HPF Confirms the First Exoplanet Discoveries from Gaia Astrometry

Posted on February 4, 2025 by Paul M Robertson

Gaia-4b is a planet orbiting the star called Gaia-4, around 244 light-years away. Gaia-4b is about twelve times more massive than Jupiter. With an orbital period of 570 days, it is a relatively cold gas giant planet. This artist impression visualises a portion of the orbital motion as determined by Gaia’s astrometric data. The star and planet are not to scale. Image credit: ESA/Gaia/DPAC/M. Marcussen

Introduction: Discovering Exoplanets with Gaia

Gaia is a spacecraft dedicated to making ultra-precise measurements of the positions and on-sky motions of billions of stars.  The technical name for this measurement is astrometry, and the astrometric data collected by Gaia is invaluable for stellar science, orbital dynamics of the Milky Way Galaxy, and studies of exoplanets.  While it just completed its full data collection mission, Gaia is just getting started in finding planets, with its first astrometric exoplanet candidates being uncovered recently.

As we discussed in a recent post, astrometry can be used to discover exoplanets through the subtle wobbles of a star under the gravitational influence of one or more orbiting planets.  While the measurement is somewhat different, the physics behind this technique are identical to those used for the radial velocity technique employed by HPF.  While astronomers have attempted to discover exoplanets using astrometry for decades, only Gaia currently has the measurement precision and sample size to do so in great numbers.

However, the astrometric exoplanet detections from Gaia remain candidates until they have been confirmed with other techniques such as with the radial velocity method.  Today, we announced the discovery of Gaia-4b and Gaia-5b, which were initially detected by Gaia and confirmed with a collection of radial velocity spectrometers, including HPF.  These planets represent the first confirmed exoplanets discovered by Gaia astrometry!

The Astrometric Technique

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 around the common center of mass of the star-planet system.  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.

Illustration of astrometry and radial velocity

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

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 hundreds to thousands of exoplanets, particularly large planets on wide orbits.

However, as mentioned above, candidate exoplanets uncovered by the astrometric technique are vulnerable to false positives, or signals that look like exoplanets, but are not.  In particular, if a pair of binary stars with nearly equal masses have just the right orbit, it can create an astrometric signal that is difficult to distinguish from that of a single star orbited by an exoplanet.

The best way to separate true exoplanets from false positives is to confirm with Doppler spectroscopy.  If the object is in fact a binary star, we will see spectra from both stars in our data.  Otherwise, our radial velocity measurements will confirm and refine the measurement of the orbit predicted by astrometry.

Discovering Gaia 4b and 5b

Members of our team have been observing a collection of Gaia exoplanet candidates using HPF and its sister spectrometer NEID.  Most of them–upwards of 75 percent–are indeed false positives created by binary stars.  However, a few have turned out to be genuine exoplanets, and we have continued to observe them to refine our estimates of their orbital properties.

Orbit of Gaia-4b

Left: radial velocity measurements of Gaia-4 from the HPF, NEID, and FIES spectrometers, confirming the orbit of the eccentric giant exoplanet Gaia-4b. Right: Model of the orbit of Gaia-4b as seen in the plane of the sky.

In today’s announcement, we have revealed the first two substellar objects confirmed by our survey: Gaia-4b and Gaia-5b.  Both objects are more massive than our own Jupiter, and follow eccentric orbits around low-mass stars.  In fact, Gaia-5b, at a mass of 21 Jupiter masses, falls into a class of objects known as brown dwarfs: bodies too massive to be considered planets, and not massive enough to trigger hydrogen fusion and become a star.  Both giant planets and brown dwarfs are exceedingly rare in low-mass star systems.

Several previously-known exoplanets have been confirmed with astrometry–including with Gaia.  However, Gaia-4b is now the first confirmed exoplanet discovered by Gaia.  It will certainly not be the last; Gaia data are still being released in stages, and it is expected that we are only seeing the tip of the iceberg in terms of the planets it will discover.  As those planets emerge, spectrometers like HPF will continue to play a critical role in confirming their discoveries.

Find Out More

The technical details of today’s announcement are described in an article in The Astronomical Journal, led by HPF team member Gudmundur Stefansson.  We encourage you to check it out!

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HPF Discovers a Giant Exoplanet in a Highly Disturbed Orbit

Posted on July 17, 2024 by Paul M Robertson

A video showing the extreme orbit of TIC 241249530b.  As the planet makes its closest approach to the star, it becomes brighter due to the increase in incident stellar radiation.  Video credit: Abigail Minnich, Penn State University

Today, the HPF Science Team, alongside the Science Team for HPF’s sister spectrograph NEID, announced the discovery of the exoplanet TIC 241249530b, a giant exoplanet with some truly remarkable properties. The planet follows an oblong, highly eccentric orbit around its star; if we compare it to objects in the Solar System, its orbit more closely resembles that of a comet than those of our planets. Furthermore, its orbit is almost exactly backwards from what we would expect based on theories of planet formation. Beyond being an interesting oddity in itself, TIC 241249530b offers a unique glimpse into the process of planetary orbital evolution, helping to explain the existence of “hot Jupiter” exoplanets.

An in-depth discussion of this discovery can be found on the NEID Science Blog, and we encourage you to check it out there!  For all of the technical details, see the research manuscript published today in Nature, led by HPF Science Team member Arvind Gupta.

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

Posted on November 30, 2023 by Paul M Robertson


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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