The exoplanet β Pictoris b plows a path through its star's disk of gas and dust in this illustration.

ESO/L. CALÇADA

Oh baby! Astronomers hope to glimpse giant newborn planet’s rings and moons during rare transit

Astronomers are staring at a nearby star in hopes of seeing a giant baby of a planet pass across its face, perhaps accompanied by dust clouds, rings, or newborn moons. Last week, the newest and tiniest telescope joined the vigil, when the French-built PicSat rode into orbit on an Indian rocket. It will be able to continuously monitor the star, β Pictoris, until chances of seeing the once-in-20-year transit event diminish in a few months' time. "We can't miss this. We would be kicking ourselves," says astronomer Matthew Kenworthy of Leiden University in the Netherlands.

Astronomers have seen thousands of exoplanets transit, or cross the face of their stars, eclipsing a fraction of their light. But β Pictoris, a bright star just 63 light-years away, is special. It is a natural laboratory for how solar systems form because it is only 24 million years old—the "equivalent of a baby of a few weeks," says Sylvestre Lacour of the Paris Observatory.

In 1984, astronomers observed a disk of gas and dust around it, the first protoplanetary disk to be seen. The disk, viewed nearly edge on, was warped and had gaps, a sign of planets in the making. But it wasn't until 2009 that researchers spied the faint glow of a hot, young giant planet, 10 times the mass of Jupiter, in a roughly 20-year orbit. Now dubbed β Pictoris b, it is one of only a handful of exoplanets to be imaged directly.

The discovery could explain why, in 1981, β Pictoris's light dimmed erratically by up to 6% over 2 weeks, then brightened again. Another transit may have passed unnoticed 2 decades later, and the newfound planet appeared to be heading for yet another one in 2017 or 2018. Recent calculations suggest that the transit will be a near miss. But the planet's large "Hill sphere"—a zone of gravitational influence that may contain planetary rings, clouds of material, or newly formed moons—may yet reveal itself in dips in the light of β Pictoris.

To catch those dips, astronomers needed to monitor the star 24 hours a day over most of a year—too big a commitment for most observatories. So Lacour and his colleagues decided to build a small one of their own. In 3 years, with €1.5 million from the European Research Council, they built PicSat, a 5-centimeter space telescope in a satellite only slightly larger than a toaster. "It was risky and not everyone believed in it," Lacour says.

Kenworthy, along with Eric Mamajek of NASA's Jet Propulsion Laboratory in Pasadena, California, decided to observe from the ground. They built two washing machine–size robotic observatories, dubbed bRing, sited in Australia and South Africa.

A few existing telescopes have also joined the hunt: the Bright Target Explorer Constellation, five microsatellites designed to study luminous stars; and a 40-centimeter telescope that's part of the Antarctic Search for Transiting ExoPlanets, which can watch β Pictoris continuously during the darkness of the southern winter. "We had to ensure we didn't drop the ball and had at least one scope on the star during the transit," Kenworthy says. The researchers also lined up agreements with larger telescopes to swing into action if they did see something.

PicSat, scheduled for launch in September 2017 but delayed by a launcher failure, finally reached orbit on 12 January. The Paris team is now checking its health. Sadly, it is joining the party toward its end; the transit of the Hill sphere is expected to end in February. "Maybe we won't see anything. We knew from the start it was risky," Lacour says.

Kenworthy remains optimistic. "We're not discounting anything," he says. Even a null result will imply that just 24 million years after its birth, the baby planet has already cleared out its Hill sphere. And once the predicted transit of β Pictoris b is over, the astronomers will keep watching the star and its planet nursery, hoping to see something else, like the fleeting transit of a smaller baby planet.