Image of the Week


Astrometric orbit of the exoplanet-host star HD81040


Figure 1: Astrometric orbit of HD81040 (left panel) as determined by Gaia. North is up and East is left. The sky-projected orbit model about the system barycentre marked with an "x" is shown in grey and astrometric normal-points after subtraction of parallax and proper motion are shown in black. Only one-dimensional ("along-scan") astrometry was used, therefore the shown offsets are projected along Gaia's instantaneous scan angle, whose orientation is also indicated by the error-bars. The star's modelled parallax and proper motion is shown in the top-right panel by the solid curve, where open circles indicate the times when the star crossed the Gaia field-of-view. The arrow indicates the direction of motion. The bottom right panel shows the post-fit residual normal-points, whose small root-mean-square disperson of 65 micro-arcsecond indicates that the chosen astrometric model reproduces the data well. Normal-points are computed at every field-of-view transit of the star from the ~9 individual CCD transits and are only used for visualisation, whereas the data processing uses individual CCD-transit data. The solution parameters and their uncertainties are shown by insets on the very left and in the top right panel. Credit: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO.


Many exoplanets are known to orbit stars in the solar neighbourhood but for only a few it was so far possible to measure the tiny corresponding motion of the host star in the plane of the sky. And when it has been achieved, e.g. with the Hubble Space Telescope (McArthur et al. 2010), the Very Large Telescope (Sahlmann et al. 2016), or Very Long Baseline Interferometry (Curiel et al. 2020), it required significant investments in terms of observing resources and the researchers' ingenuity and perseverance.

In contrast, Gaia automatically delivers measurements of non-single stars because those are being observed just as any other source in the sky, and Gaia's astrometric precision is often sufficient to detect the signatures of orbital motion caused by giant exoplanets. When the orbital period is comparable to the time span of Gaia's observations the planet's tug on the star can become apparent as an excess displacement on top of the stellar parallax and proper motion.

The robust detection of astrometric orbits requires a sufficiently large timespan and dedicated processing steps, which is why non-single star solutions have so far been absent from Gaia data releases. This will change dramatically with the upcoming third data release which will contain hundreds of thousands of non-single star solutions, from astrometry, photometry, and spectroscopy.

Figure 1 shows the orbital solution, to be released in Gaia DR3, for the bright (G ​​​​7.6) star HD81040 located at a distance of about 35 pc from the Sun. The ~400 individual astrometric measurements of this source collected over ~900 days were processed with a pipeline designed to identify small-amplitude Keplerian motion, which resulted in this high-significance detection.

In 2006, radial-velocity variations of HD81040 revealed the presence of a super-Jupiter planet orbiting the star every 1000 days (Sozzetti et al. 2006). Comparison of the orbital parameters in common between the radial-velocity and astrometry solutions shows that Gaia has independently detected the star's reflex motion caused by the same orbiting planet. Since Gaia is observing in a visible bandpass and this system is not young, we can safely assume that the light contribution by the planet is negligible, hence the orbits of the star and the system's photocentre coincide. As expected, that orbital motion is tiny and has a semi-amplitude of ~0.4 mas, i.e. about 3 orders of magnitude smaller that the star's annual proper motion. To give a sense of absolute scale, an astronaut's footprint on the moon as seen from Earth corresponds to about half of the angular orbit size.

The astrometric period is about 150 days (~1.3σ) too short compared to the radial-velocity period, which is much more precise. This difference is marginally significant and can probably be attributed to the Gaia measurement timespan of 900 days that does not fully cover one orbital revolution of the planet. In Figure 2 we show that when using the radial-velocity period and assuming a primary mass of 0.96 ± 0.04 MSun the prediction based on the Gaia solution and the measured radial-velocities agree well.


Figure 2: The radial-velocity curve of HD81040 predicted by the Gaia astrometry solution (black curve, offset by the systemic velocity) and the measured radial velocities from Sozzetti et al. 2006 (black symbols). Because the radial-velocity period is much more precise and the radial-velocity data were taken >10 years before the Gaia data, the radial-velocity period was used for the purpose of this figure. The grey region shows the 1-σ-equivalent confidence interval of the Gaia solution. Credit: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO.


Since the Gaia astrometry determined the orbital inclination, we can resolve the sin i ambiguity of the radial-velocity solution and determine the actual planet mass of 8.04-0.54+0.66 MJupiter, which is roughly one Jupiter-mass higher than the lower limit from radial-velocities. The given uncertainties were computed using Monte-Carlo resampling and take into account all solution parameter uncertainties and covariances.

Recently, the astrometric orbit of HD81040 was also constrained by combining the available radial velocities with the change in absolute proper motions between Hipparcos and Gaia EDR3 (Li et al. 2021). Their solution parameters are generally in good agreement with the Gaia orbit from epoch astrometry, except for the ascending node, which is not very well constrained by their solution. The inclination bimodality in the Li et al. solution is resolved by the Gaia orbit, which unambiguously establishes the retrograde configuration (i > 90 deg) as being the correct one.

This orbital solution is part of the first long-awaited results on exoplanets from Gaia. It is a small appetiser for a larger sample of astrometric orbits that will be released in Gaia DR3. We stress that no epoch astrometry will be released in Gaia DR3, i.e. the data points shown in Figure 1 will generally not become publicly available before Gaia DR4.

Whereas Gaia's data release 3 was generated from the first 34 months of data, Gaia is now in the eighth year of science operations and its astrometry will eventually lead to the discovery and characterisation of thousands of giant exoplanets and substellar companions orbiting nearby stars (e.g. Perryman et al. 2014, Sozzetti et al. 2014, Holl et al. 2021). This will make it possible to investigate the masses and orbital architectures of exoplanetary systems on a large scale and explore the dynamics of multi-planet systems and circumbinary planets.

Gaia's data release is expected in the second quarter of 2022. More information on Gaia's future releases can be found from the Gaia release scenario page.


Credits: ESA/Gaia/DPAC/CU4-CU3, Johannes Sahlmann (RHEA Group for ESA, ESAC); members of Development Unit 437 (Extrasolar planets): Berry Holl, Damien Ségransan, Domenico Barbato, Jean-Baptiste Deslisle, Nicolas Unger (University of Geneva), Alessandro Sozzetti, Paolo Giacobbe, Mario G. Lattanzi (INAF - Osservatorio Astrofisico di Torino); the CU4/NSS team; the CU3 teams.


Published: 31/01/2022


Image of the Week Archive


13/06 Gaia Data Release 3

10/06: MK classification of stars from BP/RP spectrophotometry across the Hertzsprung-Russell diagram

09/06: BP/RP low-resolution spectroscopy across the Hertzsprung-Russell diagram

27/05: Cepheids and their radial velocity curves

23/05: The Galaxy in your preferred colours

19/05: GaiaXPy 1.0.0 released, a tool for Gaia's BP/RP spectra users

11/05: Systemic proper motions of 73 galaxies in the Local group

28/03: Gaia query statistics

16/03: Gaia's first photo shooting of the James Webb Space Telescope

08/03: Gaia's women in science - coordination unit 8

25/02: Not only distances: what Gaia DR3 RR Lyrae stars will tell us about our Galaxy and beyond

11/02: Gaia's women in science

31/01: Astrometric orbit of the exoplanet-host star HD81040

12/01: The Local Bubble - source of our nearby stars

05/01: A Milky-Way relic of the formation of the Universe


23/12: Signal-to-Noise ratio for Gaia DR3 BP/RP mean spectra

22/12: The 7 October 2021 stellar occultation by the Neptunian system

01/12: Observation of a long-predicted new type of binary star

24/09: Astrometric microlensing effect in the Gaia16aye event

22/09: the power of the third dimension - the discovery of a gigantic cavity in space

16/09: An alternative Gaia sky chart

25/08: Gaia Photometric Science Alerts and Gravitational Wave Triggers

09/07: How Gaia unveils what stars are made of

23/06: Interviews with CU3

27/04: HIP 70674 Orbital solution resulting from Gaia DR3 processing

30/03: First transiting exoplanet by Gaia

26/03: Apophis' Yarkovsky acceleration improved through stellar occultation

26/02: Matching observations to sources for Gaia DR4


22/12: QSO emission lines in low-resolution BP/RP spectra

03/12: Gaia Early Data Release 3

29/10: Gaia EDR3 passbands

15/10: Star clusters are only the tip of the iceberg

04/09: Discovery of a year long superoutburst in a white dwarf binary

12/08: First calibrated XP spectra

22/07: Gaia and the size of the Solar System

16/07: Testing CDM and geometry-driven Milky Way rotation Curve Models

30/06: Gaia's impact on Solar system science

14/05: Machine-learning techniques reveal hundreds of open clusters in Gaia data

20/03: The chemical trace of Galactic stellar populations as seen by Gaia

09/01: Discovery of a new star cluster: Price-Whelan1

08/01: Largest ever seen gaseous structure in our Galaxy

20/12: The lost stars of the Hyades
06/12: Do we see a dark-matter like effect in globular clusters?
12/11: Hypervelocity star ejected from a supermassive black hole
17/09: Instrument Development Award
08/08: 30th anniversary of Hipparcos
17/07: Whitehead Eclipse Avoidance Manoeuvre
28/06: Following up on Gaia Solar System Objects
19/06: News from the Gaia Archive
29/05: Spectroscopic variability of emission lines stars with Gaia
24/05: Evidence of new magnetic transitions in late-type stars
03/05: Atmospheric dynamics of AGB stars revealed by Gaia
25/04: Geographic contributions to DPAC
22/04: omega Centauri's lost stars
18/04: 53rd ESLAB symposium "the Gaia universe"
18/02: A river of stars
21/12: Sonification of Gaia data
18/12: Gaia captures a rare FU Ori outburst
12/12: Changes in the DPAC Executive
26/11:New Very Low Mass dwarfs in Gaia data
19/11: Hypervelocity White Dwarfs in Gaia data
15/11: Hunting evolved carbon stars with Gaia RP spectra
13/11: Gaia catches the movement of the tiny galaxies surrounding the Milky Way
06/11: Secrets of the "wild duck" cluster revealed
12/10: 25 years since the initial GAIA proposal
09/10: 3rd Gaia DPAC Consortium Meeting
30/09: A new panoramic sky map of the Milky Way's Stellar Streams
25/09: Plausible home stars for interstellar object 'Oumuamua
11/09: Impressions from the IAU General Assembly
30/06: Asteroids in Gaia Data
14/06: Mapping and visualising Gaia DR2

25/04: In-depth stories on Gaia DR2

14/04: Gaia tops one trillion observations
16/03: Gaia DR2 Passbands
27/02: Triton observation campaign
11/02: Gaia Women In Science
29/01: Following-up on Gaia
19/12: 4th launch anniversary
24/11: Gaia-GOSA service
27/10: German Gaia stamp in the making
19/10: Hertzsprung-russell diagram using Gaia DR1
05/10: Updated prediction to the Triton occultation campaign
04/10: 1:1 Gaia model arrives at ESAC
31/08: Close stellar encounters from the first Gaia data release
16/08: Preliminary view of the Gaia sky in colour
07/07: Chariklo stellar occultation follow-up
24/04: Gaia reveals the composition of asteroids
20/04: Extra-galactic observations with Gaia
10/04: How faint are the faintest Gaia stars?
24/03: Pulsating stars to study Galactic structures
09/02: Known exoplanetary transits in Gaia data
31/01: Successful second DPAC Consortium Meeting
23/12: Interactive and statistical visualisation of Gaia DR1 with vaex
16/12: Standard uncertainties for the photometric data (in GDR1)
25/11: Signature of the rotation of the galactic bar uncovered
15/11: Successful first DR1 Workshop
27/10: Microlensing Follow-Up
21/10: Asteroid Occultation
16/09: First DR1 results
14/09: Pluto Stellar Occultation
15/06: Happy Birthday, DPAC!
10/06: 1000th run of the Initial Data Treatment system
04/05: Complementing Gaia observations of the densest sky regions
22/04: A window to Gaia - the focal plane
05/04: Hipparcos interactive data access tool
24/03: Gaia spots a sunspot
29/02: Gaia sees exploding stars next door
11/02: A new heart for the Gaia Object Generator
04/02: Searching for solar siblings with Gaia
28/01: Globular cluster colour-magnitude diagrams
21/01: Gaia resolving power estimated with Pluto and Charon
12/01: 100th First-Look Weekly Report
06/01: Gaia intersects a Perseid meteoroid
18/12: Tales of two clusters retold by Gaia
11/11: Lunar transit temperature plots
06/11: Gaia's sensors scan a lunar transit
03/11: Celebrity comet spotted among Gaia's stars
09/10: The SB2 stars as seen by Gaia's RVS
02/10: The colour of Gaia's eyes
24/09: Estimating distances from parallaxes
18/09: Gaia orbit reconstruction
31/07: Asteroids all around
17/07: Gaia satellite and amateur astronomers spot one in a billion star
03/07: Counting stars with Gaia
01/07: Avionics Model test bench arrives at ESOC
28/05: Short period/faint magnitude Cepheids in the Large Magellanic Cloud
19/05: Visualising Gaia Photometric Science Alerts
09/04: Gaia honours Einstein by observing his cross
02/04: 1 April - First Look Scientists play practical joke
05/03: RR Lyrae stars in the Large Magellanic Cloud as seen by Gaia
26/02: First Gaia BP/RP deblended spectra
19/02: 13 months of GBOT Gaia observations
12/02: Added Value Interface Portal for Gaia
04/02: Gaia's potential for the discovery of circumbinary planets
26/01: DIBs in three hot stars as seen by Gaia's RVS
15/01: The Tycho-Gaia Astrometric Solution
06/01: Close encounters of the stellar kind
12/12: Gaia detects microlensing event
05/12: Cat's Eye Nebula as seen by Gaia
01/12: BFOSC observation of Gaia at L2
24/11: Gaia spectra of six stars
13/11: Omega Centauri as seen by Gaia
02/10: RVS Data Processing
12/09: Gaia discovers first supernova
04/08: Gaia flag arrives at ESAC
29/07: Gaia handover
15/07: Eclipsing binaries
03/07: Asteroids at the "photo finish"
19/06: Calibration image III - Messier 51
05/06: First Gaia BP/RP and RVS spectra
02/06: Sky coverage of Gaia during commissioning
03/04: Gaia source detection
21/02: Sky-background false detections in the sky mapper
14/02: Gaia calibration images II
06/02: Gaia calibration image I
28/01: Gaia telescope light path
17/01: First star shines for Gaia
14/01: Radiation Campaign #4
06/01: Asteroid detection by Gaia
17/12: Gaia in the gantry
12/12: The sky in G magnitude
05/12: Pre-launch release of spectrophotometric standard stars
28/11: From one to one billion pixels
21/11: The Hipparcos all-sky map
15/10: Gaia Sunshield Deployment Test
08/10: Initial Gaia Source List
17/09: CU1 Operations Workshop
11/09: Apsis
26/08: Gaia arrival in French Guiana
20/08: Gaia cartoons
11/07: Model Soyuz Fregat video
01/07: Acoustic Testing
21/06: SOVT
03/06: CU4 meeting #15
04/04: DPCC (CNES) 
26/03: Gaia artist impression 
11/02: Gaia payload testing  
04/01: Space flyby with Gaia-like data
10/12: DPAC OR#2. Testing with Planck
05/11: Galaxy detection with Gaia
09/10: Plot of part of the GUMS-10 catalogue
23/07: "Gaia" meets at Gaia
29/06: The Sky as seen by Gaia
31/05: Panorama of BAM clean room
29/03: GREAT school results
12/03: Scanning-law movie
21/02: Astrometric microlensing and Gaia
03/02: BAM with PMTS
12/01: FPA with all the CCDs and WFSs
14/12: Deployable sunshield
10/11: Earth Trojan search
21/10: First Soyuz liftoff from the French Guiana
20/09: Fast 2D image reconstruction algorithm
05/09: RVS OMA
10/08: 3D distribution of the Gaia catalogue
13/07: Dynamical Attitude Model
22/06: Gaia's view of open clusters
27/05: Accuracy of the stellar transverse velocity
13/05: Vibration test of BAM mirrors
18/04: L. Lindegren, Dr. Honoris Causa of the Observatory of Paris
19/01: Detectability of stars close to Jupiter
05/01: Delivery of the WFS flight models
21/12: The 100th member of CU3
17/11: Nano-JASMINE and AGIS
27/10: Eclipsing binary light curves fitted with DPAC code
13/10: Gaia broad band photometry
28/09: Measuring stellar parameters and interstellar extinction
14/09: M1 mirror
27/08: Quest for the Sun's siblings
Please note: Entries from the period 2003-2010 are available in this PDF document.