Kevin Hardegree-Ullman
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kevinkhu.bsky.social
Kevin Hardegree-Ullman
@kevinkhu.bsky.social
2.6K followers 260 following 280 posts
Research Scientist at Caltech/IPAC-NExScI studying all things related to exoplanets! Opinions are my own. https://kevinkhu.com
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kevinkhu.bsky.social
Awesome, another paper of mine has reached 100 citations! 📜
A screenshot of a list of 6 papers on the Astrophysics Data System (ADS) with Kevin Hardegree-Ullman as the first author. The second paper on the list has a red circle around the citation count, which is 100.
October 24, 2025 at 9:33 PM
kevinkhu.bsky.social
Thank you Colorado State University! Congrats on 60 years of the Madison-Macdonald Observatory! 🔭
About 100 people lined up to look through several telescopes at the Madison-Macdonald Observatory.
October 18, 2025 at 3:37 AM
kevinkhu.bsky.social
The NASA #Exoplanet Archive now has Gaia DR3 IDs! This should hopefully make your work and planning observations easier!

exoplanetarchive.ipac.caltech.edu
An image of the Gaia spacecraft logo in red with text above it that says Gaia DR3 IDs.
October 16, 2025 at 10:47 PM
kevinkhu.bsky.social
It has been a while since I just got to "play" with a telescope! 🔭
The Madison-Macdonald Observatory on the Colorado State University campus in Fort Collins. A 14-inch Celestron telescope on an equatorial mount inside a room with a roll-off roof and red lights on. The telescope is pointing at a twilight blue sky. A door is open and Dobsonian telescopes line the wall.
October 15, 2025 at 4:01 AM
kevinkhu.bsky.social
Do you even lift?
Two turtles (larger round one on the left, very flat one on the right) sitting on a rock with some moss on it in the middle of a very green pond.
October 3, 2025 at 1:52 AM
kevinkhu.bsky.social
The Arecibo sky bridge, as seen from the secondary support structure in 2013. Sadly, this bridge no longer exists. 😞
A fisheye lens image looking through a support structure for the Arecibo Radio Observatory secondary mirror toward the sky bridge used to walk onto the secondary structure. A support tower is seen near the end of the bridge.
September 26, 2025 at 7:10 PM
kevinkhu.bsky.social
The original source material from Murderbot on Apple TV+.
A screenshot of the show-within-a-show "The Rise & Fall of Sanctuary Moon" in the Apple TV+ series Murderbot.
September 24, 2025 at 11:34 PM
kevinkhu.bsky.social
I gave a talk about my paper last month with this as the title slide. Can you tell I recently watched #Murderbot? 🔭 #Exoplanets
A heavily modified version of a screenshot of the show-within-a-show "The Rise & Fall of Sanctuary Moon" in the Apple TV+ series Murderbot. The title now reads "The Rise & Fall of Sub-Neptune" to reflect a recent paper by Kevin Hardegree-Ullman in which the population of sub-Neptune exoplanets experiences a rise toward cool stars, then a fall when you reach stars cooler than about 3750 Kelvin.
September 24, 2025 at 11:34 PM
kevinkhu.bsky.social
On the way to daycare, my 3 year old saw me bringing my Hot Wheels Mars Rovers to work today.

3 yo: I want to go to Mars... with you. We go on rocket ship to Mars.

Me: 🥹
A gold-plated 1997 Hot Wheels Sojurner Mars Rover and a 2012 Hot Wheels Curiosity Mars Rover sitting on the box for Sojourner.
September 17, 2025 at 4:34 PM
kevinkhu.bsky.social
Our now published paper can be found here! iopscience.iop.org/article/10.3...

#exoplanets
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.
August 26, 2025 at 6:24 PM
kevinkhu.bsky.social
Quite astonishing that we can be observing right now ~50 km from where this volcano is spewing its guts. This is our last night of August IRTF observations with @sakhee-space.bsky.social! 🌋 🔭
A picture of a computer screen showing telescope controls on the left and a volcano erupting on the right.
August 23, 2025 at 5:46 AM
kevinkhu.bsky.social
Albert Climbstein
A young child climbing on a statue of Albert Einstein sitting on a bench at Griffith Observatory.
August 17, 2025 at 9:05 PM
kevinkhu.bsky.social
Pebble accretion models predicted a drop in “small” planet formation somewhere around M dwarfs, but models will need to be refined based on our observational result. Importantly, super-Earth and sub-Neptune #exoplanets should be modeled separately.

6/9
Occurrence rates versus host star temperature compared to three planet formation models. The models don't quite fit the data, but it is difficult to make a direct comparison because of different ranges in the models.
August 11, 2025 at 1:01 AM
kevinkhu.bsky.social
We computed planet occurrence rates separately for super-Earth and sub-Neptune #exoplanets for host stars between 3200 K and 6900 K. The sub-Neptunes peak around 3750 K and drop toward cooler stars! However, the super-Earths do not drop off!

5/9
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.
August 11, 2025 at 1:01 AM
kevinkhu.bsky.social
Now, it is important to separate the “small” #exoplanet population into super-Earths and sub-Neptunes, and luckily we can use the planet radius valley to do just that.

4/9
A scatter plot of exoplanet orbital period in days on the x-axis and planet radius in Earth radii on the y-axis. The period range is between 1 and 40 days, and the planet size range is between 1 and 4 Earth radii. K2 planets have a black ring around the dots, and Kepler planets do not. The planets are separated into Super-Earths (green) and Sub-Neptunes (blue) by the planet radius valley, an underdensity of planets that separate the two planet populations.
August 11, 2025 at 1:01 AM
kevinkhu.bsky.social
K2 observed nearly an order-of-magnitude more M dwarfs than Kepler, and consequently, K2 found 3.5x more small (super-Earth and sub-Neptune sized) #exoplanets with host stars cooler than 3700 K than Kepler.

3/9
A histogram plot of host star effective temperature for Kepler (orange) and K2 (blue) small exoplanet hosts. The temperature range goes between 3000 and 7000 K. An inset plot of stars between 3200 K and 4000 K shows that K2 observed 3.5 times more small exoplanets than Kepler.
August 11, 2025 at 1:01 AM
kevinkhu.bsky.social
Combining missions for #exoplanet demographics requires placing them on the same footing. Comparable pipelines for finding and vetting planets from Kepler and K2 have been built, but the stars also had to be placed on the same scale.

2/9
A plot showing 6 Hertzsprung-Russell (HR) diagrams (stellar effective temperature versus absolute K-band magnitude). The top row contains 3 plots from an old analysis from Kepler (left), K2 (middle), and the overplotting of both (right). The bottom row shows HR diagrams from the new analysis from Kepler (left), K2 (middle), and combined from both (right). F, G, and K stars are shown in blue, and M dwarfs are shown in orange.
August 11, 2025 at 1:01 AM
kevinkhu.bsky.social
I timed it perfectly to the second! New paper coming soon to the top of the Sunday evening/Monday morning arXiv post!
A screen capture that says: arXiv:submit/6690322 From: Kevin Hardegree-Ullman Date: Thu, 7 Aug 2025 14:00:00 EST This is honestly a very boring picture because it is all text.
August 7, 2025 at 6:12 PM
kevinkhu.bsky.social
My 3 year old has probably spent more time observing than some astronomers I know. 🔭
A blonde child wearing a blue and red Spiderman shirt looking at some computer screens that are displaying telescope control interfaces.
August 6, 2025 at 2:20 PM
kevinkhu.bsky.social
In case you were curious, or for some reason need to prove this, The Magic School Bus and Billy Nye the Science Guy were funded by the U.S. Department of Energy. They were also funded by the National Science Foundation, but that was shown in the credits. 🧪
A screenshot of the US Department of Energy Office of Science Portfolio Analysis and Management System (PAMS) showing search results that list the Department of Energy grants that helped fund the kids TV shows The Magic School Bus and Bill Nye the Science Guy. The initial grant awards for these shows started one month apart, in July 1992 for The Magic School Bus, and June 1992 for Bill Nye the Science Guy.
July 29, 2025 at 10:07 PM
kevinkhu.bsky.social
The law of super glue: no matter how hard you try to avoid it, you will always get it on your fingers.
Meme of Jeff Goldblum portraying Dr. Ian Malcolm in Jurassic Park with the text "Super glue, uh, finds a way."
July 19, 2025 at 3:55 AM
kevinkhu.bsky.social
HomeGoods has some strong Jupiter vibes today.
A spherical desk lamp with orange swirl tones. The lamp looks very much like Jupiter. A spherical paper weight with orange, white, and gray swirl tones. The paper weight looks very much like Jupiter.
July 4, 2025 at 10:00 PM
kevinkhu.bsky.social
Preparing for Gaia DR4 and Roman. #Exoplanets

(Cartoon adapted from The Oatmeal comic and planet illustrations from Martin Vargic.)
A 4 panel cartoon from The Oatmeal depicting a person opening a laptop in the first frame, giant exoplanets rushing out of the laptop toward the person in the second frame (labeled Gaia), the person drowning in various colored exoplanets in the third frame (labeled Gaia+Roman Microlensing), and just a full frame of various colored and sized exoplanets in the last frame (labeled Gaia+Roman Transits).
June 27, 2025 at 9:11 PM
kevinkhu.bsky.social
Come say hi to us at the @caltechipac.bsky.social and NExScI booth this week at #AAS246! We have lots of cool swag for exoplanets and the projects/missions IPAC supports!
A conference booth setup for Caltech/IPAC and the NASA Exoplanet Science Institute (NExScI).
June 9, 2025 at 4:56 AM
kevinkhu.bsky.social
While you're at it, check out these American Innovation coins from Florida and Texas. It just goes to show how much impact NASA has on these two states that the US Mint decided that a Space Shuttle and Astronaut/ISS should feature prominently on a coin. #FundNASA 🔭🧪
Screenshot of an American Innovation $1 Coin page from the US Mint. On the left is a $1 golden coin from Florida featuring a NASA Space Shuttle. On the right is a $1 golden coin from Texas featuring a NASA Astronaut in front of the International Space Station.
June 3, 2025 at 5:08 PM