Image of the Week

Known EXoplanetary transits In GAia Data


Figure 1: Gaia folded light-curve of WASP-19b in the bands RP, G and BP. The light-curves are shifted to zero transit phase, and folded by the known period. The vertical dashed lines mark the times of transit ingress and egress. (Image credit: ESA/Gaia/DPAC/CU7)


During its nominal five-year mission Gaia is expected to sample a billion Milky-Way stars, on average 70 times each. The Gaia scanning law was optimised for Gaia's main science goal that is precise astrometry and is therefore not optimal for detecting transiting planets. However a test carried out on some known exoplanets suggests that these type of events can be seen in the Gaia data as well, provided enough data is gathered in that area of the sky.

Here the results are shown for two examples of known transiting planets observed by Gaia during the mission's first year. Given that only one year of data was available for use in this test, these results provide an encouraging and tantalising hint of what will eventually be possible with the full set of data from the five-year nominal mission.

The first example is WASP-19b, which is a planet detected in 2009 by Hebb et al. (ApJ, 708, 224). Its orbital period  is 0.7888400+/-3 • 10-7 days, the transit duration is 0.066 days and the transit depth is 0.022 magnitude. During its first year, Gaia sampled WASP-19b only 24 times. Fortuitously, one of those samples occurred during a transit. In Figure 1 the Gaia light-curve can be seen in the G, BP and RP bands, phase-folded according to the known period (using the mid-transit time as the reference epoch). The vertical dashed lines represent the transit ingress and egress, the points in time where the transit starts and ends respectively. The one measurement during transit is indeed 0.02 magnitude fainter than the rest, as expected according to the known transit parameters.


Figure 2: Gaia folded light-curve of WASP-98b in the bands RP, G and BP. The light-curves are shifted to zero transit phase, and folded by the known period. The vertical dashed lines mark the times of transit ingress and egress. (Image credit: ESA/Gaia/DPAC/CU7)


The second example is WASP-98b, which was detected by Hellier et al. in 2013 (MNRAS, 440, 1982). It has a period of 2.962640+/-1.3 • 10-6 days, a transit duration of 0.079 days and a depth of 0.028 magnitude. Again, out of 20 samples, one sample occurred during transit and is apparently a little more than 0.02 magnitudes fainter than the rest. The Gaia light-curve can be seen in Figure 2.

Transit signals are expected to be achromatic in stars that are not blended with background stars, meaning that the transit should appear similar in all bands. In both cases presented here, we see that the sample that occurs in transit is similarly fainter in all bands - G, BP and RP - and thus exhibiting the expected achromaticity. Obviously, one sample out of a total of about 20 samples does not qualify as a transit detection, however if we recall that this happened in only one year it shows that in five years we can expect many more cases. It may very well be that there are already previously unknown planetary transits within the current dataset, but we cannot use one or two samples in transit to claim a detection.

Significant variability in times other than those of the transit ('out-of-transit variability') can be seen in both cases. This is understandable once the scale of the Y-axis is taken into account. Planetary transit signals are quite weak - of the order of 0.01-0.02 magnitude in the best cases. Stellar light can be variable at this level due to various stellar processes. The samples that occur in transit in these two cases are still fainter. When more data  becomes available, detection will become evident by the grouping together of a few samples in the folded ligthcurve, even if one or two samples are almost as faint.

As data accumulates during the coming years of the Gaia mission, we expect to detect transiting exoplanets using algorithms similar to the BLS (Kovács, Zucker & Mazeh 2002, A&A, 391, 369) and potentially using auxiliary ground based observations (Dzigan & Zucker 2013, MNRAS, 428, 3641). Thus, Gaia might also serve as an all-sky survey for transiting exoplanets. In DPAC we aim to fully exploit Gaia's potential in this respect. While we wait for data to accumulate to detect hitherto unknown planets, we continue to perform checks for validation purposes on the signatures of known transiting planets in the current Gaia dataset and produce figures such as those presented here.

A background story on exoplanets was published simultaneously by ESA to give a more general overview of exoplanetary research performed.

Credits: ESA/Gaia/DPAC/CU7/TAU, Shay Zucker (Tel Aviv University), Leanne P. Guy (University of Geneva), Laurent Eyer (University of Geneva) and the whole CU7 team

[Published: 09/02/2017]


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
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