Planets May Have More Time to Form Than Previously Thought

A recent study suggests that protoplanetary disks may tend to linger longer than we thought, meaning that planets likely have at least 5 million years to form before their building materials vanish.

Disk Dispersal Deadlines

illustration of a protoplanetary disk being evaporated by a nearby massive star

One way that protoplanetary disks are dispersed is by radiation and winds from massive stars, as shown in this illustration. [NASA/JPL-Caltech]

Planets arise from gaseous disks called protoplanetary disks. While the details of planet formation are hidden from view within these dusty disks, the big picture is clear: the timeline for planet formation is set by the lifetime of the disk — once the disk disperses, planet formation must come to a halt. Determining how long planets have to form should be a simple task, then: researchers can measure the ages of star clusters and determine whether the stars in those clusters have disks, thus establishing a cutoff point at which disks typically disperse.

In reality, however, this technique has produced a wide range of estimates for the lifetimes of protoplanetary disks, and thus widely varying constraints on how long planets have to form — and the shortest estimates, in the 1.0–3.5 million year range, set a tight deadline for models of planet formation to meet.

Plot of disk fraction as a function of cluster age and distance

Fraction of stars with disks as a function of cluster age and distance. More distant clusters tend to have smaller disk fractions. [Adapted from Pfalzner et al. 2022]

Young, Massive, and Misleading?

In a recent publication, a team led by Susanne Pfalzner (Jülich Supercomputing Center and Max Planck Institute for Radio Astronomy, Germany) suggested that careful application of existing techniques can provide a little more wiggle room for modelers, lengthening the typical lifetime of a protoplanetary disk. Researchers often study disks around stars in clusters, since it’s more straightforward to determine their ages than stars outside of clusters. However, it’s easier to identify young, compact clusters than it is to find old, dispersed clusters, especially at large distances from Earth. Since bright, massive stars are easier to detect at large distances, studies biased toward younger clusters are also biased toward more massive stars — which are known to have shorter-lived disks.

As a demonstration of this effect, Pfalzner and coauthors examined how the results of previous studies varied with the properties of the clusters in each study’s sample. They found that samples containing mostly distant (>650 light-years away), young clusters resulted in short estimates for disk lifetimes, while samples containing nearby, old clusters were linked to long disk lifetimes.

Modelers Everywhere Breathe a Sigh of Relief

Plot showing the effect of stellar mass and initial disk fraction on the median disk lifetime in star clusters

The effect of stellar mass and initial disk fraction (IDF) on the median disk lifetime in star clusters. [Adapted from Pfalzner et al. 2022]

To counteract this issue, the team constructed a new sample that is evenly balanced between young and old clusters that are located within 650 light-years of Earth. Analysis of this sample suggested a median disk lifetime of 6.5 million years, with a substantial fraction of disks enduring for 10–20 million years — meaning that in many star systems, planets have far longer to form than expected.

While this result provides much-needed leeway for our models of planet formation, there are still plenty of open questions to explore. For example, it’s important to pin down the fraction of stars that are born with disks; assuming that all stars are initially shrouded in disks implies typical disk lifetimes in the 5–6 million year range, while allowing for a small fraction of stars to be born diskless would allow planets 8–10 million years to form around low-mass stars and 4–5 million years to form around high-mass stars. Regardless of the exact timeframe, understanding how high-mass stars form planets under stricter timescales than low-mass stars will remain a challenging question to answer.


“Most Planets Might Have More than 5 Myr of Time to Form,” Susanne Pfalzner et al 2022 ApJL 939 L10. doi:10.3847/2041-8213/ac9839