(Image: NASA/JPL-Caltech/T. Pyle (SSC))

The exo-asteroids are lurking. While no asteroid has yet been found in another solar system, an analysis of observations from NASA’s Kepler spacecraft shows shadowy hints of their presence.

In our own solar system, asteroids called Trojans settle in the placid gravitational oases called Lagrange points that precede and follow a planet as it circles the sun. These stable points emerge as a consequence of the balance of forces between planets and their stars.


Jupiter, with its great mass, hosts the most Trojans, but similar asteroids have also been seen around Neptune, Uranus, Mars and even Earth.

We expect that Trojans should flank planets in other solar systems, too – we just hadn’t found them yet, probably because they’re so small.

Now an analysis of the full set of Kepler observations is showing faint signs of Trojans. To tease them out, Michael Hippke of the Institute for Data Analysis in Neukirchen-Vluyn, Germany, and Daniel Angerhausen of the Goddard Space Flight Center in Maryland looked at data from every planet Kepler has seen.

Starlight dip

Kepler finds planets by detecting a dip in starlight as planets cross, or transit, in front of their star. Asteroids should block some starlight too, but not as much.

The dip in starlight caused by Trojan asteroids in a single system would be buried in random wiggles, they reasoned – but laying many transits on top of each other should make them stand out.

From the combined observations of more than 1000 planet-hosting stars, the team found dips in starlight of about two parts per million in the parts of the orbit where the Lagrange points should be, where Trojans are expected to sit. That dimming could correspond to an asteroid around each star with a radius of 970 kilometres, or many smaller bodies added up – although the results are also consistent with many planets having no Trojans at all.

That size roughly matches what we see in our own solar system. “It’s in the same ballpark as we have near Jupiter,” Angerhausen says. The observations also showed that Trojans are found mostly near planets with orbits of longer than 60 Earth days.

Finding exo-Trojans may help us understand how giant planets move through solar systems, scattering asteroids in their wake, Angerhausen says.

“I think it’s a quite clever method that they use, and the results are intriguing,” says Markus Janson of Stockholm University in Sweden. Janson searched for Trojans in individual Kepler systems in 2013, but came up empty. “The reason why I didn’t find any is that there probably aren’t very many large Trojans out there,” he says.

But confirmation of these hints will have to wait. “This is pretty much all you can squeeze out of the full Kepler data set,” Angerhausen says. Kepler’s successor, The European Space Agency’s PLATO spacecraft, may settle the question after its planned launch in 2024.

Journal reference: arxiv.org/abs/1508.00427