1. „We are not talking about cosmology...“ (A. Sozzetti)

2. The visitor from an ancient galaxy A planetary companion around an old, metal-poor Red Horizontal Branch star R. J. Klement, J. Setiawan, Th. Henning, H.-W. Rix, B. Rochau, T. Schulze-Hartung, J. Rodman (ESA/ESTEC,NL)

3. Do planets exist in external galaxies? Probably, but can we find out? Motivational background

4. Do planets exist in external galaxies? Probably, but can we find out? Motivational background Yes we can!

5. Do planets exist in external galaxies? Probably, but can we find out? Observe nearby halo stream stars! Motivational background Yes we can!

6. Target selection The Helmi stream (Helmi et al. 1999)

7. Target selection The Helmi stream (Helmi et al. 1999) SErGiO

8. SErGiO Star of Extra-Galactic Origin Stellar parametersLiterature & our results Spectral type Brightness (V) Distance Effective temperature Radius Surface gravity (log g) Stellar mass Metallicity [Fe/H] Projected rotational velocity Confirmed membership Evolutionary stage F2 9.84 mag 700 pc 6025 K 6.7 R Ͽ 2.69 0.82 M Ͽ -2.09 10.3 km/s Helmi stream RHB

9. Radial Velocity FEROS Sept. 2009 – July 2010 36 RV measurements 550 spectral lines for RV computations semi-amplitude 120 m/s uncertainty ~ 50 m/s FAP = 5.5 x 10e-6

10. Sources of RV variation Starspots Line-profile asymmetry (bisector, BVS) Ca II variations (K & H, IR triplet) Other spectroscopic activity indicator (Hα, Teff variation, …) Photometric variations If the stellar activity can be excluded Rotational modulation Stellar pulsations Rotational modulation Stellar pulsations Companion

11. Sources of RV variation periodic variations of - the BVS ~ 5 days - CaII λ8498 ~ 6 days → rotational modulation → V rot ~60 km/s SuperWASP photometry (~3600 data points) → stellar pulsations

12. Planet: SerGiO jr. Setiawan, Klement et al., submitted ParameterValue Period Amplitude Eccentricity Semimajor axis Minimum mass (m sin i) 16.2 days 120 m/s 0.28 0.116 AU 1.3 M(Jup) The estimated minimum mass places the companion in the substellar regime.

13. What is interesting? Sergio has an extra-galactic origin Sergio has the lowest metallicity among known planet host stars, [Fe/H] = -2.09 ± 0.25 dex

14. What is interesting? The planet has survived the RGB phase during the stellar evolution. Probably, at the beginning the planet had a larger orbit, but then spiraled in to a closer orbit The star is a fast rotator. Sign that the inner planets have been engulfed already(?)

15. Conclusions Planets can indeed form around metal- poor stars as low as [Fe/H] ~ - 2 Under certain circumtances planets can survive the red giant phase. The orbits may be changed during the stellar evolution. SErGiO jr is the first planet detected in a tidal stellar stream and probably the first known exoplanetary system with an extragalactic origin.