Building a planet is a mysterious process. New worlds emerge from the disk of leftover dust and gas that swirls around an infant star, but it’s still not clear how planets form or how quickly they grow.

Astronomers have typically had difficulty peering into the dusty planet nurseries formed from newborn stars. But in the past few years, they’re starting to get glimpses—and important clues as to exactly how planets form. Image credit: Shutterstock/Neo Edmund.

Now researchers have added another puzzling discovery to the mix. On average, these planet-forming disks appear to contain far less material than the planets they spawn, a recent study in Astronomy & Astrophysics suggests (1). “We expected the disks to be more massive than the planets that formed in them,” says astronomer Carlo Manara of the Garching, Germany-based European Southern Observatory, part of the team behind the study. “But the disks are smaller—less massive—than what is needed.”

The cocoon of dust and gas around a newborn star mostly contains hydrogen molecules and helium along with traces of carbon, oxygen, and other elements. Once the star’s rotation has pulled this cocoon into a disk, planets have only about 3 million years to form before stellar winds blow off most of the hydrogen gas, leaving only dust behind. Somehow, “gas giant” planets such as Jupiter or Saturn must be created before that gas disappears.

These dusty nurseries have long hidden nascent worlds from curious astronomers. But over the past few years, the Atacama Large Millimeter/submillimeter Array (ALMA), a group of radio telescopes on the Chajnantor plateau in northern Chile, has probed the disks’ millimeter-sized dust particles and measured their abundance with incredible precision. In 2014, ALMA captured images of a protoplanetary disk in unprecedented detail, and a few hundred more disks have been observed since then. Meanwhile, astronomers have found thousands of exoplanets outside our own solar system since the mid-1990s.

The recent study directly compared the masses of more than 1,500 confirmed exoplanet systems with the masses of about 120 disks, the first time that such a large comparison has been done. It found that the disks contained only about 20% of the average mass of the exoplanet systems. So where did these planets get the rest of their ingredients from?

Building Blocks The answer may lie in the various planet-building blueprints that scientists have proposed. The most massive planets, far bigger than those in our solar system, are thought to form rather like stars. Gas and dust in the disk simply collapse in on themselves to form these gigantic worlds, a process known as gravitational instability. “All you need to do is get enough gas and have it cool rapidly enough to collapse down,” says Katherine Kretke, a senior research analyst at the Southwest Research Institute’s (SwRI’s) Boulder, CO office who models planet formation. “We see stars forming like this all the time.” Planets created by gravitational instability would grow relatively quickly and retain a lot of leftover gas and dust in their atmospheres. But gravitational instability builds only the most massive planets. For smaller worlds, the material found spinning in the disk around the star begins to clump together, gathering or accreting more dust until they become a larger planet. Most researchers today think that most planets form this way, except the most massive super Jupiters, says Alessandro Morbidelli at Nice Observatory in France, who worked with Manara and Nice colleague Tristan Guillot on the Astronomy & Astrophysics study. Both models have their problems. The gravitational instability process happens so fast that it can only produce giant planets several times larger than Jupiter, according to astronomers’ calculations. “You could never form the Earth with gravitational instability,” says Morbidelli. In contrast, the slow sweeping up of material is too sluggish to generate a Jupiter. By the time a small rocky core has formed, the gas required for a gas giant’s atmosphere is long gone. Somehow, the cores of some worlds must form fast enough to pick up a lot of gas. Worse, the process just assumes the presence of a large collection of planetesimals with no indication of where the cores came from. “Maybe we don’t wait around for 2 million years to start growing our planets, but instead things go right away.“ —Kevin Walsh Astronomers needed to figure out how the rocky planetesimals formed, and in recent years a subclass of the accretion model known as pebble accretion has bridged the gap (2). In this model, the cores start off as chunks that range from micron-sized dust grains to meter-sized boulders. As the growing cores interact with one another, larger planetesimals shove their smaller siblings out of the disk, stifling their growth and ensuring they remain too puny to collect much gas. This arrested development offers a way to grow the planets of the solar system, both small and large, as well as the most massive exoplanets. Despite the success of this model, Kretke cautions that astronomers should not restrict themselves to a single blueprint for planet formation. “As we learn more about planets, we see that there is a really a diverse population,” she says. “Probably some planets do form by different mechanisms.” ALMA in Chile captured this disc of rotating, leftover material surrounding the young star HD 163296. The gaps between the rings are likely the result of a depletion of dust; astronomers also found less gas in the middle and outer gaps. Both features suggest the presence of newly formed planets. Image credit: ESO, ALMA (ESO/NAOJ/NRAO); A. Isella; B. Saxton (NRAO/AUI/NSF).

Missing Mass While theorists have been trying to figure out how to build a planet, observers have been busy hunting them down. Today, more than 5,000 potential and confirmed exoplanets have been spotted (for the latest count, see exoplanets.org/). ALMA’s insights about the dust within protoplanetary disks have also helped astronomers infer how much gas the disk carries. Adding dust and gas together provides the total mass of the disk, which—as Manara, Morbidelli, and Guillot found—is much less than astronomers expected. Understanding why this discrepancy arises could help develop detailed blueprints for planet formation. One possibility is that astronomers have underestimated the amount of gas in the disks. Molecular hydrogen does not emit or absorb much radiation, so it is difficult to measure directly. Instead, astronomers estimate the amount of hydrogen based on the quantity of dust or the amount of another gas, carbon monoxide. Edwin Bergin, an astronomer at the University of Michigan in Ann Arbor, is pioneering a third approach. He hopes to make more accurate hydrogen estimates by tracking hydrogen deuteride (3). This molecule contains a heavier isotope of hydrogen, called deuterium, and has a much stronger signature in far-infrared wavelengths than molecular hydrogen itself. Bergin has been stymied by the loss of the European Space Agency’s Herschel Space Observatory in 2013, which was ideal for making observations at this wavelength. However, NASA’s Stratospheric Observatory for Infrared Astronomy (SOFIA), a modified jumbo jet, will soon be outfitted with an instrument that can spot hydrogen deuteride’s signal, Bergin says. A second explanation for the missing mass is that planet-forming disks are strewn with rubble that telescopes have not been able to see. ALMA can only observe dust that is a few millimeters in size, so anything larger—say a kilometer-sized asteroid—is effectively invisible. If the dust starts coagulating almost immediately, there could be a wealth of asteroids hiding in the disks, which would suggest that planetary cores are forming even faster than astronomers thought. “Maybe we don’t wait around for 2 million years to start growing our planets, but instead things go right away,” says Kevin Walsh of SwRI, who models planet formation. There are signs of this in our solar system, says Walsh. Iron meteorites suggest that relatively large asteroids had grown and melted within a few million years, for example, and Mars formed within an estimated 2 to 4 million years after the birth of the solar system. Morbidelli and his colleagues raise a third option: that something keeps feeding the disks like a river feeds a lake. Gas and dust from the vast tracts of space between the stars could be captured by a new star’s gravity and drawn into the disk. “We know that disks form [inside larger] clouds of dust and gas,” Manara says. “It may well be that some of the material gets to the disk at later stages and gives the disk a larger amount of material available to form the planet.” Walsh and his colleague Harold Levison, also at SwRI, agree the disk could be nourished by an external source. Levison says that previous models of planetary formation have tended to ignore this and thinks the new results may prompt theorists to adjust their models to include the interstellar medium. “It’s going to force us now to address this limitation on our models, which we knew was there all along,” he says. However, Bergin isn’t so sure that material from the interstellar medium makes a big difference. In-falling material should have its own radio-wave signature, he says, and those haven’t been widely observed. Instead, he thinks the composition of our solar system shows that planetary building blocks are assembled as cores at an extremely rapid pace. Even as the debate continues, this interplay between theories and observations will clearly be crucial to understanding how planets are born. Telescopes such as ALMA can continue to measure the composition and masses of disks, and if SOFIA can be used to study hydrogen deuteride, some of the missing mass might be found in the near future. Theorists have already begun to model how the molecular cloud surrounding the disk can influence it. “The only thing we can hope for,” says Kretke, “is more data, more modeling.”