ALMA Minds the (Planet‑Forming) Gap

For the first time, astronomers have imaged a key stage in planet formation, witnessing the gas streams that signal two gas giant planets sweeping up material around a star.

A few million years after starbirth, leftover debris encircles the newborn fireball, feeding the growing star and any planets it hosts. But growing planets can carve large gaps in the disk, wreaking havoc with the food supply.

One such gap cuts the disk around young star HD 142527, stretching from 10 astronomical units (the size of Saturn’s orbit) to 140 a.u. (three times the size of Pluto’s orbit). The large gap poses a problem: the star ought to have gobbled up all of the inner disk by now, yet the disk persists. In theory, planets in the gap could initiate gas streams, which would replenish the inner disk and feed the central star. But theory remained unconfirmed by observations.

Now Simon Casassus (Universidad de Chile, Chile) and his colleagues have imaged the star system using 19 antennas that are part of the Atacama Large Millimeter/submillimeter Array (ALMA), which sits 16,400 feet high in the Chilean Atacama Desert. Even using only a fraction of the 66 dishes that will eventually make up the still-under-construction radio array, Casassus and his team achieved sharp images of HD 142527’s gaseous surroundings.

ALMA observed an outer, horseshoe-shaped gas ring around the star. The gravitational effect of planets is probably responsible for the distinctive shape. The outer ring had been imaged before by the Submillimeter Array, but in much coarser resolution. While the SMA hadn’t been able to resolve much inside the horseshoe disk, eagle-eyed ALMA discerned two gas “bridges” crossing the gap from the outer to the inner disk. ALMA even measured the gas speed as it spools from the outer disk and slams into the inner disk, feeding the central star in the process.

“Astronomers have been predicting that these streams must exist, but this is the first time we’ve been able to see them directly,” Casassus said in a press release.

A planet, by the force of its gravity, pulls each stream from the outer disk, but most of the gas doesn’t end up on the planet. Instead, much of the gas overshoots, passing the planet and slamming into the inner disk.

Casassus’s team also discovered diffuse gas in the disk gap — more evidence that the gap was cleared by planets and not by something more massive, like a companion star. Though the team attempted to detect the planets themselves, they had no luck — the gas giants are likely enshrouded in dust and gas that would make seeing them directly impossible for current instruments.

However, imaging the planets was not crucial to the study’s findings. Just by observing the gas streams and the residual gas in the cavity, the astronomers were able to measure roughly how much gas was flowing from the outer disk into the inner one. Since the star is accreting gas at a steady rate, the evidence is heavily in favor of planet-led gas streams. In future ALMA observations, the authors hope to measure just how much gas is streaming, to get a better idea of the mass of the planets themselves.