About

Christopher Chyba is a Professor of Astrophysical Sciences at Princeton University. His role involves teaching and research within the Department of Astrophysical Sciences. His office is located in Peyton Hall, and he can be contacted via email at [email protected] His work focuses on astrophysics, contributing to the academic community through his involvement in the department's activities and research initiatives.

Research topics

• Aerospace engineering
• Engineering
• Aeronautics
• Engineering ethics
• Biology
• Astrobiology

Selected publications

• Measuring Apsidal Clustering

  ArXiv.org · 2026-04-28

  articleOpen access

  The decade-long debate over the existence of apsidal clustering in the outer solar system is poised for reignition given the plethora of distant trans-Neptunian object (TNO) discoveries expected from the forthcoming Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST). Here, we present a new conditional-likelihood method to measure apsidal clustering that is insensitive to uneven survey footprints. We calculate the long-term orbital stability of distant TNOs, which allows us to expand the known sample of relevant objects from 21 to 25. We apply our new method to this up-to-date sample, showing that the significance of the apsidal clustering in the outer solar system has fallen from $2.7σ$ to $1.9σ$, and that the direction of clustering is not well constrained. This new method is suitable for application to the growing sample of known TNOs, and the results will reveal whether the evidence for a hypothetical Planet X from apsidal clustering is real or spurious.

  PublisherOA PDF

• Revealing Callisto’s Near-subsurface Thermophysical Properties with ALMA Calibration Data

  The Planetary Science Journal · 2026-01-01

  articleOpen access

  Abstract Thermal images at different wavelengths probe varying subsurface depths of planetary bodies and therefore can inform us about their compositions, thermophysical properties, and impact histories. We identified six archival observations of Callisto obtained by the Atacama Large Millimeter/submillimeter Array (ALMA) between 2012 July 17 and 2012 November 4 at wavelengths of 0.43–0.47 mm (701.9–641.5 GHz). These wavelengths are shorter than those of nearly all other Callisto ALMA data and are sensitive to subsurface emission at depths (the upper ∼centimeter) between those sounded by millimeter and infrared observations. We estimate the disk-averaged brightness temperature as 133 ± 15 K and use a thermophysical mixture model to find that Callisto’s thermal emission is best fit by a ∼50%–50% two-component thermal inertia mixture of Γ low ∼15–40 J m −2 K −1 s −1/2 and Γ high ∼1200–2000 J m −2 K −1 s −1/2 , consistent with recent ALMA observations of Callisto at longer wavelengths. Finally, we present several previously unpublished Galileo Photopolarimeter/Radiometer (PPR) observations of Callisto and derive thermal inertia and spectral emissivity maps using the same model. Altogether, these ALMA and PPR maps improve our understanding of the thermal properties and spatial distribution of Callisto’s shallow subsurface regolith and demonstrate the value of ALMA flux density calibrator data for extending frequency coverage of existing science data.

  PublisherDOI

• Measuring Apsidal Clustering

  The Astrophysical Journal · 2026-05-15

  articleOpen access

  Abstract The decade-long debate over the existence of apsidal clustering in the outer solar system is poised for reignition given the plethora of distant trans-Neptunian object (TNO) discoveries expected from the forthcoming Vera C. Rubin Observatory’s Legacy Survey of Space and Time. Here, we present a new conditional-likelihood method to measure apsidal clustering that is insensitive to uneven survey footprints. We calculate the long-term orbital stability of distant TNOs, which allows us to expand the known sample of relevant objects from 21 to 25. We apply our new method to this up-to-date sample, showing that the significance of the apsidal clustering in the outer solar system has fallen from 2.7 σ to 1.9 σ and that the direction of clustering is not well constrained. This new method is suitable for application to the growing sample of known TNOs, and the results will reveal whether the evidence for a hypothetical Planet X from apsidal clustering is real or spurious.

  PublisherDOI

• Measuring Apsidal Clustering

  arXiv (Cornell University) · 2026-04-28

  preprintOpen access

  The decade-long debate over the existence of apsidal clustering in the outer solar system is poised for reignition given the plethora of distant trans-Neptunian object (TNO) discoveries expected from the forthcoming Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST). Here, we present a new conditional-likelihood method to measure apsidal clustering that is insensitive to uneven survey footprints. We calculate the long-term orbital stability of distant TNOs, which allows us to expand the known sample of relevant objects from 21 to 25. We apply our new method to this up-to-date sample, showing that the significance of the apsidal clustering in the outer solar system has fallen from $2.7σ$ to $1.9σ$, and that the direction of clustering is not well constrained. This new method is suitable for application to the growing sample of known TNOs, and the results will reveal whether the evidence for a hypothetical Planet X from apsidal clustering is real or spurious.

  PublisherDOI

• Experimental demonstration of electric power generation from Earth's rotation through its own magnetic field

  Physical Review Research · 2025-03-19 · 2 citations

  articleOpen access1st authorCorresponding

  Earth rotates through the axisymmetric part of its own magnetic field, but a simple proof shows that it is impossible to use this to generate electricity in a conductor rotating with Earth. However, we previously identified implicit assumptions underlying this proof and showed theoretically that these could be violated and the proof circumvented. This requires using a soft magnetic material with a topology satisfying a particular mathematical condition and a composition and scale favoring magnetic diffusion, i.e., having a low magnetic Reynolds number <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:msub><a:mi mathvariant="normal">R</a:mi><a:mi mathvariant="normal">m</a:mi></a:msub></a:math> [Chyba and Hand, ]. Here we realize these requirements with a cylindrical shell of manganese-zinc ferrite. Controlling for thermoelectric and other potentially confounding effects (including 60 Hz and RF background), we show that this small demonstration system generates a continuous DC voltage and current of the (low) predicted magnitude. We test and verify other predictions of the theory: voltage and current peak when the cylindrical shell's long axis is orthogonal to both Earth's rotational velocity <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:mi mathvariant="bold">v</d:mi></d:math> and magnetic field; voltage and current go to zero when the entire apparatus (cylindrical shell together with current leads and multimeters) is rotated <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:msup><f:mn>90</f:mn><f:mo>∘</f:mo></f:msup></f:math> to orient the shell parallel to <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"><g:mi mathvariant="bold">v</g:mi></g:math>; voltage and current again reach a maximum but of opposite sign when the apparatus is rotated a further <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"><i:msup><i:mn>90</i:mn><i:mo>∘</i:mo></i:msup></i:math>; an otherwise-identical MnZn ferrite cylinder generates zero voltage at all orientations; and a high-<j:math xmlns:j="http://www.w3.org/1998/Math/MathML"><j:msub><j:mi mathvariant="normal">R</j:mi><j:mi mathvariant="normal">m</j:mi></j:msub></j:math> cylindrical shell produces zero voltage. We also reproduce the effect at a second experimental location. The purpose of these experiments was to test the existence of the predicted effect. Ways in which this effect might be scaled to generate higher voltage and current may now be investigated.

  PublisherDOI

• Limits on Stellar Flybys in the Solar Birth Cluster

  The Astrophysical Journal Letters · 2025-10-23 · 1 citations

  articleOpen accessCorresponding

  Abstract The orbits of small bodies in the outer solar system are particularly sensitive to gravitational perturbations, including stellar flybys. Stellar clusters, with low velocity dispersions and high number densities, can be the source of strong and frequent flybys. As a result, we can infer what properties of the solar birth environment would be incompatible with the structure of the outer solar system observed today. Here, we explore with n -body simulations the implications of the low inclinations ( i &lt; 20 ∘ ) of the distant sednoids (objects with perihelia q &gt; 40 au and semimajor axes a &gt; 400 au) for the properties of the solar birth cluster. We find that the existence of these orbits, if they were in place in the Sun’s birth cluster phase, would limit the product of the stellar number density and the Sun’s residence time in the birth cluster to ≲5 × 10 3 Myr pc −3 , as compared to the weaker limit ≲5 × 10 4 Myr pc −3 implied by the low inclinations of the cold classical Kuiper Belt.

  PublisherDOI

• The Inner Kernel of the Classical Kuiper Belt

  ArXiv.org · 2025-11-10

  preprintOpen access

  The `kernel' of the classical Kuiper belt was discovered by Petit et al. (2011) as a visual overdensity of objects with low ecliptic inclinations and eccentricities at semimajor axes near 44 AU. This raises the question - are there other structures present in the classical Kuiper belt? If there are, clustering algorithms applied to orbits transformed into free elements may yield the best chance of discovery. Here, we derive barycentric free orbital elements for objects in the classical Kuiper belt, and use the Density-Based Spatial Clustering of Applications with Noise (DBSCAN) algorithm to identify a new structure, which we dub the inner kernel, located at $a \sim 43 \mathrm{\; AU}$ just inward of the kernel ($a \sim 44 \mathrm{\; AU}$), which we also recover. It is yet unclear whether the inner kernel is an extension of the kernel or a distinct structure. Forthcoming observations, including those by the Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) may provide further evidence for the existence of this structure, and perhaps resolve the question of whether there are two distinct structures.

  PublisherOA PDFDOI

• Orbit of a Possible Planet X

  The Astrophysical Journal · 2025-01-07 · 7 citations

  articleOpen access

  Abstract The plausibility of an unseen planet in the outer solar system, and the expected orbit and mass of such a planet, have long been a topic of inquiry and debate. We calculate the long-term orbital stability of distant trans-Neptunian objects (TNOs), which allows us to expand the sample of objects that would carry dynamical information about a hypothetical unseen planet in the solar system. Using this expanded sample, we find statistically significant clustering at the ∼3 σ level for TNOs with semimajor axes &gt; 170 au in the longitude of perihelion ( ϖ ), but not in inclination ( i ), argument of perihelion ( ω ), or longitude of node (Ω). Since a natural explanation for clustering in ϖ is an unseen planet, we run 300 N -body simulations with the giant planets, a disk of test particles representing Kuiper Belt objects, and an additional planet with varied initial conditions for its mass, semimajor axis, eccentricity, and inclination. Based on the distribution of test particles after 1–2 Gyr, we compute relative likelihoods given the actual distribution of ϖ as a function of semimajor axis for distant TNOs on stable orbits using a significantly larger sample than previous work. We find the best-fit unseen planet parameters to have mass m p = 4.4 ± 1.1 M ⊕ , semimajor axis a p = 290 ± 30 au, eccentricity e p = 0.29 ± 0.13, and inclination i p = 6 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover> <mml:mrow> <mml:mo>.</mml:mo> </mml:mrow> <mml:mrow> <mml:mtext>°</mml:mtext> </mml:mrow> </mml:mover> </mml:math> 8 ± 5 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover> <mml:mrow> <mml:mo>.</mml:mo> </mml:mrow> <mml:mrow> <mml:mtext>°</mml:mtext> </mml:mrow> </mml:mover> </mml:math> 0. Only 0.06% of the M. E. Brown &amp; K. Batygin Planet Nine reference population produce probabilities within 1 σ of the maximum within our quadrivariate model, indicating that our work identifies a distinct preferred region of parameter space for an unseen planet in the solar system.

  PublisherDOI

• Measuring the mean plane of the distant Kuiper belt

  Monthly Notices of the Royal Astronomical Society Letters · 2025-08-12 · 3 citations

  articleOpen access

  ABSTRACT In the absence of any unseen planetary-mass bodies in the outer Solar system, the mean plane of the distant Kuiper belt should be the same as the plane orthogonal to the angular momentum vector of the Solar system – the invariable plane. Here, we measure the mean plane of the non-resonant Kuiper belt across semimajor axes $50{\!-\!}400 \rm{\,\,au}$. We introduce a new method to measure the mean plane that we demonstrate to be independent of observational bias. In particular, our results are not biased by surveys that look only at limited areas on the celestial sphere. We find a warp relative to the invariable plane at semimajor axes of $80{\!-\!}400 \rm{\,\,au}$ (98 per cent confidence) and $80{\!-\!}200 \rm{\,\,au}$ (96 per cent confidence), but not at $50{\!-\!}80 \rm{\,\,au}$ or $200{\!-\!}400 \rm{\,\,au}$. If it is not spurious, a possible explanation for this warp is an unseen planet in the outer Solar system. With n-body simulations, we find that a planet with mass between that of Mercury and the Earth, semimajor axis in the range ${\sim} 100{\!-\!}200 \rm{\,\,au}$, and inclination ${\gtrsim} 10^{\circ }$ to be the most likely cause of the warp; however, parameters outside of these ranges are still possible. Such a body is distinct in both mass and semimajor axis from the various versions of an unseen planet invoked to explain apsidal clustering in the outer Solar system. The Vera C. Rubin Observatory’s Legacy Survey of Space and Time is expected to confirm or deny the existence of the warp reported here, and might detect the planet that may produce it.

  PublisherOA PDFDOI

• Is the Distant Kuiper Belt Warped or Flat?

  2025-07-09

  preprintOpen accessSenior author

  At large semimajor axes ($a \gtrsim 50 \mbox{\;AU}$), the mean plane of the Kuiper belt is expected to be the ``invariable plane,'' which is orthogonal to the angular-momentum vector of the known solar system. However, if there were an additional unseen planet or planets in the outer solar system, the expected mean plane of the distant Kuiper belt could differ from the current definition of the invariable plane. While there is a general consensus that the mean plane of the Kuiper belt is consistent with the invariable plane for $50\mbox{\;AU}\lesssim a \lesssim 100 \mbox{\;AU}$, not much is known about the mean plane of the Kuiper belt at $a \gtrsim 100 \mbox{\;AU}$. We measure the mean plane of the Kuiper belt at semimajor axes of $100 - 400 \mbox{\;AU}$ and investigate the effects of hypothetical unseen planets of various masses and orbits on the mean plane.

  PublisherDOI

Frequent coauthors

• Paul J. Thomas

  42 shared

• Carl Sagan

  University of Manitoba

  41 shared

• K. P. Hand

  Jet Propulsion Laboratory

  37 shared

• Kevin Zahnle

  29 shared

• Christopher P. McKay

  Ames Research Center

  28 shared

• C. B. Phillips

  Jet Propulsion Laboratory

  10 shared

• W. R. Thompson

  9 shared

• E. Pierazzo

  9 shared

Labs

• Department of Astrophysical Sciences, Princeton UniversityPI