB telescopes are sited at the south pole with a narrow field of view. This is ideal for cosmology but not so good for planet hunting – they might not be pointing in the direction of Planet Nine. Future telescopes will look over wider patches of the sky, upping the chances of catching the planet. But it is possible current observations could hit the jackpot. “There is an outside chance that Planet Nine is already in someone’s CMB experiment,” says Cowan.

Saturnian searches

But these big bang telescopes aren’t our only option for finding Planet Nine. Agnès Fienga of the Nice Observatory in France and her colleagues have been using data from NASA’s Cassini probe, which has been exploring Saturn and its moons for the past 10 years, to pinpoint the planet’s potential location.

Astronomers have used radio-ranging data from the probe to make a model of the motion of all the large bodies in the solar system. Fienga’s team tried adding Planet Nine into the mix and found they could rule out the planet’s existence in around half of its potential orbit, as its tug from these locations would have shown up in the Cassini data.

The probe’s mission is due to end in 2017, but the team estimate that extending its life to 2020 would narrow the search even further. “It could help extend the forbidden zone,” says Fienga. Data from NASA’s New Horizons probe, which flew by Pluto last year, and Juno, due to arrive at Jupiter this July, could also rule out other parts of the orbit, she says.

“These are very clever ideas,” says Batygin. “It is wonderful to see that members of the community are presenting their own proposals on how to best optimise the observational search for Planet Nine. This is exactly what Mike and I hoped for.”

Cowan thinks the hunt for Planet Nine has fired astronomers’ imaginations, and by combining methods we should soon have a definitive discovery, or another explanation that rules out the planet. “I don’t think it will be very long, I’d say it’ll be two or three years,” he says.

So if Planet Nine does exists, what should we call it? Some, including Cowan, have already suggested “Bowie”, after rock star David Bowie who died shortly before Batygin and Brown’s paper was published. But the International Astronomical Union, which governs the naming of the cosmos, has strict rules on naming and prefers to reference ancient myths rather than modern celebrities, so it remains to be seen whether Bowie will be allowed. “I guess you could argue that maybe he is a mythological creature,” says Cowan.

Journal references: arxiv.org/abs/1602.05963, Astronomy & Astrophysics DOI: 10.1051/0004-6361/201628227