Avoiding violent neighbors key to planet formation in star clusters
According to the results of a new study, stars orbiting near the centers of massive clusters have a much harder time forming planets than those located at the edge. The discovery was made possible by the powerful viewing capabilities of the Hubble Space Telescope, and could help astronomers unravel the secrets surrounding star formation in the Milky Way.
We are lucky enough to have evolved on a rocky planet that is sheathed in a dense protective atmosphere capable of shielding us from the radiation pouring out from our parent star – a relatively placid main sequence stellar body known as a yellow dwarf. But we don’t orbit in isolation. Our solar system is mature, and has given rise to a population of diverse planets and moons that tread their own complex paths around the Sun.
But this wasn’t always the case. To find out how star systems like our own came to form astronomers must look to less developed stars that are still in the planet forming stage. To this end, a group of scientists using the Hubble Space Telescope’s Wide Field Camera 3 have sought to shed light on the conditions that are favorable to planet formation around young stellar bodies orbiting in a massive star cluster called Westerlund 2.
These clusters are like stellar beehives, and it is thought that our own Sun formed in one before migrating outward into the relative quiet of the cosmic void.
Westerlund 2 is one of the most massive young star clusters in the Milky Way. It is located 20,000 light-years from Earth in the Sagittarius arm of our spiral galaxy, and comprises many thousands of stars. Systems like these are a goldmine for astronomers hoping to understand the dynamics surrounding stellar bodies much younger than our Sun, known as pre-main sequence stars.
The three-year near-infrared study of Westerlund 2 revealed that the massive, bright stars that clustered near the center of the vast cosmic structure were mysteriously free of the dense clumps of gas needed to form planets. However, smaller stars observed on the outskirts of the cluster hosted huge amounts of planet-forming material that could settle into an accretion disk.
Hubble is not able to spot material orbiting a star in Westerlund 2 directly, but is able to track dips in the light of a star as the material passes across the stellar body’s surface relative to Earth. By tracking the frequency and amount of time that a star dims, scientists can infer how much mass is present around a star.
No dips in apparent brightness were seen in stars orbiting within four light-years of the cluster’s center. However, 1,500 of the 5,000 stellar bodies observed with masses the equivalent to 0.1 – 5 times the mass of the Sun that were located on the fringes of the cluster tracked light fluctuations. Five percent of this population had dips in light lasting 10 – 20 days before returning to their previous apparent luminosity.
"We think they are planetesimals or structures in formation," comments lead researcher Elena Sabbi, of the Space Telescope Science Institute in Baltimore. "These could be the seeds that eventually lead to planets in more evolved systems. These are the systems we don't see close to very massive stars. We see them only in systems outside the center."
The materials that caused the dips in light are likely formed into a clumpy disk that would be orientated edge on to Earth, and the watching Hubble telescope.
The massive interior stars could have each other to blame, as well as their location, for their lack of planetary forming potential. Observations suggest that there are at least 30 truly monstrous stars lurking at the heart of Westerlund 2 with a mass the equivalent to 80 times that of our Sun. These huge stars are extremely active, and have a relatively short life span when compared to smaller stellar bodies.
The colossal amount of ultraviolet radiation and solar winds that pour forth from the leviathans could be quickly dispersing any clouds of cosmic dust and gas that may have been present around the inner stars. The chaotic influence of the huge stars may also be altering the makeup of orbital dust clouds in neighboring systems.
"You may still have a disk, but the stars change the composition of the dust in the disks, so it's harder to create stable structures that will eventually lead to planets,” explains Sabbi. “We think the dust either evaporates away in 1 million years, or it changes in composition and size so dramatically that planets don't have the building blocks to form."
Westerlund 2 presents an exciting prospect for follow up observations with the upcoming James Webb Space Telescope, and the planned Nancy Grace Roman Space Telescope, both of which will boast impressive infrared observing capabilities.
So, if you happen to be a pre-main sequence stellar body with a mass between 0.1 – 5 Suns, it might be best set up camp on the outskirts of your star cluster if you plan on having any kids.
The paper has been published in The Astrophysical Journal. You can see a fly-through of the Westerlund 2 star cluster in the video below.