The TESS Mission
For the past five years, NASA’s Transiting Exoplanet Survey Satellite (TESS) has surveyed the sky searching for new planets orbiting our closest stellar companions. To accomplish this colossal task, this satellite stares at hundreds of thousands of stars waiting for a planet to pass in front of it blocking some of the light. This technique is known as the transit method, in which small periodic dips in a star’s light indicate the presence of a transiting object (Fig. 1). The depth of the transit is directly proportional to the size of the planet. The overall symmetric transit shape however, is consistent regardless of planet size – with the notable exception of the TOI-3884 system.
Fig 1. Cartoon of the transit method courtesy of NASA. As the planet passes in front of the star, the star dims slightly giving a symmetric transit curve.
TOI-3884b – A Super-Neptune Around a Low-Mass M-dwarf Star
A potential planet orbiting the star, TOI-3884, every 4 days was first flagged back in 2020. At this point, the only observations of this target relied on 30-minute exposures (imagine opening your camera shutters for 30 minutes at a time) taken by TESS. Because of this only 2-3 points were observed during the 1.5-hour transit. Interestingly, these dips blocked 3-4% of the star’s light – a significant fraction as most planets block less than 1% of light. This large several percent transit depth could only be explained by a Neptune or larger gas giant planet orbiting a small star known as a M-dwarf or red dwarf. The existence of a large planet around a low-mass star like TOI-3884 is surprising (with only three others known: TOI-3235b, TOI-519b and TOI-5205b, also confirmed by HPF).
However, simply observing a transit is not enough to confirm a planet. Small stars and brown dwarfs are also known to create similar deep transits. One reliable way to confirm this object is by measuring its mass. We began following up this planet with radial velocities using the Habitable-zone Planet Finder to derive a mass measurement. From these RV observations, we confirmed this object has a mass of 32 times that of Earth (~2 times Neptune’s mass). By all appearances, we discovered the first super-Neptune orbiting the lowest mass star known to date. A straightforward discovery, until we observed the transit.
A Persistent Asymmetric Transit
To confirm the object was transiting the star in question, we observed 3 transits from the ground-based telescope 0.3 m TMMT in Las Campanas, Chile. While we found the planet did orbit the correct star, the transit shape was…wonky. Instead of a clean symmetric dip (as shown in Fig. 1), the transit appeared skewed, slightly shallower in the first half, deeper in the second half (Fig.2). Odd, but perhaps an issue with the instrument? We observed it again, and again. Same result each time. We then decided to observe it with the 3.5 m Apache Point Observatory (APO) telescope using a fast exposure rate (one image per 20 seconds). Not only do we observe the same skewed transit, there appears clear structure in the transit shape! TESS observations in 2022 confirm this shape. Whatever is distorting the transit must be constant as it appears at the same time across every transit for nearly 2 years.
Fig. 2. Various transits of TOI-3884b observed with TESS (top row), the 0.3 m TMMT (middle row), and the 3.5 m ARC Telescope at APO (bottom row). The expected symmetric transit shape is plotted in red. The light blue points highlight the persistent bump that is apparent in every transit for the past 2 years.
The Culprit: A Large Starspot
Starspot crossing events, or when the planet passes over a star spot during transit, are notorious for creating bumps in a transit (Fig 3.). As the planet passes over the colder and darker spot, it suddenly blocks a little less light thus creating a slight rise in the transit shape. Once the planet reaches the hotter and brighter photosphere, it blocks a little more light creating a deeper shape. How much light is blocked during the spot crossing depends on the temperature of the spot – cooler spots are darker leading to more contrast and a bigger bump. For TOI-3884b, the amplitude of this bump leads to spot temperature 300 K cooler than the average stellar temperature (2900 K versus 3200 K). A value originally predicted by previous models and confirmed with this new data!
Fig. 3. Our transit model when including three different spots (red circles) on the star. The orbit of the planet is shown with the black line. As it passes over a spot, less light is blocked creating a bump in the below transit. In order to match our transit shape the star must be tilted so that we are observing its pole and TOI-3884b must possess a misaligned (polar) orbit.
However, the spot is only part of the story. We needed to explain how the spot appears at the exact same location in every transit for 2 years. This led to three hypotheses. First, the star could be rotating very slowly such that the spot has not rotated far on the surface. HPF spectra however, suggest the star rotates every 10 days. Second, the star’s rotational period could be exactly the same as the planet’s orbital period. This however implies that the spot would be rotating in and out of view every several days and the star would become brighter (no spot) and fainter (with a spot) over the 4 days. TESS shows the brightness of TOI-3884 to be constant over multiple observations ruling out this hypothesis as well (Fig. 4). The most promising hypothesis is if the star is inclined from our line of sight, such that we were observing its pole, and the spot was located on the pole (Fig. 3). Thus, as the star rotated, the spot never rotated in and out of view from our viewpoint. Pole spots on low-mass M-dwarf stars are not uncommon, especially young active stars like TOI-3884. They are also known to exist for months to years, it is feasible to observe this spot over multiple years.
Fig 4. If the spot was on the star’s equator, it would rotate in and out of view creating large changes in the amount of light (flux) we see (red model). However, TESS does not observe these changes meaning the star must be oriented such that most of the spot does not rotate out of view (blue model).
The only way for the planet to cross the spot, however, is if its orbit was significantly inclined from the star’s rotational equator. In other words, unlike our solar system with all the planets essentially lining up along our Sun’s equator, TOI-3884b’s orbit is inclined such that it nearly passes over its star’s pole! How TOI-3884b ended up on this skewed orbit is still a mystery, though TOI-3884b joins the small population of known misaligned Neptunes.
Future Prospects of the TOI-3884 System
Discovering a gas giant orbiting a low-mass star is in itself a surprising discovery! Understanding how TOI-3884b formed and evolved on its misaligned orbit remains an ongoing challenge. The atmosphere of this planet may hold tentative clues surrounding its origin. We show that TOI-3884b is one of the most promising planets for atmospheric characterization using the James Webb Space Telescope especially for planets cooler than 500 K (Fig. 5). The consistent spot crossing event also enables a unique look into the structure of M-dwarf pole spot and their impacts on the observed atmospheric spectrum of a planet.
Fig. 5. Simulation of what the atmospheric spectrum of TOI-3884b could look like with JWST. Water and methane is expected to be the most dominant molecules in the spectrum, and deviations from this could yield new insights into the atmospheres of giant planets and the impact of starspots on these measurements.
The TOI-3884 system is a rare find – a misaligned super-Neptune consistently transits a pole-spot along our line of sight. Without the assistance of HPF, we would have struggled to unravel this unique puzzle. However, while the mystery surrounding the skewed shape is solved (with the help of HPF), ongoing monitoring of this system with HPF will yield new one-of-a-kind insights into the planet, the star, and the system as a whole.
You can read more about the TOI-3884 here.
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