Australasian Science Magazine

Astronomers may have seen a “baby Earth” forming, and have found that Saturn’s moons may be younger than the dinosaurs.

The images show a planet-forming disc around TW Hydrae, a Sun-like star close to Earth. The planet is about 10 million years old, making it a veritable newborn, floating in a ring of planet-forming material. Combined with its close proximity to Earth, TW Hydrae has become a popular study subject for researchers.

The developing planet is about 175 light years away and has a face-on orientation that provides a rare, undistorted view of the complete disc. Most profiles give a more oblique and indistinct outline.

“Previous studies with optical and radio telescopes confirm that this star hosts a prominent disc with features that strongly suggest planets are beginning to coalesce,” said lead researcher Sean Andrews. “The new ALMA images show the disc in unprecedented detail, revealing a series of concentric dusty bright rings and dark gaps, intriguing features that suggest a planet with an Earth-like orbit is forming there.”

Pronounced gaps near the star are 3 billion and 6 billion km from the central stars, which are similar distances from the Sun to Uranus and Pluto. These are also likely the result of particles that came together and formed planets before sweeping their orbits clear of dust and gas before corralling the remaining material into bands.

Scientists believe that studying young star systems like TW Hydrae can help them learn about our own solar system’s past. It provides a snapshot of when things got started around here about 5 billion years ago. “TW Hydrae is quite special. It is the nearest known proto-planetary disc to Earth and it may closely resemble the solar system when it was only 10 million years old,” co-author David Wilner said.

By studying TW Hydrae, astronomers hope to get a better understanding of Earth’s evolution and prospects for similar systems throughout the galaxy. Astronomers are now trying to find out how common these features are in discs around other young stars and how they may change in the future.

New research suggests that the birth of Saturn’s icy moons, as well as its famous rings, may have taken place a mere 100 million years ago, more recent than the reign of many dinosaurs.

“Moons are always changing their orbits. That’s inevitable,” says Matija Cuk, principal investigator at the SETI Institute. “But that fact allows us to use computer simulations to tease out the history of Saturn’s inner moons. We find that they were most likely born during the most recent 2% of the planet’s history.”

While Saturn’s rings have been known since the 1600s, there’s still debate about their age. The straightforward assumption is that they are primordial, as old as the planet itself, which is more than four billion years. However, in 2012 French astronomers found that tidal effects are causing them to spiral to larger orbital radii comparatively quickly. The implication, given their present positions, is that these moons, and presumably the rings, are a recent phenomena.

Cuk’s team used computer modelling to infer the past dynamic behaviour of Saturn’s icy inner moons. While our own moon has its orbit around Earth to itself, Saturn’s many satellites have to share space with each other. All of their orbits slowly grow due to tidal effects, but at different rates.

This close “orbital resonance” means that even small moons with weak gravity can strongly affect each other’s orbits, making them more elongated and tilting them out of their original orbital plane. By comparing present orbital tilts and those predicted by computer simulations, the researchers could determine by how much the orbits of Saturn’s moons grew.

The relatively small orbital tilts found indicate that they haven’t crossed many orbital resonances, so they must have formed not far from where they are now. But how long ago was their birth?

NASA’s Cassini mission helped to answer this question when it saw ice geysers on Saturn’s moon Enceladus. Assuming that the energy powering these geysers comes directly from tidal interactions, then the tides within Saturn are quite strong. Analysis suggests these would move the satellite by the small amount indicated by the simulations in only about 100 million years.

This would date the formation of the major moons of Saturn, with the exception of more distant Titan and Iapetus, to the relatively recent Cretaceous Period:the era of the dinosaurs.

“So the question arises, what caused the recent birth of the inner moons?” asks Cuk. “Our best guess is that Saturn had a similar collection of moons before, but their orbits were disturbed by a special kind of orbital resonance involving Saturn’s motion around the Sun. Eventually, the orbits of neighbouring moons crossed, and these objects collided. From this rubble, the present set of moons and rings formed.”

If this result is correct, Saturn’s bright rings may be younger than the heyday of the dinosaurs, and we are fortunate to witness them today.