HPF Discovers a Close-In Neptune Orbiting a Very Low-Mass Star


Artist’s rendering of the Neptune-mass exoplanet orbiting the very low-mass star LHS 3154.  Video Credit: Abigail Minnich

Introduction

As the name implies, the Habitable-zone Planet Finder was designed primarily to search for Earthlike planets orbiting nearby stars.  Its sensitivity to near-infrared light allows us to search for planets around the smallest, coolest stars in the Milky Way Galaxy.  There is still a lot we don’t know about these tiny stars, so as we search for Earthlike planets we also want to get a better census of their overall planet population.  That way, we can better understand how those planets originally formed.

HL Tau disk

A protoplanetary disk orbiting the young star HL Tau. The dark gaps in the disk may be caused by newly-formed planets collecting material. Image credit: Ralph Bennett – ALMA (ESO/NAOJ/NRAO)

Stars originally form from large clouds of gas and dust in interstellar space.  The cloud collapses to form the star, and a small fraction of the original cloud remains as a disk of material orbiting the newborn star.  Over the first few million years of the system’s life, the gas and dust coalesces into planets.  Smaller stars tend to have smaller disks, so our current theories of planet formation predict that tiny M dwarf stars should form fewer planets, and in particular should form fewer large, gaseous planets like those found in our outer Solar system.  Earlier surveys for planets orbiting M stars indeed found fewer giant planets.  But to ensure we have a firm grasp on the physics of planet formation around all stars, we need to study more targets, and for longer periods of time.

In our HPF survey for M dwarf exoplanets, we have discovered a Neptune-sized exoplanet orbiting very close to an extremely low-mass star, LHS 3154.  This discovery comes as a surprise in light of the aforementioned planet formation theories, and forces us to consider modifications to the theory at the low-mass end of the stellar sequence.

LHS 3154

LHS3154 RVs

HPF radial velocity measurements of LHS 3154, mapped to the orbital phase of its Neptune-sized planet.  The red curve shows the best fit model, and the grey curves show 1- and 3-sigma confidence intervals. ‘P’ shows the best fit orbital period in days and its uncertainties, and K shows the semi-amplitude in meters per second.

The star LHS 3154 is a target of the HPF survey for exoplanets orbiting M dwarf stars near the Solar system.  It has a mass just 11 percent that of the Sun, and puts out only 0.1% as much energy as the Sun.  A star must have at least 8 percent the mass of the Sun to sustain nuclear fusion in its core, so LHS 3154 is among the smallest stars we could expect to find.  At such a small size, we place LHS 3154 into a category of objects (somewhat unimaginatively) called “very low-mass stars,” or VLM stars.

VLM stars such as LHS 3154 have very little history in exoplanet surveys, as they are too faint and red for optical spectrometers.  Their energy spectrum peaks well into the near-infrared, making them ideal targets for HPF.  We started surveying the star in early 2020, and quickly noticed the star exhibiting a characteristic wobble like we would expect if it hosted one or more planets.  We watched it carefully, as VLM stars frequently exhibit signals from starspots in their atmospheres that can look a lot like planets.  But over time, it became clear that the signal remained constant, and was not connected to the star’s magnetic activity.  We had a real planet!

LHS 3154b View

Artistic rendering of the possible view from LHS 3154b towards its low-mass host star. Given its large mass, LHS 3154b likely has a Neptune-like composition. Image credit: Thomas Klimek, Penn State

A Planet Out of Place

M dwarf stars are known to host plenty of planets, but they tend to be on the smaller side.  Again, this is consistent with what we understand about star and planet formation; the protoplanetary disks we have measured around young M stars typically only have enough solid material to form a few rocky planets the size of Earth or so.  The planet we found orbiting LHS 3154–named LHS 3154b–does not meet this description: it has a mass similar to that of Neptune, more than 13 times the mass of Earth.  Furthermore, it follows a tight orbit around the star, at just 2 percent the separation between Earth and the Sun.

LHS 3154b comparison

Relative sizes of the star LHS 3154 and its planet, with the Sun/Earth system for comparison. The large size of the planet LHS 3154b relative to its host star is unusual among known exoplanets.  Image credit: Thomis Klimek, Penn State

None of our current theories of planet formation predict such a planet should be found around a VLM star!  We performed computer models of the core accretion process, where small bits of solid material in a protoplanetary disk collide and grow into planet-sized objects.  While LHS 3154 is too old to still have a protoplanetary disk, we can make some reasonable estimates of what properties it once had, and simulate the planets it would be likely to form.  Even when making optimistic assumptions about the geometry of the disk, and the amount of solid material it contained, our models rarely produced planets as massive as LHS 3154b on such close-in orbits.

There are a few possibilities for what’s going on here.  Either LHS 3154b is an extreme outlier in the Galactic planet population, or our models of planet formation are incomplete.  The only way to determine which is to keep searching for planets.  If we never find any more planets like LHS 3154b, then it may just be the rare extreme outcome of a well-understood process.  But if we find more like it, we’ll have to revise our models to account for their formation.

Interestingly, despite LHS 3154b’s extreme proximity to its host star, the star’s faintness means the planet actually orbits near the inner edge of the system’s liquid-water Habitable Zone (HZ).  We don’t expect that the planet is habitable for Earthlike life; it is massive enough that any lifeform would experience deadly atmospheric pressure long before it reached the planet’s surface.  But this discovery still holds important implications for habitable planets in the system.  Having a massive planet so close to the habitable zone potentially means any other planets in the HZ would be gravitationally destabilized by LHS 3154b.  So this might not be the best system to search for life!

Find Out More

Our study of LHS 3154 is described in a research article led by HPF Team member Gudmundur Stefansson, which was recently published in Science.  Check it out for all the details!

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

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

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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