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A particular trait that astronomers want to understand better in the evolution of planets around other stars is how their orbits change. In an ideal model system, all orbits are 2 perfectly uniform spheres circling a shared center of mass. However, nature tends to be more complex than that. Deviations from the idealized model allow scientists to better understand these systems, including their geometric arrangements in space and the possible presence of an unseen partner planet.

A team of astronomers recently conducted an extensive survey of the exoplanet TrES-1 b to determine how its orbit has changed over the past 20 years, since its discovery in 2004. The team chose TrES-1 b because it belongs to a category of relatively easy-to-observe exoplanets known as hot Jupiters. Hot Jupiters are exoplanets that are approximately the size of our Solar System’s gas giant Jupiter, but orbit close to their host stars, sometimes so close that they complete an entire revolution in mere days. TrES-1 b orbits its host, a star just under 90% the mass of the Sun, every 3 days. This short orbital period allows astronomers to observe many of its passages around the star, making it easier to measure changes in its orbit.

The team first compiled data on how much light TrES-1 b blocks when it passes in front of its host star from Earth’s perspective. This is known as fitting a transit light curve. Most of this light data came from ground-based telescopes, including observations made by citizen scientists. They also found relevant data in catalogs from the Transiting Exoplanet Survey Satellite, the Hubble Space Telescope, and the Spitzer Space Telescope. These data enabled the team to measure precisely how long it takes for TrES-1b to orbit its host.

They also found that another team of astronomers had used Spitzer’s Infrared Array Camera to observe 5 eclipses of TrES-1 b behind its host star relative to Earth in 2004. Additionally, they identified 4 other studies from 2004 to 2016 with detailed measurements of how the light from TrES-1 b’s host star is affected by its tugging over the course of its orbit, which is called its radial velocity. The astronomers combined the transit light curves, eclipses, and radial velocity measurements to obtain a more comprehensive picture of TrES-1 b, which they compared with statistical models to understand its long-term behavior.

They attempted to fit 5 different models to their observations of TrES-1 b to see which model most closely matched the data. The first model simulated the planet with a constant, circular orbit around its star. The next used a constant, slightly squished or eccentric orbit. The third used a circular orbit that was shrinking or decaying over time. The fourth used a decaying, slightly eccentric orbit. And the last used a slightly eccentric orbit that also changed in orientation over time with respect to its star, referred to as precessing.

The team found that, regardless of which data subset they used, the best-fitting explanation for their data was that TrES-1 b had an eccentric, precessing orbit. They also found that the decaying orbit models fit better than either of the constant orbit models. This suggested that, regardless of how the exoplanet’s orbit is changing, the data don’t support any explanation claiming there is no real change in its orbit.

In addition, the team explained that the speed at which this exoplanet’s orbit is changing suggests another planet in the system is pulling on it. They estimated that this other planet is at most 25% the size of Jupiter with an orbital period of at most 7 days. However, this claim comes with the caveat that they couldn’t find direct evidence for such a planet in the data, except through its effects on TrES-1 b. They did find evidence for another exoplanet in the system, which they designated TrES-1 c, but this exoplanet has a wide, eccentric orbit that is unlikely to be the driver for TrES-1 b’s changing orbit.

Ultimately, the team concluded that a multi-faceted approach to studying exoplanet orbital timing reveals dynamics that single-faceted observations and models could miss. They suggested that advances in studying the long-term behavior of exoplanets will require long-term monitoring, more precise radial velocity measurements, and complex simulations of multiple objects in a gravitational system.

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Study Information

Original study: TrES-1 b: A Case Study in Detecting Secular Evolution of Exoplanet Orbits

Study was published on: September 2, 2025

Study author(s): Simone R. Hagey, Billy Edwards, Angelos Tsiaras, Aaron C. Boley, Anastasia Kokori, Norio Narita, Pedro V. Sada, Filip Walter, Robert T. Zellem, Napaporn A-thano, Kevin B. Alton, Miguel Ángel Álava Amat, Paul Benni, Emmanuel Besson, Patrick Brandebourg, Marc Bretton, Mauro Caló, Martin Valentine Crow, Jean-Christophe Dalouzy, Marc Deldem, Tõnis Eenmäe, Stephane Ferratfiat, Pere Guerra, Gary Vander Haagen, Ken Hose, Adrian Jones, Yves Jongen, Didier Laloum, Stefano Lora, Alessandro Marchini, Jacques Michelet, Matej Mihelčič, Johannes Mieglitz, Eric Miny, David Molina, Mario Morales Aimar, Raphael Nicollerat, Ivo Peretto, Manfred Raetz, François Regembal, Robert Roth, Lionel Rousselot, Mark Salisbury, Darryl Sergison, Anaël Wünsche, Jaroslav Trnka

The study was done at: The University of British Columbia (Canada), Netherlands Institute for Space Research (The Netherlands), Aristotle University of Thessaloniki (Greece), University College London (UK), The University of Tokyo (Japan), Instituto de Astrofísica de Canarias (IAC) (Spain), Universidad de Monterrey (Mexico), Stefanik Observatory (Czech Republic), Czech Astronomical Society (Czech Republic), California Institute of Technology (USA), National Astronomical Research Institute of Thailand (Thailand), UnderOak Observatory (USA), Acton Sky Portal (private observatory) (USA), Observatoire des Baronnies Provençales (France), Laboratoire d’Astrophysique des Baronnies: Science Citoyenne et Action pour la Nuit (France), Tartu Observatory of Tartu University (Estonia), Observatori Astronòmic Albanyà (Spain), Observatoire de Vaison-La-Romaine (France), AAVSO (USA), MarSEC (Marana Space Explorer Center) (Italy), University of Siena (Italy), Observatorio de Sencelles (Spain), Bundesdeutsche Arbeitsgemeinschaft für veränderliche Sterne e.V. (BAV) (Germany), Hvězdárna Slaný (Slaný Observatory) (Czech Republic)

The study was funded by: Natural Sciences and Engineering Research Council of Canada, University of British Columbia, Li Tze Fong Fellowship

Raw data availability: Available from the ExoClock project, VarAstro, AAVSO, the VisieR database, and the Mikulski Archive

Featured image credit: Comparison of exoplanet TrES-1 b with Jupiter by Aldaron is licensed under CC BY-SA 3.0

This summary was edited by: Halimat Chisom Atanda