How to make a star system

Since astronomers first stared up at the wandering stars they eventually recognized as planets, they have mostly used our home solar system as the basis for how such celestial families form. And then, twenty years ago, astronomers started finding the first planetary systems other than our own. And all understanding went out the window.

Our solar system has no Jupiter-size world orbiting close to our star. And while rare relative to other kinds of planets, we see hot Jupiters in abundance around others stars. The Sun also hosts no planets between the sizes of Earth and Neptune. Yet these appear to be overwhelmingly common in other systems. And we have no giant planets orbiting past Pluto. Yet direct imaging observations reveal multiple examples of these as well.  

Perhaps most perplexing is that astronomers have found no true solar system analog in their searches. They know of no system other than our own that has neatly arrayed rocky small planets, followed by gas giants, then slightly smaller ice giants, and then small rocks again.

So, we are clearly not a typical example. But could we be common in the galaxy? Astronomer Ruth Murray-Clay closed a fantastic 227th meeting of the American Astronomical Society in Kissimee, Florida, addressing just this question.

Exoplanet searches are still not fully sensitive to Earth-sized planets on Earth-length orbits around Sun-like stars. Or Jupiter-sized planets on Jupiter-length orbits. And so on down the line. So, despite not seeing any systems like ours, we could still be common. But the wealth and diversity of other types of systems means that any models or understanding of how planets form must answer for all kinds of planet families. And that is quite the challenge.

Astronomers are sure of some basics: Stars form in clouds of dust and gas called a nebula, and as they gather mass and collapse down into a massive ball of gas, they also form a disk of gas and dust that eventually becomes planets.

But what causes the diversity of worlds? Why do some nascent disks yield solar systems like ours, and others the alien systems we observe? And, because astronomers have to tie their questions into observations, the only solid answers they can obtain, how can researchers use the large, easy-to-spy worlds to tell them about the existence of Earth-sized planets?

One of the key factors that determines how big a planet can get is essentially how much material a budding planetesimal can crash into before the disk disappears. This is how astronomers explain the small planets in our solar system: They form in the hot part of the disk near the star where it is harder for material to clump together and impossible for some icy materials to exist at all. Farther out, material is cooler and more “sticky,” and planets form more readily.

But this is highly dependent on the mass of the disk. Disks with less material form different planets at different distances from their stars. So Murray-Clay developed a system to predict what kinds of planets should form depending on the total mass of the disk and how far the developing planets are from their star. Among other things, her work suggests that if a system includes mini-Neptunes, it should not also include giant planets. And if it includes super-Earths, giants are also likely.

Many observed extrasolar systems fit into Murray-Clay’s framework. But more importantly, her work creates more predictions, and she encouraged future observers to test these.

One or two systems are not enough to validate or dismiss her work. But astronomy is already entering the age where researchers have dozens, sometimes hundreds of systems for comparison. And we will only learn more as future exoplanet hunting telescopes come online.