Image of the Week


Pluto Stellar Occultation

  Download an animated .gif image here  

On 19 July 2016, Pluto passed in front of the faint star UCAC4 345-180315, offering a rare chance to study the atmosphere of the dwarf planet as the star gradually disappeared and then reappeared behind Pluto.

This stellar occultation was only visible from a narrow strip stretching across Europe, similar to the totality path that a solar eclipse lays down on our planet’s surface. Precise knowledge of the star's position was crucial to correctly compute where on Earth the event was visible, so the exceptional early release of the Gaia position for this star, which was ten times more precise than previously known, was instrumental to the successful monitoring of this rare event.

Astronomers observed the event using telescopes across Europe, the Middle East and northern Africa. It was one of the largest Pluto occultation campaigns ever organised in Europe, mobilising tens of observers, both amateurs and professionals. To further analyse the data, researchers were also aided by an improved determination of Pluto’s orbit after NASA's New Horizons mission had flown by the dwarf planet in 2015.

Prediction was the main bottleneck for a successful campaign. Pluto subtends a mere 100 milliarcsecs (mas) diameter on the sky, roughly equivalent to a one Euro coin observed at a distance of 100 kilometres.

Initial predictions - relying on ground-based measurements of the star - put the shadow centrality sweeping over mid-Europe (Fig. 1), but with uncertainties as large as 50 mas, equivalent to about 1,500 km when projected onto Earth's surface. This situation, which was common to many occultation events before the Gaia data release, makes the task of choosing the telescopes that will look at the event much more difficult, with the risk of missing some relevant observations.

The release of the star position by the Gaia project drastically improved that accuracy, down to a few mas level, and corresponding to less than 100 km on the ground. This release was combined with an improved Pluto orbital solution based on the radio tracking of the New Horizons spacecraft that also reached an accuracy of about 100 km. This placed the centrality of the event over the Middle East and northern Africa, and triggered alerts in Israel, Morocco and the Canary islands (Fig. 2).

From the many observations collected in Europe and from some of the sites aforementioned, it appears that the reconstructed geometry of the event is amazingly close – less than 100 km – to the latest prediction (Fig. 3).


Figure 4: images collected at the Observatory Valle d’Aosta, Saint Barthélémy showing Pluto's motion in one day.


Images collected with a professional 81-cm telescope at Valle d’Aosta, in the Italian Alps, illustrate Pluto's motion in the sky from night to night (Fig. 4), and show the event itself (Fig. 5).



Figure 5: the Pluto occultation on 19 July 2016

The gradual disappearances and reappearances of the star into Pluto's atmosphere, as observed from various sites (Fig. 6), combined with a theoretical model (Fig. 7) eventually provide Pluto's atmospheric pressure as of July 2016 (Fig. 8).


Figure 6: gradual disappearances and reappearances of the star into Pluto's atmosphere, as observed from various sites



Figure 7: theoretical model. In blue: simultaneous fit to the data using a Pluton atmospheric model

It confirms the monotonic pressure increase since 1988, a paradoxical result since Pluto has been continuously receding from the Sun since then, thus cooling down. This should cause a drastic drop of pressure as the gaseous nitrogen condenses on the icy surface of the planet. Clearly, more complex seasonal processes must be at play to explain such results. A hint for a "pause" of that increase in 2016 is in fact suggested by the July 19 event. If real, this pause could be the onset of Pluto's atmospheric collapse, where less solar flux available means the start of an atmospheric "freezing".


Figure 8: one full year after the NASA New Horizons flyby, Pluto's atmospheric pressure may have reached its maximum and started its decrease as it moves away from the Sun. However, analysis of more data from the July 19 event is required to reduce the 1-σ error bar (in blue), and confirm (or not) this trend.


However, the error bar displayed in blue in Fig. 8 being at 1-σ level, no firm conclusion can be drawn at this time. Surely, a more accurate measurement will be obtained from this observation as more data is analysed (and not only four light-curves, as in Fig. 7).

Note that the best observing sites for detecting a "central flash" were in Morocco. This flash is caused by the focusing of the stellar rays towards observers that are less than about 50 km away from the shadow centre. The flash, which lasts for a few seconds only, is a good proxy of Pluto’s global atmospheric shape, and then, its wind regimes. Moreover, the flash intensity is very sensitive to the possible presence of hazes lingering a few kilometres above Pluto's surface.

Although two stations were mobilised in Morocco, thin clouds in the terrestrial sky scattered the bright moonlight into the instruments, and prevented the sufficient signal-to-noise ratio necessary to detect the brief flash. However, the accuracy of the prediction is a good sign for future observations. Campaigns planned well in advance, possibly involving small transportable telescopes at the right spots, will allow researchers a detailed view of those - up to now - rare events. This will reveal more about Pluto's atmosphere, something that New Horizons cannot do anymore after its swift flyby of last year.

This is also a good omen for the tens of stellar occultations by dwarf planets and large TNO's that will be detected per year thanks to Gaia's unprecedented star catalogue, opening the way for discovering more objects with an atmosphere, ring systems, or other unexpected features.

Credits: ESA/Gaia/DPAC/B. Sicardy (LESIA, Observatoire de Paris), D. Berard (LESIA, Observatoire de Paris), E. Meza (LESIA, Observatoire de Paris), R. Leiva (LESIA, Observatoire de Paris), A. Carbognani (Osservatorio Astronomico Valle d'Aosta - OAVdA, Italy), P. Tanga (Observatoire de la Côte d'Azur, Nice)

[Published: 14/09/2016]


Image of the Week Archive

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
Please note: Entries from the period 2003-2010 are available in this PDF document.