TOI-3785 b: A Low-Density Neptune Orbiting an M-dwarf Star

Posted on July 13, 2023 by Jessica Libby-Roberts
By Luke Powers

The planet now known as TOI-3785 b was observed in 2019 by the Transiting Exoplanet Survey Satellite (TESS). TESS observed periodic dips in the star’s brightness (known as a transit) that occurred every 4.67 days. As TOI-3785 b moves in its nearly circular orbit, there are predictable times (one orbital period) when this planet will partially obstruct our view of the host star, thus blocking a small percentage of its light. In the case of TOI-3785 b, approximately 1% of the host star’s light was blocked – motivating further investigation of this potential planet. From this photometric transit we could constrain planetary size, leading us to the conclusion that this planet is similar in size to Neptune (Figure 1).  

 

Figure 1: Transit observations of TOI-3785 b with red points showing 10-minute bins. The top row shows individual transits of TOI-3785 b observed with the ARCTIC instrument on the 3.5-meter Apache Point Telescope (left) and with the CCD camera on the 0.6m Red Buttes Observatory Telescope (RBO; right). Bottom row shows the multiple transits observed by TESS in Sector 20 (1800 second exposure time; left) and Sector 47 (120 second exposure time; right) stacked on top of each other. During transit, TOI-3785 b blocks out about 1% of the total light from its host star as it passes in front of it.

 

Figure 2: Phase folded radial velocity points from both HPF (dark red) and NEID (gold) with the best fit model plotted in blue along with the 1, 2, and 3 sigma uncertainty contours. As TOI-3785 b orbits, it causes its star, TOI-3785, to move with a speed of 20 m/s towards and away from us.

Even in light of this transit data, a planetary sized object does not necessarily equate to an exoplanet. Further investigations are required to rule out other possibilities such as white dwarfs, brown dwarfs,  or eclipsing binary stars – any of which could create a similarly deep transit. Deriving a constraint on this object’s mass is required to rule out other non-planetary objects. Ground-based follow-up of the host star’s radial velocity (motion of the star as it is being tugged around by the object)  using the Habitable-zone Planet Finder and NEID spectrographs revealed a surprising mass estimate (Figure 2). This larger-than-Neptune-sized exoplanet only exhibited a mass 80% that of Neptune (15 Earth masses)! This leaves us with a density of 0.61 grams per cubic centimeter, meaning that if this planet were to be submerged in water, it would float!

Unique Mass and Radius Reveals an Addition to a Rare Planetary Population

These estimations of the planet’s mass and radius, when viewed in the context of its cool and small M-dwarf host star, show intriguing results (Figure 3). When comparing it to all confirmed exoplanets, a surprising dearth of large planets around cool stars arises – notably when compared to similarly sized planets around hotter stars. In particular, we identify TOI-3785 b as belonging to a rare class of Neptune-like planets that orbit M-dwarf stars, only of which 8 have been confirmed. The discovery of a new planet in this parameter space, especially one with a lower density, has strong implications for our understanding of how gas giant planets form around low-mass and low-temperature stars.

Figure 3: Left: Planet radius plotted against planet mass for all planets with a well-measured mass. Planets orbiting smaller M-dwarfs are denoted with the color of their host star. Planets orbiting larger stars are plotted in gray. TOI-3785 b is circled while all the other known Neptunes orbiting M-dwarfs are numbered. Right: Planetary radius plotted against stellar temperature showing the clear lack of Neptunes around cooler stars.

In our recent paper on the confirmation of this planet, we explore the likely formation pathways rare planets such as TOI-3785 b may have taken. Observing the lack of confirmed Neptune-like planets around cool hosts leads us to question what we are missing about planet formation and/or evolution that leads M-dwarfs not to favor large planet formation. In this publication, we explore a planetary formation model that may be responsible for this Neptune shortage. The most popular and well accepted model for planetary formation is core accretion, in which a dense ring of gas and dust surrounding a star begins to collapse into a single point along an orbital path through the collision of planetesimals. This small collection of mass will begin to accrete heavier material onto its surface, eventually leading to a planet inside of a balanced orbit. In the case of ample material and undisturbed formation time, a planet may continue to grow until a significant size is reached, allowing for accelerated mass accumulation to occur known as runaway accretion. This is a common description of how massive Jupiter-sized planets came to be. In the case of TOI-3785 b, this planet did not achieve the same mass as many gas giants. Why? We proposed that planetary cores fit for accretion on the level of gas giants require much more time to form around M-dwarfs due to the lower mass of their protoplanetary disks. This creates a situation in which these cores do not have sufficient time to grow to their critical mass, and therefore accrete only into mid-ranged Neptune-like planets. This lack of surrounding material may be one possible explanation; another equally plausible explanation is that a core poised for runaway accretion could have been denied sufficient material from a disruption event that caused either disk dispersion or planetary migration to occur. Either scenario would prevent this protoplanet from developing into a Jupiter-mass planet. This suggests that M-dwarf Neptunes may arise from either lack of material or from external disturbances. These rare disturbances that promote mid-ranged planetary formation may be the reason only a handful of M-dwarf Neptunes have been confirmed.

The Neptune Desert and a Need for Caution Concerning M-dwarfs

The “Neptune Desert” was first proposed in Mazeh et al. (2016) in their paper Dearth of short-period Neptunian exoplanets: A desert in period-mass and period-radius planes . In this publication they highlight an extreme lack of confirmed exoplanets that exhibit mid-ranged radii (4-7 Earth radii) as well as short orbital periods (< 10 days). This promotes several questions regarding the formation and evolution of these short-period Neptune planets and why this dearth is observed. 

Figure 4: Left: The Neptune Desert reported by Mazeh et al. (2016) in orbital period – planet radius space. The dashed line denotes the defined desert. Red points are planets orbiting M-dwarfs with TOI-3785 labeled. Right: We reorient the Neptune Desert bounds to insolation (light received by the planet) – planet radius space. The dark dashed lines are the redefined Neptune Desert. Since TOI-3785 is a much cooler star, TOI-3785 doesn’t receive as much light even though it has a similar orbital period to many planets orbiting hotter stars. By redefining the Neptune Desert, we show that TOI-3785 b is not in the Neptune Desert. The only M-dwarf planet in the Desert is TOI-532 b.

TOI-3785 b falls within this radius-period space (Figure 4). With a radius of 5.14 Earth radii and a period of 4.67 days, TOI-3785 b is found to exist in the middle of this desert. However, before we celebrate an addition to the Neptune Desert, TOI-3785 b must be viewed not only in the context of its planetary characteristics but its host star’s as well. As mentioned previously, the star TOI-3785 is an M-dwarf, meaning that it possesses a lower mass and cooler temperature compared to other stellar types. The lower mass and luminosity of TOI-3785 presents a problem when placing planetary targets within the Neptune Desert. The lower temperature of TOI-3785 will cause planets of Neptune size to receive less stellar radiation, allowing them to retain their atmospheres and radii at shorter orbital periods. This places short period targets within the Neptune Desert not due to their unique formation pathways, but due to the reduced starlight they receive as a characteristic of their host star’s temperature. 

In our publication we highlight the necessity to consider a planet in context of its host star when placing targets inside the Neptune Desert. To quantify this, we reorient the Neptune Desert to Radius-Insolation space. Insolation (the amount of stellar radiation a planet receives) is chosen as it encapsulates both the radiative temperature of a host star as well as a planet’s proximity from its host. In this reoriented desert, we find that TOI-3785 b as well as nearly all other M-dwarf hosted exoplanets are shifted outside this new desert. Moving forward, we encourage caution when placing targets inside the Neptune Desert, as planetary correlations depend upon several crucial variables.    

Future Work

Now that the initial work for this planet is complete, we highlight the possibilities of what further investigations could achieve. For example, the most intriguing aspect of this planet is its atmosphere. With a high Transmission Spectroscopy Metric (TSM), we identify TOI-3785 b to be the most promising target for atmospheric follow-up of low temperature M-dwarf planets. Learning more about its atmospheric composition will aid in achieving a better understanding of the formation timeline of Neptune-like exoplanets. With the James Webb Space Telescope (JWST) recently coming online, this presents an ideal opportunity to dive into the atmosphere of a rare planet such as this.

You can read more about TOI-3785 b in our recent publication, led by HPF team member Luke Powers.

 

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New Science, and a History Lesson: a Giant Planet Orbiting Gliese 463

Posted on June 15, 2023 by Paul M Robertson

Introduction: the HET/HRS M dwarf Survey

HPF is the latest and greatest in Doppler searches for exoplanets orbiting the Galaxy’s smallest stars, but it is certainly not the first, even at Texas’ McDonald Observatory.  Before HPF came on sky, astronomers at McDonald Observatory–led by HPF Team members Michael Endl, Bill Cochran, Phillip MacQueen, and Paul Robertson–carried out an M dwarf exoplanet survey using the High Resolution Spectrometer, or HRS.  Like HPF, HRS was installed on the 10-meter Hobby-Eberly Telescope.  HRS was not a purpose-built planet hunting machine, and thus lacked a lot of HPF’s features like near-infrared wavelength coverage, extreme environmental stabilization, or a laser frequency comb calibrator.  Still, HRS could make sensitive measurements of stellar motions, and its survey of 100 M dwarfs placed important constraints on the types and numbers of exoplanets that form around these small stars.

Today, we will look at the last exoplanet discovered by the HRS M dwarf exoplanet survey.  Even though the last observations from that survey took place in 2013, the planet was not confirmed and announced for nearly a decade due to its long orbital period.  The planet is a gas giant orbiting an M star at a large distance from its host star.  It is a fascinating precursor to the ongoing HPF survey, and adds to the insight HPF is providing into the planets orbiting these tiny stars.  If you’re interested in all the technical details, check out the recent publication led by HPF Science Team member Michael Endl.

Gliese 463: The Ultimate Cliffhanger

Gliese 463 (abbreviated GJ 463) is an M star that lies about 60 light years from Earth, and is half as massive as the Sun.  It is a little bigger, hotter, and bluer than the stars we target with HPF, which made it perfect for the HRS spectrometer’s optical wavelength coverage.

We watched GJ 463 with HRS for a little over 5 years.  Over that time, it was clear that something was orbiting the star, but we could never pin down its orbit.  The reason is that the star’s velocity never “turned around,” indicating we had observed a full orbit.  Without a complete orbit, it was impossible to say whether the companion was a giant planet, or a more massive object (like a star) in a more distant orbit.

Possible orbits of GJ 463b

Radial velocity measurements of GJ 463 from HET/HRS (blue) and Keck/HIRES (red) at the time of the HET shutdown. Possible orbit models to these data are shown in green and purple. These candidate orbits imply radically different objects orbiting the star, so we could not make any conclusions based on these data.

Unfortunately, we were never able to finish the orbit with HRS.  Before we could, the Hobby-Eberly Telescope was shut down in 2013 for two years in order to upgrade it for the HETDEX survey to better understand the mysterious dark energy that drives the accelerating expansion of the Universe.  At the time of the shutdown, our data looked tantalizingly close to turning around, but we couldn’t be sure.  Hence, our tentative discovery of a giant exoplanet orbiting GJ 463 was left hanging in limbo.

An Assist from Keck

Luckily, the Hobby-Eberly Telescope was not the only telescope we used to observe Gliese 463.  Around the same time, we were using the HIRES spectrometer on the 10-meter Keck I Telescope on Maunakea in Hawaii to measure masses of transiting exoplanets discovered by the CoRoT satellite.  When our CoRoT targets had set below the horizon, we used HIRES to observe some of our HET/HRS survey targets, including GJ 463.

Our CoRoT observing program ended around the same time as the HET shutdown, but we were able to grab the occasional glance at GJ 463 thanks to a favor from our collaborators in the California Planet Search program.  Still, the orbit never definitively turned around.  Clearly, we were looking at a very long-period orbit.

It was only in 2019 that we finally broke through.  Members of our team were using Keck/HIRES as part of our HPF project studying very rapidly rotating M dwarfs.  Again, our primary targets had set for the night, so we turned Keck back to GJ 463.  This time, we found that the star had finally turned around.  Our models showed that the orbiting companion is a giant planet like Jupiter, on an orbit that takes nearly 10 years to complete.  This is one of the most massive planets–on one of the longest orbital periods–ever discovered for an M dwarf star!

GJ 463 orbit

Full HET/HRS and Keck/HIRES radial velocities for Gliese 463. Our best model to the planet orbit is shown as a dashed black line. The light gray lines show alternate possible orbits allowed by our data.

Confirmation from Space

The newly-discovered planet, dubbed GJ 463b, is a large planet on a distant orbit from its host star.  This makes it an ideal candidate for detection using astrometry.  Astrometry as an exoplanet detection method relies on the same gravitational physics as the radial velocity technique used by HPF.  As a planet orbits its star, it tugs on it gravitationally and causes it to wobble.  With the radial velocity method, we measure the radial motion of the star: that is, the star’s movement towards and away from us.  Astrometry measures the other component of this motion: the wobble of the star in the plane of the sky, also known as the transverse velocity.  In astronomy, we frequently refer to the transverse movement of a star as its proper motion.

Astrometry explainer

When a star wobbles under the gravitational influence of its planets, its motion can be measured as radial velocity and transverse velocity. HPF and other exoplanet spectrometers measure radial velocity, while the astrometry technique measures the transverse velocity.  Original: Brews ohare Vectorisation: CheChe, CC BY-SA 3.0, via Wikimedia Commons

We are entering a new golden age of exoplanet discovery using astrometry thanks to ESA’s Gaia spacecraft.  Gaia is measuring the positions and proper motions of 100 billion stars in the Milky Way at an unprecedented level of precision.  Data from Gaia will have the sensitivity to detect exoplanets, particularly large planets on wide orbits like GJ 463b.

When we first announced the discovery of GJ 463b, we realized that it would be a good target for Gaia.  We even noticed some motion in early-release Gaia data of the star that we pointed out was most likely caused by the planet.  Recently, though, another study combined our data with more observations from Gaia and confirmed the astrometric detection of GJ 463b.  Detecting the planet with both radial velocity and astrometry is extremely valuable, as it allows us to determine its true mass, rather than just an upper limit.  From this result, we find that GJ 463b is 3.6 times the mass of Jupiter–much bigger than any planet in our Solar system, but still small enough to definitively be a planet.  Knowing the GJ 463b’s true mass makes it even more valuable for future efforts to obtain direct images of the planet.  Furthermore, GJ 463 is the first M dwarf system to have a planet’s mass measured using radial velocities and Gaia astrometry!  This represents a fun historical distinction.

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The Unusual Transit of TOI-3884b: A Pole-Spot Crossing Super-Neptune

Posted on May 16, 2023 by Jessica Libby-Roberts

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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