Finding Patterns in Planets: Researchers Explore the Demographics of Alien Worlds in K2 Data
Our galaxy is jam-packed with planets, with thousands already discovered and millions more expected to eventually come into view. Within this ever-growing bounty, surprises have abounded, revealing that our solar system only offers a small sample of the planetary possibilities.
Perhaps the biggest shocker: The most common exoplanets we are able to detect are “super-Earths” and “sub-Neptunes,” bodies bigger and smaller than their respective namesakes, and yet they are strangely absent from our own solar system.
Scaling K2, an ongoing project at the NASA Exoplanet Science Institute (NExScI) at IPAC, is delving deeper into this mystery and others regarding exoplanet demographics—the study of the numbers, types, and characteristics of alien worlds. The project’s results are expanding our understanding of our Milky Way neighborhood, showing that planet formation happens similarly across the cornucopia of worlds inhabiting the galaxy.
The K2 mission began when NASA’s Kepler Space Telescope, which operated from 2009 to 2013, lost the ability to point at its initial target set of 150,000 stars. Mission planners cleverly pivoted to use the solar radiation pushing on Kepler’s solar panels to steady the vessel enough to hunt for exoplanets around 300,000 additional stars, yielding a treasure trove of observations, especially of exoplanets orbiting cool, low-mass, orange and red dwarf stars.
As described in a series of eight peer-reviewed papers and counting, Scaling K2 has “scaled up” the possible demographic analyses by uniformly processing all of the data related to K2’s dozen-and-a-half observing runs, including archival information about the brightnesses and distances to the stars observed.
“For Scaling K2, we’re pulling data together to do some key statistics and figure out what types of planets are really out there,” said Kevin Hardegree-Ullman, an assistant research scientist at NExScI at IPAC and a leader of the Scaling K2 project. “We’re extending the legacy of both the Kepler and K2 missions and leveraging the rich datasets the spacecraft collected.”
The project began in 2018 when Jessie Christiansen, Chief Scientist of NExScI, brought on Hardegree-Ullman as a postdoctoral researcher to go through the four years’ worth of archival K2 data. That data vastly expanded on orange and red dwarf stars, both of which far outnumber Sun-like yellow dwarfs in the galaxy and serve as different cosmic environments for the development of planets.
“The original Kepler mission sampled mostly Sun-like stars, but what we had at the end of the four years of K2 was this incredibly varied, super-set of stars,” said Christiansen. “This project is enabling us to start pushing these interesting questions that Kepler had brought up, about how planets form and where they form, into these new regimes with K2.”
In lieu of the usual alphabet-soup acronyms for astronomy missions and projects, Christiansen chose to dub the project Scaling K2, a subtle mountaineering reference to the second tallest mountain on Earth, and a nod to her enjoyment of rock climbing. “With Scaling K2, we know we have all this hard work to do,” said Christiansen, “but when we get to the top, the view of exoplanet demographics is going to be amazing.”
Getting the most out of K2
As NASA’s first dedicated exoplanet-seeking mission, Kepler used the transit method, finding exoplanets when they crossed the faces of their host stars and caused a very slight dimming of the total starlight received at the telescope’s ultra-sensitive detector. In this way, the Kepler-K2 mission has added 3,000 new exoplanets—about half of the total recorded—to the rolls so far. Some thousands more still await follow-up observational confirmation.
In their inaugural Scaling K2 paper, Hardegree-Ullman, Christiansen and colleagues sought to solidify this expanding catalog by adding in measurements of stellar properties from the European Space Agency’s star-tracking Gaia spacecraft, as well as from the Chinese Academy of Sciences’ ground-based LAMOST (Large Sky Area Multi-Object Fibre Spectroscopic Telescope) project. That work led to more accurate star sizes and masses, in turn allowing more precise calculations of the sizes of around 800 K2-spotted exoplanets.
With that more stringent data, the researchers learned they could discern the so-called “exoplanet radius valley.” Previously discovered with Kepler by Benjamin “BJ” Fulton, an IPAC scientist, the valley is a gap where we see fewer planets than we would expect around the size of two Earth radii.
This size serves as the boundary between exoplanets classified as super-Earths or sub-Neptunes. Super-Earths are slightly larger than Earth in size, sub-Neptunes are from two to four Earth radii, and both have turned out to be more common than two-Earth-radii worlds. Importantly, because K2 gazed into 18 different regions of stars, these novel results “tell us something about how planet formation is fairly similar across the galaxy,” said Hardegree-Ullman.
Buoyed by these out-of-the-gate results, the researchers built up an automated planet detection pipeline for K2 data, yielding 300 more exoplanet candidates than were previously known.
Subsequent Scaling K2 papers continued to unveil more trends in the Milky Way. For instance, stars that happen to wander above and below the plane of our galaxy tend to have fewer planets than stars that spend more time nestled within the material-rich plane.
Another paper reported on the scene in the young Praesepe and Hyades star clusters, finding far higher numbers of sub-Neptune-sized, fledgling worlds orbiting scorchingly close to their stars than around older stars, suggesting that these planets significantly change as they age.
“There must be some sort of mass-loss process that happens that could strip these hot sub-Neptunes of their atmospheres and just leave a smaller planetary object behind,” said Hardegree-Ullman. “Results like these help to fill in gaps in our understanding of what sorts of planets originally and typically form in the galaxy, and how they might evolve into remarkably different bodies.”
Smaller stars, bigger planetary broods?
In their latest paper, the Scaling K2 crew bolstered the K2 datasets by adding in the original Kepler dataset, uniformly combining these sets to empower a broader, first-of-its-kind demographic analysis. By looking at about 10 times more red dwarf stars than Kepler, K2 ultimately logged three-and-a-half times more super-Earths and sub-Neptunes around such stars.
That haul enabled the researchers to examine and extend another strange result from the original Kepler mission. Kepler showed that as stars get smaller, they tend to sport more super-Earth- and sub-Neptune-sized planets packed into close, short-period orbits. “It’s counterintuitive because as you go to smaller stars, you expect there to be less material available to make planets,” said Hardegree-Ullman. “But we’ve been seeing more planets nonetheless.”
Planetary formation theorists puzzled over this smaller-stars, extra-planets paradox and eventually predicted that this baffling trend could not continue all the way down to the very smallest of stars, red dwarfs, because eventually planet-making material would indeed be exhausted. The Scaling K2 researchers bore this prediction out, finding that sub-Neptunes do become less common deeper into the smallest star regime available to K2, down to roughly mid-sized red dwarfs. Interestingly, and contrarily, super-Earths still increased in occurrence through this stellar range, posing fresh wrinkles for theorists to iron out.
“Planet formation models and theories will need to be refined to coincide with our observational result, which is that only sub-Neptunes exhibit this drop,” said Hardegree-Ullman. “Models will need to treat formation of super-Earths and sub-Neptunes separately because there is a divergence in their occurrence rates at short periods for red dwarfs.”
Scaling beyond K2
To learn more about the exoplanet abundances and varieties around even smaller red dwarfs, as well as other stars, Hardegree-Ullman and colleagues plan to explore data gathered by post-K2 missions. These include NASA’s TESS (Transiting Exoplanet Survey Satellite), in orbit since 2018, ESA’s PLATO (PLAnetary Transits and Oscillations of stars) mission, slated to launch late this year or early next, and NASA’s latest flagship mission, NASA’s Nancy Grace Roman Space Telescope, on track for launch as early as this fall.
Roman is set to undertake the Galactic Bulge Time-Domain Survey, which could yield 60,000 to 200,000 new transiting exoplanets as a bonus on top of its main observing goals. Plenty of the same small-stars-with-small-planets systems should turn up, offering more insight into exoplanet demographics in our galaxy.
“We’re looking forward to extending our analyses with data from these other missions to really nail down what’s going on with these red dwarf-orbiting planets, as well as other star types,” said Hardegree-Ullman. “Combining different types of surveys is the next big step in figuring out the big picture of planets, how they form, and adding context to the planet we call home in this vast universe.”
IPAC is a science and data center for astrophysics and planetary science located on the Caltech campus in Pasadena, CA. NASA Exoplanet Science Institute (NExScI), a part of IPAC, is a science operations and analysis service organization for NASA Exoplanet Exploration Program (ExEP) projects, and the scientists and engineers that use them. NExScI facilitates the timely and successful execution of exoplanet science by providing software infrastructure, science operations, and consulting to ExEP projects and their user communities.
The Roman Science Support Center at Caltech/IPAC in Pasadena, California will be responsible for the high-level science data processing for the Galactic Bulge Time Domain Survey, including exoplanet microlensing and general community outreach for Roman exoplanet science.
By Adam Hadhazy
Scaling K2. VIII. Short-period Sub-Neptune Occurrence Rates Peak Around Early-type M Dwarfs, The Astronomical Journal (open access)
Astrobiology
