Kevin K. Hardegree-Ullman, Ph.D.

Research: Recent Highlights

Link to all my published articles on NASA/ADS!

Link to a web/mobile-friendly Main Sequence Stellar Parameters table from Pecaut & Mamajek!

Number of stars uniformly classified: 608,327

222,088 K2 main sequence stars

286,391 main sequence stars within 120 pc

94,422 Kepler main sequence stars

Scaling K2 VIII: Short-Period Sub-Neptune Occurrence Rates Peak Around Early-Type M Dwarfs

Kevin K. Hardegree-Ullman, et al. 2025.

Synopsis: We uniformly combined data from the Kepler and K2 missions and computed planet occurrence rates for super-Earths and sub-Neptunes across the host star temperature range of 3200 K to 6900 K. The super-Earths follow a single power-law, with increasing prevalence toward cooler stars. The sub-Neptunes peak at early-type M dwarfs, around 3750 K, and drop in prevalence toward cooler host temperatures, consistent with recent pebble accretion planet formation models. Another possible drop in sub-Neptunes occurs for host stars warmer than the Sun.

A plot of planet occurrence rates in planets per 100 stars on the y-axis versus host star effective temperature in K on the x-axis between 3200 and 6900 K. Occurrence rates are computed separately for super-Earths (green) and sub-Neptunes (blue) with orbital periods between 1 and 40 days. A power law was fitted to the super-Earths and a double broken power law was fitted to the sub-Neptunes, showing breaks at 3750 K and 5758 K.

Bioverse: GMT and ELT Direct Imaging and High-Resolution Spectroscopy Assessment – Surveying Exo-Earth O₂ and Testing the Habitable Zone Oxygen Hypothesis

Kevin K. Hardegree-Ullman, et al. 2025.

Synopsis: We used the Bioverse framework to assess the capabilities of future extremely large telescopes (GMT, ELT) to probe Earth-like O₂ levels on simulated nearby habitable zone Earth-sized exoplanets via direct imaging and high-resolution spectroscopy. In a 10-year survey, between 5 and 20 planets could be probed for Earth like O₂ levels. There are six currently known habitable zone super-Earth candidates, including Proxima Centauri b, that could be searched for Earth-like O₂ levels within a week to a few months of observing time with the ELT and GMT.

(Left) A simulation of the number of planets that can be probed for Earth-like levels of O2 on Earth-sized habitable-zone planets via direct imaging and high-resolution spectroscopy using the GMT (orange) and ELT (blue). In a 10-year survey, between ~7 (GMT) and 19 (ELT) planets could be probed. (Right) Survey duration to probe Earth-like levels of O2 on known nearby habitable zone super-Earth candidates. Four of these planets could be probed for O2 with a median observing time of less than 1 week on the ELT, and most of the planets could be probed for O2 with the GMT in a median time less than a few months with the GMT. The large uncertainties on these measurements are due to unknown planet (albedo) and orbital (e.g., inclination angle) parameters.

Bioverse: A Comprehensive Assessment of the Capabilities of Extremely Large Telescopes to Probe Earth-like O₂ Levels in Nearby Transiting Habitable-zone Exoplanets

Kevin K. Hardegree-Ullman, et al. 2023. AJ 165, 267.

Synopsis: We used the Bioverse framework to simulate a survey of habitable-zone Earth-sized exoplanets orbiting nearby stars in order to assess the capabilities of future extremely large telescopes (GMT, TMT, ELT) to probe Earth-like O₂ levels via transmission spectroscopy. In the most optimistic cases, it is unlikely transmission spectroscopy will be the avenue to detect O₂. TRAPPIST-1 d-g would take at least 16 to 55 years in a very optimistic scenario.

A simulation of the expected number of years to test for Earth-like levels of O2 on the TRAPPIST-1 habitable-zone planets. The upper panel shows the expected time to measure O2 for different telescopes if full transits are required, and the lower panel shows the same results if only partial transits are required.

K2-138 g: Spitzer Spots a Sixth Planet for the Citizen Science System

Kevin K. Hardegree-Ullman, et al. 2021. AJ 161, 219.

Synopsis: We confirm the existence of K2-138 g with Spitzer data. The 3.44 R_⊕ planet orbits its host star every 41.97 days, so only two transits were identified in the original K2 data.

K2-138 g light curves from K2 (left) and Spitzer (right).

Scaling K2. I. Revised Parameters for 222,088 K2 Stars and a K2 Planet Radius Valley at 1.9 R_⊕

Kevin K. Hardegree-Ullman, et al. 2020. ApJS, 247, 28.

Synopsis: We updated stellar parameters for 222,088 K2 stars (download table here 94MB). We applied the uniformly derived stellar parameters to 299 confirmed and 517 candidate K2 planets to refine their radii and incident stellar flux (download table here 190KB), elucidating a conclusive planet radius valley in a data set other than Kepler for the first time. The image below on the left shows the individual K2 planet radii versus their incident flux relative to Earth (points). The density contours are shown behind the points. The image on the right shows the K2 density contours compared to those from the Kepler mission planets (Fulton+ 2018). The planet radius valley is at the same location in both data sets, with hints of a small slope with respect to incident flux.

Kepler planet occurrence rates as a function of spectral type.

Kepler Planet Occurrence Rates for Mid-type M Dwarfs as a Function of Spectral Type

Kevin K. Hardegree-Ullman, et al. 2019. AJ, 158, 75.

Synopsis: Using updated stellar parameters from spectra and parallaxes we computed planet occurrence rates for Kepler mid-type M dwarfs. The image below shows our revised occurrence rates compared to literature values for early-type M dwarfs and FGK dwarfs.

Catalog of New K2 Exoplanet Candidates from Citizen Scientists

Jon K. Zink, Kevin K. Hardegree-Ullman, et al. 2019. RNAAS, 3, 43.

Synopsis: We report 28 new K2 exoplanet candidates discovered by citizen scientists through the Exoplanet Explorers Zooniverse project. The (interactive!) plot below shows how the candidates compare to other K2 confirmed and candidate planets. Circle size indicates Kepler band magnitudes.

Outreach

It's a beautiful day in the local stellar neighborhood!

We're all stuck at home, mostly limited to exploring our own boring neighborhoods. Why not explore the (more interesting) local stellar neighborhood with your very own model?! The following slides are a tutorial to make a scale model of the Sun and its closest neighbors out to 10 light years. For best readability, view slides in fullscreen.

Download and print the star cut out files here:

Black & white (color the stars yourself!)

Color

If you don't have a printer, you can use the table below for sizing (best viewed on a laptop or desktop). You can also make the stars bigger (I don't recommend smaller) by multiplying the star diameters by a number larger than 1.

Common Name	Distance (Light years)	Spectral Type	Diameter Relative to Sun (inches)	Diameter Relative to Sun (cm)	Approximate Color	x (inches)	y (inches)	z (inches)	x (cm)	y (cm)	z (cm)
Sun	0.00	G2V	1.00	2.54	#FFF5F2	5.75	5.75	5.75	14.5	14.5	14.5
Proxima Centauri	4.24	M5.5V	0.15	0.39	#FFCC6F	4.75	5	8	12	12.5	20.5
Alpha Centauri A	4.37	G2V	1.22	3.11	#FFF5F2
Alpha Centauri B	4.37	K1V	0.86	2.19	#FFEEDD
Barnard’s Star	5.96	M4V	0.20	0.50	#FFC97F	5.75	2.25	5.5	14.5	5.5	14
Luhman 16 A	6.50	L8	0.11	0.28	#FF6060	3.5	6.5	8.75	9	16.5	22
Luhman 16 B	6.50	T1	0.11	0.28	#FF4080	3.5	6.5	8.75	9	16.5	22
WISE 0855-0714	7.26	Y2	0.11	0.28	#9366B4	3	8.75	6.25	7.5	22	16
Wolf 359	7.86	M6	0.16	0.41	#FFC370	1.5	7	5.25	4	18	13.5
Lalande 21185	8.31	M2	0.39	1.00	#FFC483	2	6.75	3	5	17	7.5
Sirius A	8.66	A1	1.71	4.35	#B5C7FF	4.75	10.5	7.25	12	26.5	18.5
Sirius B	8.66	DA2	0.01	0.02	#A8BDFF	4.75	10.5	7.25	12	26.5	18.5
Luyten 726-8 A	8.79	M5.5V	0.14	0.36	#FFCC6F	10	7.75	7.25	25.5	19.5	18.5
Luyten 726-8 B	8.79	M6V	0.14	0.36	#FFC370	10	7.75	7.25	25.5	19.5	18.5
Ross 154	9.70	M3.5V	0.24	0.61	#FFCE81	6.75	0.75	8	17	2	20.5

The Planet Coloring Book!

Color in the planets of the Solar System and the 55 closest confirmed planets orbiting other stars! Planet sizes are to scale, so you can see how the "exoplanets" compare to the ones we all know and love! Cut out and hang these planets anywhere, and you can even come up with your own (much more creative) planet names!

Download and print the coloring book below!

Multimedia

NASA's been pulling out of major astronomy meetings — and scientists are feeling the effects. Article on space.com, including several quotes from me, from the 246th American Astronomical Society meeting in June 2025 in Anchorage, Alaska.

The Exoplanet R(E)volution. Virtual talk given on March 4, 2021 to Embry-Riddle Aeronautical University-Prescott as part of The Jim and Linda Lee Planetarium Science Speaker Series. Versions of this talk have been given to the Central Appalachian Astronomy Club for their 7 Months of Science program on November 10, 2021 (video here), and to the Summer Science Program on July 6, 2023.

American Astronomical Society press release: Spitzer confirmation of K2-138 g.

Surely You're Joking podcast about the K2-138 exoplanet system.

Synopsis of my M dwarf dissertation research by Active Galactic Videos.

Chasing the Great American Eclipse of 2017
My article in UToledo News.

Outreach video to help explain gravity using a spandex gravity well.
Check out the video on YouTube for instructions on how to build your own!

Astrophotography

This is a collection of astronomical images I've taken recently at Ritter Observatory, the Lowell Discovery Telescope, and other areas of Arizona and Ohio. Please contact me if you want a full-resolution version or if you would like to use any of these images for any purpose.

M1 - Crab Nebula

Ritter Observatory

M13 - Hercules Cluster

Ritter Observatory

M16 - Eagle Nebula

Ritter Observatory

M27 - Dumbbell Nebula

Ritter Observatory

M42 - Orion Nebula

Ritter Observatory

M57 - Ring Nebula

Ritter Observatory

M82 - Cigar Galaxy with SN2014J

Ritter Observatory

Horsehead Nebula

Lowell Discovery Telescope

Pointing to the center of the Milky Way

Near Assumption, Ohio

Camelopardalid Meteor Shower

Near Assumption, Ohio

Observing at the LDT

Lowell Discovery Telescope

Annular Eclipse

Horseshoe Bend, Arizona - May 20, 2012

About Me