🔭 astrophysics

TOI-6692b: An eccentric 130 day period giant planet with a single transit from TESS

This paper reports the discovery and characterization of TOI-6692 b, an eccentric 130-day period giant planet initially identified as a single TESS transit candidate, whose orbital period and properties were confirmed through extensive radial velocity monitoring and ground-based photometric follow-up.

Original authors: Allyson Bieryla, Karen A. Collins, George Zhou, David W. Latham, Brad Carter, Paul Dalba, Robert Gagliano, Thomas L. Jacobs, Martti Holst Kristiansen, Daryll Lacourse, Mark Omohundro, H. M. Schwengele

Published 2026-01-26

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CC BY 4.0

Original authors: Allyson Bieryla, Karen A. Collins, George Zhou, David W. Latham, Brad Carter, Paul Dalba, Robert Gagliano, Thomas L. Jacobs, Martti Holst Kristiansen, Daryll Lacourse, Mark Omohundro, H. M. Schwengeler, Khalid Barkaoui, Rafael Brahm, Douglas A. Caldwell, Jeffrey D. Crane, Tansu Daylan, Sarah Deveny, Yadira S. Gaibor, Michael Gillon, Thomas Henning, Keith Horne, R. Paul Butler, Jason D. Eastman, Steve B. Howell, Emmanuel Jehin, Eric L. N. Jensen, Andres Jordan, Michelle Kunimoto, Colin Littlefield, Lena Parc, Samuel N. Quinn, Malena Rice, Joseph E. Rodriguez, Richard P. Schwarz, Ramotholo Sefako, Stephen A. Shectman, Avi Shporer, Abderahmane Soubkiou, Gregor Srdoc, Michal Steiner, Marcelo Tala Pinto, Johanna Teske, Trifon Trifonov, Solene Ulmer-Moll, Cristilyn N. Watkins, Sharon X. Wang, Jhon Yana Galarza, Samuel W. Yee

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: Finding a "Ghost" Planet

Imagine you are watching a busy highway from a high tower. You spot a giant truck driving by, but you only see it for a split second before it disappears around a bend. You know it's there, but you have no idea when it will come back. Maybe it's a daily truck, maybe it's a weekly one, or maybe it's a monthly one.

This is exactly what astronomers faced with TOI-6692b.

The NASA TESS space telescope acted like that tower. It spotted a giant planet (a "Jupiter" roughly the size of our own Jupiter) passing in front of its star. But because the planet takes a long time to orbit (130 days), TESS only saw it once before the observation window closed. It was a "single transit" candidate—a ghost sighting that needed confirmation.

The Challenge: The Long Wait

Most planets we find orbit quickly, like a race car on a short track. You can watch them pass by many times in a single night. TOI-6692b, however, is like a slow-moving freight train on a very long, winding track. It takes 130 days to complete one lap.

Because the team only saw it once, they had a massive guessing game to play. They knew roughly when it might come back, but the uncertainty was huge. It was like trying to predict exactly when a train will arrive at a station when you only know it left the previous station 130 days ago, and your clock is off by several days.

The Detective Work: Two Teams, One Goal

To solve the mystery, the astronomers launched a two-pronged investigation:

1. The "Weight" Check (Radial Velocities)
Since they couldn't wait 130 days to see the planet pass again, they used a different trick. They listened to the star. As the giant planet orbits, its gravity tugs on the star, making the star wobble slightly.

The Analogy: Imagine a mother spinning a child around her hand. The mother (the star) has to lean back and forth to keep her balance. By measuring how much the star "leans" (using powerful telescopes like the 6.5m Magellan), the team could calculate the planet's mass. They confirmed it was indeed a giant planet, weighing about 0.6 times the mass of Jupiter.

2. The "Stakeout" (Ground-Based Photometry)
Once they knew the planet's mass and had a rough idea of its orbit, they had to catch it passing in front of the star again to confirm the exact timing.

The Analogy: This was like a global game of "Whac-A-Mole." The planet's transit (passing in front of the star) lasts for over 11 hours. No single telescope on Earth can watch a star for 11 hours straight because the Earth rotates, and the sun comes up.
The Solution: The team used a network of robotic telescopes around the world (in Chile, South Africa, and Australia). As one telescope lost sight of the star because the sun came up, another telescope in a different time zone picked it up. They kept the "watch" going for two weeks, hoping to catch the planet mid-transit.

The Result: A Near-Miss and a New Clue

The team didn't get a perfect view. They missed the exact moment the planet started or finished crossing the star (the "ingress" and "egress"). However, they did catch a blip in the data—a slight dip in brightness that looked like the planet was in the middle of crossing the star.

The Outcome: Even though it wasn't a perfect photo, this "blip" was enough to tighten the clock. They refined the planet's orbit from a rough guess to a very precise schedule: 131.125 days.

What We Learned About the Planet

It's a "Warm" Jupiter: It's not a "Hot Jupiter" (scorching close to its star) or an "Ice Giant" (frozen far away). It sits in a "Goldilocks" zone where it's warm but not burning.
It's a Wild Rider: The planet has a very eccentric orbit (e ~ 0.54).
- The Analogy: Most planets orbit like a perfect circle. This planet orbits like a stretched-out oval. It swings very close to its star and then zooms way out.
- Why it matters: This shape suggests the planet didn't just form there and stay put. It likely had a chaotic childhood, perhaps getting kicked into this weird orbit by bumping into other planets (a process called "planet-planet scattering").
There Might Be a Stranger in the House: The astronomers noticed the star was wobbling in a slow, steady rhythm that didn't match the known planet. It's like hearing a second, slower heartbeat. This suggests there is likely another, unseen companion (maybe another planet or a brown dwarf) far out in the system, tugging on the star.

Why This Matters

Finding planets like TOI-6692b is incredibly hard. Less than 1% of giant planets with long orbits have their mass and size confirmed. This discovery proves that even with just one blurry sighting from space, a coordinated team of ground-based astronomers can use patience and global teamwork to catch the "ghost" and confirm it's real.

It's a victory for persistence: catching a slow-moving giant in a universe that usually hides its secrets.

Technical Summary: TOI-6692 b – An Eccentric 130-Day Period Giant Planet with a Single Transit from TESS

Problem Statement
Long-period exoplanets (orbital periods $P > 100$ days) are inherently difficult to confirm and characterize. Their transits are geometrically rare and require extensive follow-up time due to their long durations and infrequent occurrence. Consequently, less than 1% of known giant exoplanets with periods exceeding 100 days have measured masses and radii. Furthermore, many candidates identified by the Transiting Exoplanet Survey Satellite (TESS) appear as "single-transit" events, making period determination and subsequent confirmation of the true orbital parameters a technically challenging and resource-intensive endeavor. This paper addresses the confirmation and characterization of TOI-6692 b, a candidate initially detected as a single transit, to contribute to the sparse population of well-constrained long-period giant planets.

Methodology
The authors employed a multi-faceted observational and modeling approach to confirm TOI-6692 b:

Photometric Discovery and Follow-up: The target was initially identified as a Community TESS Object of Interest (cTOI) by the Visual Survey Group and Planet Hunters, who detected a single transit in TESS Sector 39. To recover a second transit and refine the ephemeris, a ground-based photometric campaign was conducted over 12 days surrounding the predicted transit time. This utilized the Las Cumbres Observatory Global Telescope (LCOGT) network (CTIO, SAAO, SSO) and TRAPPIST-South. Despite the 11-hour transit duration exceeding the visibility window of any single site, a combined light curve was generated.
Spectroscopic Radial Velocity (RV) Monitoring: High-precision RVs were acquired to detect the planetary mass and confirm the orbital period. A total of 89 spectra were collected between 2021 and 2025 using four instruments:
- PFS (Planet Finder Spectrograph) on the 6.5m Magellan II telescope (45 spectra, highest precision).
- CHIRON on the 1.5m SMARTS telescope (31 spectra).
- FEROS on the 2.2m MPG telescope (6 spectra).
- CORALIE on the 1.2m Euler telescope (7 spectra).
High-Resolution Imaging: To rule out false positives caused by blended stellar companions, speckle imaging was performed using the Zorro instrument on the Gemini South 8-m telescope.
Global Modeling: The team performed a joint analysis using EXOFASTv2, simultaneously fitting TESS photometry, all available RVs, and catalog observations (Gaia DR3, 2MASS, WISE) to derive stellar and planetary parameters. Gaussian priors were applied to parallax and metallicity.

Key Results

Planetary Confirmation: TOI-6692 b is confirmed as a Jupiter-mass planet ( $0.620^{+0.080}_{-0.065} M_{Jup}$ ) with a radius of $1.042^{+0.050}_{-0.049} R_{Jup}$ .
Orbital Characteristics: The planet orbits with a period of $130.57^{+0.42}_{-0.35}$ days (refined to $131.125 \pm 0.012$ days when incorporating a marginal in-transit signal from the ground-based follow-up). The orbit is significantly eccentric ( $e = 0.537 \pm 0.061$ ).
Stellar Properties: The host star is a slightly sub-solar metallicity ( $[Fe/H] = -0.008^{+0.030}_{-0.027}$ ) star with a mass of $1.047^{+0.089}_{-0.071} M_{\odot}$ and a radius of $1.385^{+0.054}_{-0.052} R_{\odot}$ . The system is located at a distance of $310.5$ pc.
Transit Detection: The ground-based campaign did not capture a full ingress or egress but detected a possible in-transit signal (approx. 5-6 ppt depth) in the multi-night data. This detection, while not fully constraining the full transit shape, allowed for a refined ephemeris.
Long-Term Trend: A long-term linear RV trend of $-0.0280^{+0.0046}_{-0.0045}$ m/s/day was detected, suggesting the presence of an outer companion. Speckle imaging and RV modeling constrain this potential companion to a mass limit of $\sim 400 M_{Jup}$ at separations $> 20$ AU, though its orbital period remains unconstrained.
No Close Companions: High-resolution imaging confirmed TOI-6692 is a single star with no companions brighter than 5–6.5 magnitudes within 0.1" to 1.2" (30–376 AU).

Significance and Claims
The paper positions TOI-6692 b as one of only 15 known giant planets with periods $> 100$ days that possess secure mass, radius, and eccentricity measurements. The authors highlight several specific contributions:

Dynamical History: The planet's eccentricity ( $e \sim 0.54$ ) is lower than expected for planets undergoing high-eccentricity tidal migration (a mechanism often invoked for Hot Jupiters) but is consistent with outcomes from planet-planet scattering. This supports the view that long-period giants preserve their dynamical histories better than their highly irradiated, short-period counterparts.
Interior Structure: As a "warm Jupiter" with mild irradiation, TOI-6692 b serves as a laboratory for testing mass-radius relationships without the "radius anomaly" effects seen in Hot Jupiters. Its measured properties align well with theoretical models (Fortney et al. 2007).
Observational Feasibility: The successful confirmation of TOI-6692 b demonstrates the capability of ground-based photometric facilities (specifically the LCOGT network) to recover transits for single-transit TESS candidates, even when ephemeris uncertainties are large and transit durations are long. The authors note that such efforts are critical for expanding the sample of characterized long-period planets.
Atmospheric Potential: With a Transmission Spectroscopic Metric (TSM) of 23, TOI-6692 b is identified as a promising target for future atmospheric studies, particularly for understanding the chemical composition of cold-but-accessible giant planets.

The authors conclude that while confirming single-transit long-period candidates is resource-intensive, the combination of radial velocity monitoring and coordinated ground-based photometry is a viable path to expanding the census of these rare systems.