Where Do Planets Come From?
An artist’s impression of a world that shouldn’t exist at least according to the accepted theory of how planets form.
So many exoplanets! As of this writing, 3,869 of ’em, at least according to NASA. (But I’m writing this from the 233rd Meeting of the American Astronomical Society, a.k.a. the Super Bowl of Astronomy, so there could be more announced exoplanets by the time you read this.) Many stars have more than one planet, so these planets reside in a total of 2,886 systems. Clearly, the Universe knows how to make planets—and does so very efficiently—but astronomers haven’t quite got it figured out.
The leading theory is called the Core Accretion Model. It has a long history, dating back decades before we knew about other planetary systems. And it might be a tad biased toward our own solar system, which could be a problem, since, as Elizabeth Bailey from the California Institute of Technology noted in a press conference this morning, “Our own system’s architecture is not consistent with observations of other planetary systems.”
Which is to say that our solar system, with its eight planets—four terrestrial worlds, two “gas” giants, and two ice giants in increasing distance from the Sun—doesn’t look like other planetary systems we’ve discovered. Indeed, the very first planets astronomers found more than 20 years ago were what we have nicknamed “hot Jupiters,” giant gassy planets in orbits very close to their parent stars. And searches continue to reveal more and more and more of these objects.
Our solar system doesn’t have any hot Jupiters; instead, we just have one cold Jupiter. And implicit in the Core Accretion Model is the idea that Jupiter-sized worlds can only form far away from their parent stars—beyond the so-called frost line, where temperatures are low enough for ice to form grains. Hot Jupiters came as a huge surprise!
But astronomers found a way around that. Maybe the giant planets formed out in the cooler reaches of their planetary systems, then migrated inward toward the warmer climes closer to their stars? This has been the prevailing explanation for quite some time.
But in a paper she cowrote with Konstantin Batygin, Bailey claims to present “more evidence that hot Jupiters form close to their parent stars.” No migration required! The evidence comes in the form of accurately describing the distribution of observed planets’ masses compared to their distances from their parent stars. Basically, if planets form the way Bailey and Batygin claim, then it explains (quite elegantly and precisely) some trends that astronomers are seeing in their data.
So, bully for Bailey and Batygin! But they weren’t the only ones bashing the Core Accretion Model today…
A press release this morning from Keck Observatory claims that “astronomers have found a new exoplanet that could alter the standing theory of planet formation.” The “standing theory of planet formation” in question? You guessed it! The Core Accretion Model.
This relates to another awkward (but not unrelated) feature of the Core Accretion Model, which is, well, the whole “core” bit. The model predicts that giant planets form around a sizeable core about ten times the mass of Earth. (These cores form beyond the frost line, where they evolve into giants such as Jupiter, with a mass more than 300 times that of Earth.)
David Bennett, senior research scientist at the University of Maryland and NASA’s Goddard Space Flight Center (GSFC), describes the problem: “A key process of the core accretion theory is called ‘runaway gas accretion.’ Giant planets are thought to start their formation process by collecting a core mass of about 10 times the Earth mass in rock and ice. At this stage, a slow accretion of hydrogen and helium gas begins until the mass has doubled. Then, the accretion of hydrogen and helium is expected to speed up exponentially in this runaway gas accretion process. This process stops when the supply is exhausted. If the supply of gas is stopped before runaway accretion stops, we get ‘failed Jupiter‘ planets with masses of 10–20 Earth masses (like Neptune).”
But if the supply of gas continues, cores rapidly gobble up more material, growing to something like 80 times Earth’s mass quite rapidly. The implication is that, according to the theory, very few planets should have masses between 20 and 80 times the mass of Earth. And that’s where the big announcement comes in!
Through a painstaking simultaneous observations, astronomers at GSFC determined the mass of the planet (take a deep breath) OGLE-2012-BLG-0950Lb. With some precision, they determined that its mass is 39 times the mass of Earth. Hmm. A number suspiciously in between 20 and 80, which could nonetheless be a rarity, but…
The same researchers working on 0950Lb’s mass were also involved in a statistical analysis of observed planets—which suggests that planets with masses between 20 and 80 times that of Earth might not be rare after all!
So the GSFC scientists delivered a double whammy: a lone observation that doesn’t match the predictions of the commonly-accepted model as well as an analysis that calls the model’s predictions into question.
That plus Bailey and Batygin’s work suggests that we have a lot to learn. Today’s announcements make it clear there’s plenty of work to be done in understanding how planets form. Clearly, the Universe knows how to make planets—but astronomers have yet to figure it out.