HD 219134, an orange K-class star in Cassiopeia, is relatively close to the Sun (21 light years) and already known to have at least five planets, two of them being rocky super-Earths that can be tracked transiting their host. We know how significant the transit method has become thanks to the planet harvests of, for example, the Kepler mission and TESS, the Transiting Exoplanet Survey Satellite. It’s interesting to realize now that an entirely different kind of measurement based on stellar vibrations can also yield useful planet information.

The work I’m looking at this morning comes out of the Keck Observatory on Mauna Kea (Hawaii), where the Keck Planet Finder (KFP) is being used to track HD 219134’s oscillations. The field of asteroseismology is a window into the interior of a star, allowing scientists to hear the frequencies at which individual stars resonate. That makes it possible to refine our readings on the mass of the star, and just as significantly, to determine its age with higher accuracy.

KPF uses radial velocity measurements to do its work, a technique often discussed in these pages to identify exoplanet candidates. But in this case measuring the motion of the stellar surface to and from the Earth is a way of collecting the star’s vibrations, which are the key to stellar structure. Says lead author Yaguang Li (University of Hawaii at Mānoa):

“The vibrations of a star are like its unique song. By listening to those oscillations, we can precisely determine how massive a star is, how large it is, and how old it is. KPF’s fast readout mode makes it perfectly suited for detecting oscillations in cool stars, and it is the only spectrograph on Mauna Kea currently capable of making this type of discovery.”

Image: Artist’s concept of the HD219134 system. Sound waves propagating through the stellar interior were used to measure its age and size, and characterize the planets orbiting the star. Credit: openAI, based on original artwork from Gabriel Perez Diaz/Instituto de Astrofísica de Canarias. The 10-second audio clip transforms the oscillations of HD219134 measured using the Keck Planet Finder into audible sound. The star pulses roughly every four minutes. When sped up by a factor of ~250,000, its internal vibrations shift into the range of human hearing. By “listening” to starlight in this way, astronomers can explore the hidden structure and dynamics beneath the star’s surface.

What we learn here is that HD 219134 is more than twice the age of the Sun at about 10.2 billion years old. The age of a star can be difficult to determine. The most widely used measurement involves gyrochronology, which focuses on how swiftly a star spins, the assumption being that younger stars rotate more rapidly than older ones, with the gradual loss of angular momentum traceable over time. The problem: Older stars don’t necessarily follow this script, with their spin-down evidently stalling at older ages. Asteroseismology allows a more accurate reading for stars like this and provides a different reference point, providing that our models of stellar evolution allow us to interpret the results correctly..

We need to track this work because how old a star is has implications across the board. For one thing, understanding basic factors such as its temperature and luminosity requires a context to determine whether we’re dealing with a young, evolving system or a star nearing the transition to a red giant. From an astrobiological point of view, we’d like to know how old any planets in the system are, and whether they’ve had sufficient time to develop life. SETI also takes on a new dimension when considering stellar age, as targeting older exoplanet systems allows us to put our focus on higher priority targets.

Yaguang Li thinks the KPF work brings new levels of precision to these measurements, calling the result ‘a long-lost tuning fork for stellar clocks.’ From the exoplanet standpoint, stellar age is also quite informative. For the measurements have allowed the researchers to determine that HD 219134 is smaller than previously thought by about 4% in radius – this contrasts with interferometry measurements that measured its size via multiple telescopes. A more accurate reading on the size of the star affects all inferences about its planets.

That 4% difference, though, raises questions, and the authors note that it requires the models of stellar evolution they are using to be accurate. From the paper:

We were unable to easily attribute this discrepancy to any systematic uncertainties related to interferometry, variations in the canonical choices of atmospheric boundary conditions or mixing-length theory used in stellar modeling, magnetic fields, or tidal heating. Without any insight into the cause of this discrepancy, our subsequently derived quantities and treatment of rotational evolution—all of which are contingent on these model ages and radii—must necessarily be regarded as being only conditional, pending a better understanding of the physical origin for this discrepancy. Future direct constraints on stellar radii from asteroseismology (e.g., through potential breakthroughs in understanding and mitigating the surface term) may alleviate this dependence on evolutionary modeling.

So we have to be cautious in our conclusions here. If indeed the tension between the KPF measurements and interferometry is correct, we will have adjusted our calibration tools for transiting exoplanets but still need to probe the reasons for the discrepancy. That’s important, because with tuned-up measurements of a star’s size, the radii and densities of transiting planets can be more accurately measured. The updated values KPF has given us – assembled through over 2000 velocity measurements of the star – point to a significant aspect of stellar modeling that may need further adjustment.

The paper is Yaguang Li et al., “K Dwarf Radius Inflation and a 10 Gyr Spin-down Clock Unveiled through Asteroseismology of HD 219134 from the Keck Planet Finder,” Astrophysical Journal Vol. 984, No. 2 (6 May 2025), 125 (full text).