This Hazy Image Of A Solar System Forming Is Pretty Much A Miracle
published during a waxing gibbous moon.


Solar System Forming

TW Hydrae’s protoplanetary disk. The gaps are interpreted as newly forming planets clearing their orbits of debris. Credit: S. Andrews (Harvard-Smithsonian CfA), ALMA (ESO/NAOJ/NRAO)

Take a look at this grainy, hazy image of a star named TW Hydra. It may not look like much — It’s just a grainy orb with a couple of clear rings interrupting what would otherwise be a somewhat jumbled orange mess. But those rings are actually nascent planets, and what you are looking at is the clearest image ever of the birth of a new solar system.

It was taken by the Atacama Large Millimeter/submillimeter Array (ALMA), a massive grid of radio dishes in northern Chile that detect microwave radiation.  Sean Andrews, lead author of the study that produced the new image, said that these wavelengths are ideal because, while faint, they are pretty clearly emitted by the material in a protoplanetary disk, and, more important, completely absent in the overpowering star itself.

The image itself harkens back to an earlier time in our own cosmic history. Some 5 billion years ago, the material that made up everything in our solar system was a small region in a large nebulous cloud of gas and dust. At some point, perhaps due to the blast of a nearby supernova, or perhaps simply under the weight of its own gravity, it collapsed into itself. As centrifugal forces sped this new mass of material into a rapidly rotating orb, a thin disk of dust and gas developed around it.

Something like 99.9% of all that collapsed material coalesced in the center, forming the hot nuclear furnace that would eventually become our Sun. The stuff in that disk later become everything else — planets, moons, asteroids, comets. As clumps of rock or gas begot larger clumps of rock and gas, our solar system was born.

All this happened, of course, in an astronomical second. At least in our own solar system, the protoplanetary disk phase lasted probably only about 3 million years, Andrews said. That’s a blip compared to the 4.6 billion years our solar system has existed. That brevity is one of the reasons it is so unlikely that we humans (who have been around in our current form for maybe a mere 200,000 years) would be able to capture this moment at all.

Solar System Forming

An artist’s conception of a protoplanetary disk. Credit: University of Copenhagen/Lars Buchhave

But it’s certainly not the only reason. First, you have to find a disk that is close enough be seen. Going out to about 500 light-years away, Andrews estimates there are something like 1,000 protoplanetary disks out there. At the distance of TW Hydrae, which is about 175 light-years away, he estimates there are maybe six to ten. It’s not that protoplanetary disks are extremely rare, but finding one close enough for this kind of study is far from a given.

Second, you need to find a disk in the right orientation. In order to truly see the newly developing orbits of those growing clumps, it’s best to have a bird’s eye view of the galaxy  — something astronomers term “face-on.”  That’s pretty rare, Andrews said. “There’s no reason they’d be preferentially facing the Earth.” If you do the math, he explained, most disks would be inclined far from the ideal zero degrees for a face-on view. Luckily, TW Hydrae is inclined only 7 degrees.

And finally, you need to catch this close, face-on disk at the right point in its evolution. Andrews said that scientists estimate that a typical protoplanetary disk has a half-life of about only 3 million years. That means that clearly defined disks older than 5 million years are rare. “TW Hydrae is strange. It’s about 10 million years old,” Andrews said. “We really have no idea why it’s lasted this long.” This longevity doesn’t make it easier to image, but if the system hadn’t formed when it did and lasted through to our space age, we obviously wouldn’t get to see it.

But what, if anything, does this unlikely image tell us about how solar systems work? Astonishingly, it shows a planet forming in an orbit that is essentially identical to the orbit of Earth. When asked if this is rare, Andrews replied “No idea.” So far, he explained, we have only imaged two disks (this one included) with enough resolution to figure out orbit distances. “If I put on my optimistic hat,” he added, “I would say it’s possible that it always happens.”

Outside of the identical orbits, there is no additional data that suggests TW Hydrae’s baby planet is Earth-like. But if planets forming at such a distance are common, that has implications about finding Earth-like, life-sustaining planets elsewhere in the cosmos. And even if it doesn’t help us figure that mystery out, it’s still pretty damn cool to see such a serendipitous image, however blurry, of what our solar system’s earliest days may have been like.