One of the coolest things to see at the AAS was the abundance of young researchers leading the charge in understanding exoplanets. First confirmed in 1995, exoplanets weren’t something a lot of us older folks were ready to focus our career on when we got started. Now, there is so much data and so many opportunities, that researchers are diving in and essentially homesteading on their own awesome research topics as the field comes into its own.

Part of what transformed exoplanet research from a low-data, high-risk research field into the rich topic that it is today is the success of the Kepler space telescope. This mission was originally launched to focus on one small field of stars and observe them over and over for years as researchers waited for planets to transit in front of those stars and reveal themselves. Over several years, Kepler found star after star after star, and then its navigation system went boink, and the research it had been doing was no longer possible.

As we keep saying, however, researchers will get creative when they want data. While they couldn’t use Kepler as planned, they could use it to look at a series of fields spread around the sky that were positioned in places the telescope could still point. This redo was called K2 and yielded its own rich harvest of planetary discoveries.

The first planets discovered and studied in this data were the ones that were obvious, but as the years have passed, the software has improved, and it has become easier to find the less obvious planets trying to sneak past their stars in the data. In fact, the sample was increased by roughly a third with a much larger sample set for both the original Kepler data and the extended K2 mission. Now with this software-enabled, new trove of planets, it became possible to look at things in a statistically rigorous way, and it turns out when you look closely, the stars in the two populations aren’t identical.

The original mission looked at stars that largely live in a region of the galaxy where most of the stars would be similar in composition to our Sun. In the follow-up, they looked where the spacecraft could look and tended to examine stars that had fewer heavy atoms and less of the stuff we think of when we think about building planets like Earth. Interestingly, although maybe not surprisingly, researchers found that the planets that formed (or sometimes didn’t form) in systems with fewer heavy atoms also had fewer planets.

Now, beyond this research, they were also able to look at what kinds of planets form in what kinds of systems. This was done with the initial Kepler data, and they discovered that there is this place in solar systems that is extremely hot where larger planets just can’t exist. This is the sub-Neptune desert. They also find that there is this strange gap in radius in planets where you just don’t see things of a certain size no matter where you look. The question became: Is this a function of the composition of the stuff available to form planets or is this universal? 

Looking at the K2 data, they found that there was a slightly different distribution, but that sub-Neptune desert was still there. That gap in radius? Still there. This means that there is something going on during both the process of forming planets and perhaps during the later evolution of planets that either prevents the formation of worlds in these sizes and places or destroys them. As we talked about earlier this year, people are now starting to ask: if you start with a Neptune-like planet and you put it too close to its star, does that atmosphere get blasted away, leaving behind a giant, earthy rock? Time will tell. But now we have a whole lot more planets to yield up their story.

More Information

Penn press release

“Statistics of the Chemical Composition of Solar Analog Stars and Links to Planet Formation,” Jacob Nibauer et al., 2021 February 5, The Astrophysical Journal