Jonathan O’Callaghan writes:

Bob O’Dell wasn’t quite sure what he was looking at. It was 1992, and he had just got his hands on new images from the Hubble Space Telescope that zoomed in on young stars in the Orion Nebula. O’Dell had been hoping to study the nebula itself, an interesting region of star formation relatively close to Earth. Yet something else caught his attention. Several of the stars didn’t look like stars at all, but were instead enveloped by a dim shroud. They seemed to form a “silhouette against the nebula,” said O’Dell.

At first O’Dell and his colleagues thought they might be seeing an image artifact resulting from Hubble’s warped primary mirror, which had been molded ever so slightly into the wrong shape and would be fixed by a space shuttle mission in 1993. “We really wondered if this was a residual effect of the flawed primary mirror,” said O’Dell, who had been Hubble’s project scientist. Soon, however, they saw more and more of the phenomena in the images, even after the mirror was fixed, and realized it wasn’t a flaw at all. They were actually seeing infant disks of dust and gas surrounding young stars. They were, for the first time, witnessing the birth of planets.

O’Dell’s discovery of protoplanetary disks sparked a transformation in our understanding of planet formation. In the following decades, astronomers would realize that our classical idea of how planets form — small rocks clump into bigger rocks, which then clump further — might not be correct. For the gas giants, such as Jupiter and Saturn, a model called pebble accretion, where a dominant object gobbles up smaller rocks — would come to replace the old views of how such monstrous worlds come to be.

The rocky worlds of the inner solar system are trickier. Planetary scientists have intensely debated whether pebble accretion can explain how Earth and its neighbors arose, or whether the older view is still most likely. The clash has played out over the past few years in journal articles and even, more recently, in a castle in the Bavarian Alps.

The debate doesn’t only affect great mysteries such as the origin of Earth — and its water. The answer will also help reveal just how prevalent Earth-like worlds are across the universe. Are such worlds a cosmic fluke, merely a combination of chance events that make the prospects of life elsewhere in the universe slim? Or are habitable planets a certainty in solar systems with just the right ingredients, making us but one of many?

“It’s part of the human experience to ask how the world around us formed,” said Konstantin Batygin, a planetary scientist at the California Institute of Technology. If it formed from pebbles, it will have huge consequences for how many more worlds like ours are out there. [Continue reading…]