Astrophysical artefact in the astrometric detection of exoplanets ? Jean Schneider LUTh – Paris Observatory Work in progress ● Dynamical and brightness astrometry ● Astrophysical sources of excess brightness – Simulations – Observations ● Conclusion 12 Oct 2011 1 Context Ultimate goal: the precise physical characterization of Earth-mass planets in the Habitable Zone (~ 1 AU) by direct spectro- polarimetric imaging It will also require a good knowledge of their mass. Two approaches (also used to find Earth-mass planets): – Radial Velocity measurements – Astrometry 12 Oct 2011 2 Context Radial Velocity and Astrometric mass measurements have both their limitations . Here we investigate a possible artefact of the astrometric approach for the Earth-mass regime at 1 AU. ==> not applicable to Gaia or PRIMA/ESPRI Very simple idea: can a blob in a disc mimic the astrometric signal of an Earth-mass planet at 1 AU? 12 Oct 2011 3 Dynamical and brightness astrometry Baryc. M M * C B I Ph I I << I 1 2 2 1 Photoc. a M a = C ● Dynamical astrometry B M* D I I M ● 2 a 2 C a Brightness (photometric) astrometry Ph= − B= − I1 D I1 M* D Question: can ph be > B ? 12 Oct 2011 4 Dynamical and brightness astrometry Baryc. M M * C B I Ph I I << I 1 2 2 1 Photoc. MC a −6 a ● = B ~ 3 x 10 for a 1 Earth-mass planet M* D D I a I a = 2 − ~ 2 -6 ● Ph B Can I /I be > 3x10 ? I1 D I1 D 2 1 12 Oct 2011 5 Dynamical and brightness astrometry Baryc. M M * C B I Ph I I << I 1 2 2 1 Photoc. For 1 Jup. MC a −6 a Ph ● = B ~ 3 x 10 M* D D B 2 2 ● I2 a A RC a A RC ~ = = Ph I D 4 a D 4 aD 1 ~0.1 AU a 12 Oct 2011 6 Dynamical and brightness astrometry MC a −6 a Ph 1 Jup at 1 AU ● B = ~ 3 x 10 M* D D 2 2 I2 a A RC a A RC B ● ~ = = Ph I D 4 a D 4 aD 1 ~0.1 AU a Can Ph be larger than B at 1 AU ? 2 −6 2 2 2 Condition: A/4RC /1AU> 3.10 ~100 R Jup ==> ARC > 50R Jup Explore sources of low mass objects brighter than 50 Jupiter at 1 AU Past experience: Anything can happen in exoplanetology 12 Oct 2011 7 Astrophysical sources of brighness excess ● Simulations ''Irregular Satellite Swarms: Detectable Dust around Solar System and Extrasolar Planets'' Kennedy & Wyatt, MNRAS, 412, 2137, 2011 Satell. swarm 103 1 Jupiter 12 Oct 2011 8 Astrophysical sources of brighness excess ● Observations – Earth's dust ring (Reach, Icarus, 209, 848, 2011) About N = 10 blobs 0.1 AU x 0.1 AU 2 = 200 x 200 R jup Assuming a dust albedo 0.01, 2 3 2 ARC >~10 R Jup for each blob and at 1 AU: 2 2 1 A R a A R R ~ C = C =0.015 Jup Ph N 4 a D 12 aD D ~ B for 1MEarth at 1 AU 12 Oct 2011 9 Astrophysical sources of brighness excess ● Observations – Fomalhaut b Companion 400 brighter than a Jupiter at 100 AU Interpretation: dust cloud around a planet Open question: Can such dust clouds exist at 1 AU? 12 Oct 2011 10 Conclusion The photocenter variation can possibly be larger than the 1 Earth mass astrometric signal at 1 AU. To measure terrestrial planet masses at 1 AU, astrometric measurements must be combined with high contrast high angular resolution imaging 12 Oct 2011 11.