Watching a baby planet’s birth

The first glimpse of a new baby is a happy event. Just ask the astronomers who spotted a Jupiter-like baby planet – the youngest planet ever directly observed through a telescope.

Stephanie Sallum and her colleagues at the University of Arizona in Tucson reported their baby gas giant, seen in the very act of forming, in Nature in November. “It’s something that we’ve never had a direct view of before,” says Katherine Kretke, a planetary scientist at the Southwest Research Institute in Boulder, Colorado.

The baby gas giant supports a key theory about how planets form. And it’s helping raise expectations that within a decade, astronomers will be directly observing Earth-like planets, says Franck Marchis, an astronomer at the Search for Extraterrestrial Intelligence Institute (SETI) in California, who also works in exoplanet detection.

A mere quarter of a century ago, few astronomers thought they’d ever get to see planets orbiting other stars. They were too far away from Earth and too close to their stars to be detected. Even today, distinguishing the infant planet  – it's called LKCa 15b – was a feat.

LKCa 15b circles a young, Sun-like star about 430 light-years away. The star is only two million years old, and is surrounded by a disk of the dust and gas planets are thought to form from, hinting that it was possibly a planet nursery.

Even more tantalisingly, astronomers had spotted what looked like a large gap in its disc. Could one or more newborn planets already be circling the star, sweeping up the dust and gas as they grew?

Sallum decided to take a closer look.

With the Large Binocular Telescope in Arizona, she and her colleagues confirmed the disc’s gap, and also found three objects within it that looked suspiciously like planets.

Then, with the Magellan telescope in Chile, they learnt that one of these objects, LkCa 15b, was exuding light in the hydrogen alpha band.

Hydrogen alpha red light is emitted by hydrogen gas that has been heated to thousands of degrees –what you might expect if hydrogen was spiralling into a newly forming planet.

Magellan failed to detect hydrogen alpha emissions from the other two objects, but that might be because the emissions are masked by dust clouds, says Sallum.

Simply by detecting hydrogen alpha emissions, the new find has added weight to one popular theory about how gas giants such as our own Saturn and Jupiter formed.

According to the “core-accretion” theory, these planets start out as rocky cores. Once large enough, the baby planets are able to scoop up and cling to a thick outer coating of gases – including hydrogen via the type of inward-spiralling gas clouds that produce the hydrogen alpha emissions seen by Sallum’s team.

LkCa 15b’s sighting has also demonstrated what’s possible if you’re prepared to combine a slew of astronomical tweaks and workarounds designed to coax ever-sharper images from large, ground-based telescopes.

Adaptive optics, computer-driven secondary mirrors that flex and bend to compensate for disturbances in the Earth’s atmosphere, helped.

So did a technique known as non-redundant masking. Here, signals from LkCa 15b were passed through a sieve-like mask, turning one big aperture into an array of smaller ones. The interference patterns from the signals coming through each aperture were reconstructed to give a far more detailed picture. The technique is similar to X-ray crystallography, which enables chemists to see the fine detail of single molecules.

Another trick astronomers can use to see exoplanets is to use a coronagraph, an instrument that blocks the light from the star, making it easier to find dimmer objects nearby – like a planet.

In August this year, Marchis was one of a team of astronomers who reported in Science they had employed this technique using an instrument called the Gemini Planet Imager to spot another gas-giant exoplanet, 51 Eridani b, possibly the smallest exoplanet ever directly observed.

Combining these tools in ever-more powerful combinations could even lead to even more dramatic finds, says Marchis. For example, the Gemini Planet Finder coronagraph is currently attached to a 8.1-metre telescope – puny compared to the twin 8.5 metre-mirrors of the Large Binocular Telescope. Fitting the LBT with a coronograph might even allow it to detect Earth-like planets around certain classes of dim stars, he suggests.

Buoyed by the discovery of 51 Eridani b, Marchis is prepared to speculate still further. “Maybe we could detect water and oxygen and methane [on an exoplanet],” he says. “It’s an exciting time.”

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