🔭 astrophysics

Searching for Extragalactic Exoplanets: A Survey of the Sagittarius Dwarf Galaxy Stream with TESS

This study presents the first transit survey of the Sagittarius dwarf galaxy stream using TESS data, establishing stringent upper limits on hot Jupiter occurrence rates that are lower than those found in Milky Way globular clusters and suggesting that older, metal-poor extragalactic environments may host fewer such planets.

Original authors: William Schap, Jason Dittmann, Elizabeth Lada

Published 2026-02-18

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CC BY 4.0

Original authors: William Schap, Jason Dittmann, Elizabeth Lada

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: Hunting for "Lost" Planets

Imagine the Milky Way (our home galaxy) as a massive, bustling city. For decades, astronomers have been scanning the streets of this city, finding thousands of planets orbiting other stars. But what about the suburbs? Or the neighboring towns?

Until now, no one has ever found a planet outside our galaxy. It's like knowing every house in your city has a dog, but you have no idea if the next town over has any.

This paper is the first serious attempt to look for planets in the "suburbs" of our galaxy: a giant stream of stars called the Sagittarius Dwarf Galaxy Stream. Think of this stream as a cosmic river of stars that used to be a small, separate galaxy but got torn apart and swallowed by the Milky Way. The stars in this stream are like "ghosts" from another world, now drifting through our neighborhood.

The Challenge: Finding a Needle in a Haystack

The problem is that these "ghost stars" are incredibly dim.

The Analogy: Imagine trying to spot a firefly (a dim star) in a dark forest, but you are using a flashlight designed to find bright streetlamps. The firefly is too faint for your standard tools.
The Solution: The team used a special telescope called TESS (which usually looks at bright stars) and paired it with new, super-smart software (called eleanor and TGLC). Think of these software packages as "digital night-vision goggles" that can clean up the static and noise to see those faint fireflies clearly.

The Hunt: 15,000 Stars and One False Alarm

The team picked 15,176 stars from this stream that were bright enough to study. They watched them for a long time, looking for a tiny dip in brightness that would happen if a giant planet (a "Hot Jupiter") passed in front of the star.

The Result: They found zero planets.
The "False Alarm": They did find one signal that looked like a planet. But when they zoomed in, they realized it was a trick of the light. A nearby, brighter star (like a neighbor's porch light) was bleeding into their camera lens, making it look like the dim star was wobbling. It was a "glitch," not a planet.

The Conclusion: The "Empty" Neighborhood

Since they didn't find any planets, they couldn't say, "Here is a planet!" Instead, they calculated a probability limit.

The Analogy: Imagine you walk into a room with 15,000 people and look for someone wearing a red hat. You don't see any. You can't say "No one in the world wears red hats," but you can say, "If there were red hats here, they would have to be extremely rare—less than 1% of the people."
The Finding: They calculated that if Hot Jupiters exist in this star stream, they are less than 1% as common as they are in our own galaxy. In fact, when they compared their results to other old, metal-poor star clusters, the Sagittarius stream seemed to have even fewer planets.

Why Are They So Rare? (The "Why" Section)

The authors offer a few theories on why this "suburb" might be planet-free:

The "Old Neighborhood" Theory: These stars are very old. Hot Jupiters are like unstable tenants; over billions of years, they might spiral into their host stars and get eaten. The older the neighborhood, the more likely the "tenants" have already been evicted.
The "Poor Soil" Theory: Planets need heavy elements (like iron and gold, which astronomers call "metals") to form. This star stream is very "metal-poor" (like soil with no nutrients). It's harder to build a giant planet out of thin air.
The "Crowded vs. Empty" Theory: Sometimes, stars bumping into each other in crowded clusters can knock a planet into a tight orbit (making it a Hot Jupiter). But this stream is very spread out (low density). Without those cosmic "bumps," the giant planets might stay far away in the cold, never migrating close enough to be seen.

What's Next?

This study is just the beginning. The team estimates that to actually find one of these rare extragalactic planets, they would need to scan about 80,000 stars with this level of precision.

The Takeaway:
This paper is a "dry run." It proves we can look for planets outside our galaxy using current technology. Even though they didn't find a planet this time, they set the rules for the next hunt. If they find one in the future, it will be a historic discovery. If they keep finding nothing, it tells us that the universe might be very different in older, metal-poor environments, and that giant planets might be a luxury of "young" and "rich" galaxies like our own.

1. Problem Statement

To date, no exoplanets have been definitively detected outside the Milky Way. While occurrence rates for "Hot Jupiters" (gas giants with short orbital periods) are well-constrained within the Milky Way (0.4–0.6%), these rates in extragalactic environments remain unknown.

The Challenge: Direct transit surveys of distant dwarf satellite galaxies are impossible with current technology (TESS/Kepler) because their stars are too faint.
The Opportunity: The Milky Way's merger history has created tidal streams of extragalactic stars (e.g., the Sagittarius Dwarf Galaxy Stream) that reside within the Milky Way halo. These stars are closer than those in satellite galaxies but are generally fainter than the typical targets of TESS.
The Goal: To perform the first transit survey of an extragalactic-origin stellar stream to constrain the occurrence rate of Hot Jupiters in a low-metallicity, extragalactic environment.

2. Methodology

Target Selection

Source: The authors utilized a catalog of 773,612 halo stars identified as potential members of the Sagittarius (Sgr) dwarf galaxy stream by P. Ramos et al. (2022).
Criteria:
- Magnitude: $T_{mag} \leq 17$ (pushing beyond the standard TESS Quick-Look limit of 13.5).
- Membership Probability: $>50\%$ likelihood of belonging to the Sgr stream.
- Stellar Type: Main sequence stars with radii $<2 R_{\odot}$ (focusing on FGK stars).
Final Sample: 15,176 stars.

Data Reduction & Light Curve Processing

Standard TESS pipelines (SPOC) are ineffective for stars fainter than $T_{mag} \approx 13.5$ . The authors employed two specialized Python packages to generate light curves for these dim stars:

eleanor: Uses variable apertures and background subtraction.
TESS-Gaia Light Curve (TGLC): Incorporates Gaia DR3 data to model and subtract Point Spread Functions (PSFs) of contaminating neighbors.

Strategy: Both methods were run on all targets. The final analysis selected the light curve with the highest precision for each star.
Noise Filtering: A Poisson-like noise threshold was established (anchored at $T_{mag}=16$ ) to reject light curves with excessive scatter or contamination. Data points $>5\sigma$ from the median were removed, and data was binned to a 30-minute cadence.

Signal Detection & Vetting

Algorithm: Box Least Squares (BLS) periodograms were generated for periods between 0.6 and 10 days.
Thresholds: Candidates required a signal-to-noise ratio $>3\sigma$ above the median periodogram noise and an estimated radius $\leq 2 R_{Jup}$ .
Vetting: Candidates were visually inspected to reject sinusoidal variability (eclipsing binaries) and artifacts.
Injection Recovery: To determine survey completeness, 10,000 synthetic planet signals (radii 1–2 $R_{Jup}$ , periods 0.6–10 days) were injected into the light curves of 2,481 stars. Recovery rates were calculated to correct for detection efficiency.

3. Key Results

Detection Status

Zero Planets Detected: No confirmed exoplanets were found in the survey of 15,176 stars.
False Positive: One candidate signal (TIC 92223525, $R \approx 1.76 R_{Jup}$ , $P \approx 7.21$ days) was identified. However, detailed vetting revealed the signal originated from a nearby, brighter eclipsing binary (TIC 92223526) contaminating the aperture by ~11.5%. This candidate was rejected.

Occurrence Rate Upper Limits

Using the null detection and injection-recovery efficiencies, the authors calculated upper limits for the Hot Jupiter occurrence rate ( $\eta$ ):

General Limit: For planets with $R = 1\text{--}2 R_{Jup}$ and $P = 0.6\text{--}10$ days, the 68% confidence upper limit is <1.01%.
Specific Limits (1 $\sigma$ ):
- For $R = 1.5\text{--}2 R_{Jup}$ and $P < 10$ days: <0.37%.
- For $R = 1.4\text{--}2 R_{Jup}$ and $P < 8$ days: <0.78% (at 95% confidence).

Comparison with Milky Way Populations

The Sgr stream limits were compared to globular clusters (GCs) in the Milky Way halo, which share low-metallicity characteristics:

47 Tucanae: Previous surveys found an upper limit of <0.57% for similar parameters. The Sgr stream limit (<0.37%) is lower.
M4: Previous surveys found an upper limit of <0.81%. The Sgr stream limit (<0.78%) is slightly lower.
Metallicity Correlation: The Sgr stream has a metallicity of [Fe/H] $\approx -2.3$ to $0.0$ (average $\sim -1$ ). The observed low occurrence rates align with the known positive correlation between stellar metallicity and giant planet formation.

4. Significance and Discussion

First Extragalactic Constraints: This study provides the first quantitative constraints on the Hot Jupiter occurrence rate in an extragalactic stellar population.
Metallicity Dependence: The results support the hypothesis that Hot Jupiter formation is strongly inhibited in low-metallicity environments. The observed rates are consistent with power-law predictions based on the metallicity of the Sgr stream.
Alternative Explanations for Low Rates: The authors discuss other factors that may suppress Hot Jupiters in the Sgr stream:
- High Binary Fraction: The Sgr stream has a binary fraction ~2x higher than the Milky Way, which can disrupt protoplanetary disks or suppress planet formation.
- Stellar Age: Older stellar populations may have experienced orbital decay, causing Hot Jupiters to spiral into their host stars or be engulfed as stars evolve.
- Low Stellar Density: Unlike dense clusters where dynamical interactions might drive migration, the low density of the stream may inhibit the migration of Cold Jupiters into Hot Jupiter orbits.
Future Requirements: To detect a single extragalactic Hot Jupiter with current TESS efficiency, a survey of approximately 79,189 stars is required. With perfect detection efficiency, this drops to 11,467 stars.

5. Conclusion

The survey of the Sagittarius Dwarf Galaxy Stream yielded no confirmed extragalactic planets, resulting in an upper limit on the Hot Jupiter occurrence rate of <1.01%. This limit is lower than or comparable to limits found in Milky Way globular clusters, suggesting that the low metallicity and specific dynamical history of the Sgr stream inhibit the formation or survival of Hot Jupiters. Future surveys targeting additional extragalactic streams with larger sample sizes are necessary to either detect the first extragalactic planet or definitively confirm the suppression of giant planets in low-metallicity extragalactic environments.