HPF discovers a warm super Neptune – TOI-1728b

The Discovery

A component of HPF’s on-sky time is used to follow up on potential planets discovered by NASA’s Transiting Exoplanet Survey Satellite (TESS), launched in 2018. TESS is nominally a 2-year mission designed to conduct an all-sky survey to find transiting exoplanets. It spends one year in each celestial hemisphere, which is divided into 12-13 sectors that TESS monitors for 27 days each.  You can see more about TESS and its survey strategy in the video below.

One of the planet candidates discovered by TESS in its second year was TOI-1728b from Sector 20. Orbiting an “early M” star (it has a surface temperature of around 4,000 Kelvin; the Sun’s surface temperature is approximately 6,000 Kelvin), the depth of the transit represented a planet larger than Neptune, with a radius 5 times larger than Earth’s.  The planet completes an orbit around its star every 3.5 days.

As we have recently discussed on this blog, transforming a planet candidate into a full-fledged, well-characterized exoplanet takes a lot of follow-up observing from the ground.  Here, we’ll show you how we confirmed the discovery of TOI-1728b, and measured its mass with HPF.

Transit Follow up

We followed up this object with the 0.4 m PlaneWave telescope located on the roof of Davey Lab at University Park, Penn State in order to catch more transits of the planet.

CDK0.4m

PSU 0.4 m on Davey lab. Credit: Gudmundur Stefansson

This transit follow up helped us to confirm the TESS transit, and obtain an independent measurement unaffected by spurious background stars which can affect the TESS detection due to its larger pixel size. To insulate us from potential bad weather (as can often happen at State College), we also followed up on this target from the 17″ Perkin telescope at Hobart and William Smith Colleges in Geneva, NY, to obtain a similar result. Shown below is what the transit shape looks like from the three different instruments –

TOI-1728

Transit light curves obtained for TOI-1728 from TESS, Davey 0.4 m, and 17″ Perkin telescope.

HPF follow up

As we worked to catch more transits, we also started observing this target as part of our M dwarf TESS follow up program with HPF, to measure the planet’s mass via the star’s Doppler shifts.  For a transiting planet, it is especially useful to use Doppler measurements to estimate its mass, as that information can be combined with the radius (which we got from the transit) to estimate the planet’s density.  The density is the first hint at a planet’s bulk composition: that is, is it rocky like Earth, or dominated by a gaseous atmosphere?  As an example, consider two planets of the Solar system.  Earth, which is composed mostly of rock and metal, has a bulk density of about 5.5 grams per cubic centimeter.   The gas giant Jupiter, on the other hand, has a bulk density of just 1.3 grams per cubic centimeter!  To compare the densities of exoplanets, astronomers typically use the mass-radius diagram, which allows us to examine how planets’ compositions vary with size, temperature, and other physical properties.

TOI-1728b falls in a region of Mass-Radius (M-R) space for M dwarf planets that is occupied by two other Neptune-sized planets:  GJ 3470b and GJ 436b. Using our HPF Doppler measurements, we measure the mass of the planet to be around 26 Earth masses, thereby being more massive than Neptune, which is only 17 times as massive as Earth.

M-R

Mass-Radius plane for M dwarf planets showing TOI-1728b. TOI-1728b is both more massive and larger than the other two Neptunes orbiting M dwarfs (GJ 3470b and GJ 436b).

Searching for signs of atmospheric escape

GJ 3470b was studied by an HPF team led by Joe Ninan, where we detected helium escaping from the planet’s atmosphere. Given the similarities between these exo-Neptunes, we decided to search for signs of atmospheric escape in TOI-1728b using the same technique.

We did not find any evidence for helium escape in the planetary atmosphere, and placed an upper limit of 1.1% for helium escape measured in the near-infrared atomic absorption line. For comparison, our detection in GJ 3470b was at 1.5%.  The lack of detection is interesting for the following reasons: as can be seen in the comparison chart below, TOI-1728b receives slightly more energy (insolation) from its star than the other two comparable planets, and at the same time, has a lower  bulk density. Both of these factors favour atmospheric escape, potentially making the detection easier. This lack of detection could be due to differences in the composition of the planet, as well as a relative lack of high-energy radiation from the star compared to GJ 3470b.

Comparing the planetary radius and insolation for M dwarf planets. TOI-1728b orbits an earlier type (hotter) host star, and therefore receives more insolation than the other sub-Jovian (smaller than Jupiter) M dwarf planets.

Conversely, TOI-1728b could be similar to GJ 436b, where the CARMENES team also report no detection of helium atmospheric escape using similar methods. GJ 436b does have a detection of hydrogen escape as seen from an ultraviolet absorption feature with the Hubble Space Telescope (HST).  Therefore, TOI-1728b represents a good opportunity for further follow up observations with HST to potentially detect hydrogen atmospheric escape. If HST also shows no hydrogen escape, that information would be very informative for understanding how stars dislodge atoms from their planets’ atmospheres.

Potential for transmission spectroscopy

While our HPF observations immediately place constraints on any gas escaping from TOI-1728b, they also set the stage for future measurements of the planet’s real atmosphere with HST and the upcoming NASA flagship James Webb Space Telescope (JWST).  TOI-1728 is bright enough to facilitate detailed observations with space telescopes, and now our HPF mass measurement indicates the planet’s density is low enough that plenty of starlight should be filtering through its atmosphere to create a detectable signal.  Hopefully, we are just starting to learn what this exciting planet is made of!

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HPF Team Member Gudmundur Stefansson Runner-Up in IAU Dissertation Prize

We are thrilled to announce that HPF Team Member Dr. Guðmundur Stefánsson was recently recognized as one of two runners-up for the International Astronomical Union’s (IAU) Division B (Facilities, Technology, Data Science) PhD Prize to Recognize Excellence in Astrophysics.  This is an extremely competitive award, which is open to astronomers all over the world.

Stefansson Portrait

HPF Team Member Guðmundur Stefánsson

Dr. Stefánsson’s PhD dissertation represents a culmination of all the research he performed as a graduate student at Penn State, and includes his contributions to the HPF instrument’s development such as its vacuum chamber construction and outfitting, and its unprecedented environmental control.

Dr. Stefánsson graduated with a PhD in Astronomy & Astrophysics from Penn State in 2019 and is now a Henry Norris Russell postdoctoral fellow at Princeton University. The entire HPF team congratulates Dr. Stefánsson on this well-deserved recognition.

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Eternal spotshine of the spinning red suns

Introduction

Much of our time on sky with HPF is dedicated to discovering exoplanets, and measuring their detailed properties.  To do this, we concentrate our efforts on older, magnetically quiet stars to minimize noise and make the planets easier to detect.  But M dwarf stars are astrophysically interesting in their own right, and we have reserved a small amount of our observing time to studying younger, more active M stars.  In this post, we’ll look at a new result our team has just published, from an experiment in which we looked at some very active stars in an attempt to understand differences in Doppler signals originating from exoplanets and magnetic activity.

“An Easy Target”

Back in 2018, when we were just getting HPF on sky, a team of astronomers analyzing archival radial velocity data from the Keck/HIRES and HARPS spectrometers announced a fascinating discovery.  Examining years’ worth of Doppler measurements of AD Leonis–a very young and active M dwarf star–they found a periodic signal at around 2.2 days, which matches the star’s rotation period.  Ordinarily, one would dismiss a Doppler signal at the rotation period because it is almost certainly caused by magnetic starspots rotating across the stellar surface.  However, the HIRES and HARPS data appeared to show a signal that was consistent for years at a time, which is not expected of starspots.  Spots should disappear within a few rotations of the star, so the hypothesis presented for AD Leo was that this signal represented a giant planet whose orbit was coupled to the rotation of the star.  If true, this would be a rare and exciting discovery!  But the specter of starspots left this possibility stuck in “candidate planet” limbo.

This type of situation is exactly what HPF, with its near-infrared wavelength coverage, was built for.  We expect starspot signals to be chromatic: that is, the size of the signal will change depending on the wavelengths (or colors) you observe them with.  This is due to the fact that the color contrast between a cool starspot and the surrounding stellar atmosphere is greater at visible wavelengths than in the infrared.

SDO Solar colors

Multi-wavelength images of the Sun, as captured by NASA’s Solar Dynamics Observatory (SDO). Various features of the Sun’s atmosphere appear more prominently at certain wavelengths. Image Credit: NASA Goddard Space Flight Center

On the other hand, a genuine exoplanet should show no such wavelength dependence.  So if the signal for AD Leo is due to a real planet, it should look exactly the same for HPF as it did for the optical-wavelength HIRES and HARPS.  The nice thing about the AD Leo signal is that it is so massive as to be easily detected by HPF, even if it didn’t hit its full measurement precision goals straight out of the gate.  Given the timing of the announcement of the planet candidate, it seemed like an obvious choice for HPF’s first science project.  Once HPF was set up and running, we immediately started collecting observations of AD Leo.

Too much of a good thing

In 2018, we watched AD Leo until it set for the season, and were extremely encouraged.  Our data showed a signal that matched extremely well to the HIRES and HARPS data, as we would expect if the signal were caused by a planet.  We were confident enough in the status of the planet that we wrote a complete draft of a research journal paper titled “Near-infrared radial velocities confirm AD Leonis b,” and were on the verge of submitting it for publication.

But then we looked at our next target, and pulled the emergency brake.  Because we saw the same thing again.

G 227-22 is another young, rapidly-rotating M star.  The MEarth team, who are looking for planets that transit nearby M dwarfs, showed that it completes a rotation every 6 hours, which is remarkably fast.  It was another early science target for HPF, because the TESS satellite is collecting an enormous amount of data for it, which is especially valuable for studying stars and their planets.  What complicated our interpretation of AD Leo is that HPF radial velocities of G 227-22 also showed a signal at the stellar rotation period that did not change or disappear, even over many stellar rotations.

To see two such similar rotation-period signals among HPF’s early set of targets could not be coincidence.  We saw two possible interpretations for this result, neither of which seemed especially intuitive:

  1. Both AD Leo and G 227-22 have giant exoplanets in orbits coupled to those stars’ rotation.  But for us to find two such objects among our first two targets would suggest that nearly all young M dwarfs must have this type of planet!  This seemed inconsistent with previous studies of M stars.
  2. These signals are caused by magnetic starspots.  However, in order for them to produce the Doppler signals we saw, they would have to behave quite differently from what we see on the Sun.

It seemed impossible to convincingly distinguish between these possibilities without more data.  So our results for AD Leo were frustratingly put on hold.

A multi-color Doppler campaign

HET and Keck

The Hobby-Eberly (left) and Keck I and II (right) telescopes.

Either of the two possibilities above are quite interesting from an astrophysics perspective, so we were determined to settle the debate.  The approach we decided to take was to mount an intensive, multi-wavelength Doppler observing campaign for AD Leo, G 227-22, and two more similar stars (GJ 1245B and GJ 3959).  We set out to get both visible and near-infrared radial velocity measurements, which involved enlisting two 10-meter telescopes: in addition to HET, which houses HPF, we enlisted the HIRES spectrometer on the Keck I Telescope in Hawaii.  The idea was that if any (or all!) of these stars hosted spin-orbit coupled planets, their visible and infrared Doppler signals should look the same.  Our experiment benefited from TESS observing these stars at around the same time, so our targets received quite a bit of attention during the 2019 observing season!

Spots that won’t quit

Once we completed our observations, it became apparent that none of our targets seem likely to host a planet orbiting at the period of the stars’ rotation.  The signals appear to shift in phase, and typically have smaller amplitudes in the infrared wavelengths, as we would expect from starspots.  Even the AD Leo signal finally gave way in 2019–the infrared signal was much smaller than we saw in 2018, and another team has confirmed that it was also smaller at optical wavelengths.

AD Leo Seasons

HPF radial velocities of AD Leo from 2018 (left) and 2019 (right). The shrinking amplitude confirms the signal is caused by stellar magnetic activity, not a planet.

Still, it’s interesting to look in more detail at what caused us and other teams to suspect planets in the first place.  These spots do not act like what we observe on the Sun!  What we found is that the signals from these spots stay constant for 6 months to a year, typically.  But since these stars complete a rotation every few days or less, that means these signals persist for hundreds–or even thousands–of stellar rotations.  For comparison, Sunspots typically disappear within one or two rotations. Below, we show the brightness of G 227-22, as measured by TESS.  The consistency of the brightness variations suggests the spot patterns on the star stay fixed over time.

Phase curves for G 227-22

Brightness measurements of G 227-22, as measured by TESS. Moving from bottom to top, we see how the star dims as regions with more starspots rotate into view. From left to right, we see that the brightness distribution does not change over time, suggesting the spot pattern is fixed over many stellar rotations.

This adds to the challenge of finding exoplanets with the Doppler technique, because if we can’t rely on starspot signals to degrade over short periods of time, then we need more information to help separate real planets from false positives.  Once again, we learn that there’s no such thing as an “easy target,” especially when dealing with M dwarfs.

Really, no planets?

Probably not!  But we’ll leave you with a little cliffhanger, just for fun.  Remember in the previous section where we said that the observed starspot signals “typically” had smaller amplitudes in the infrared?  Well, there was one target–GJ 3959– for which the optical and infrared signals match up surprisingly well.  Take a look:

GJ 3959 RVs

Optical Keck/HIRES (blue) and near-infrared HPF (red) radial velocities of GJ 3959, shown at the phase of the star’s rotation. The optical and infrared signals match in phase and amplitude.

We were almost tempted to label this signal a planet candidate.  However, our colleagues on the CARMENES team have also looked at this star, and observed evidence of wavelength-dependent Doppler shifts.  Thus, we concluded that this signal is most likely caused by starspots, and the spots may have changed size between the times we observed with HIRES and HPF, leading to the signals coincidentally matching.  However, there’s one more chance to determine whether this might be the real deal: TESS is observing this star as we write, and the first data are scheduled to come down any day now.  If there is a planet here, it has a high probability of transiting, and thus being detected by TESS!  If that happens, you’ll see us frantically updating this post.

Where can I learn more?

A research article describing this experiment in full detail has been accepted for publication in the Astrophysical Journal.  Prior to publication, you can read the pre-print on arXiv.

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