EDP Sciences
Free access
Volume 433, Number 2, April II 2005
Page(s) L21 - L24
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:200500025

A&A 433, L21-L24 (2005)
DOI: 10.1051/0004-6361:200500025


A planet-sized transiting star around OGLE-TR-122

Accurate mass and radius near the hydrogen-burning limit
F. Pont1, C. H. F. Melo2, F. Bouchy3, 4, S. Udry1, D. Queloz1, M. Mayor1 and N. C. Santos5

1  Observatoire de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland
    e-mail: frederic.pont@obs.unige.ch
2  European Southern Observatory, Casilla 19001, Santiago 19, Chile
3  Laboratoire d'Astrophysique de Marseille, Traverse du Siphon, BP 8, 13376 Marseille Cedex 12, France
4  Observatoire de Haute Provence, 04870 St Michel l'Observatoire, France
5  Centro de Astronomia e Astrofísica da Universidade de Lisboa, Tapada da Ajuda, 1349-018 Lisboa, Portugal

(Received 28 January 2005 / Accepted 12 February 2005 )

We report the discovery and characterisation of OGLE-TR-122b, the smallest main-sequence star to date with a direct radius determination. OGLE-TR-122b transits around its solar-type primary every 7.3-days. With $M=0.092\pm0.009 \ M_\odot$ and $R=0.120^{+0.020}_{-0.013} \ R_\odot$, it is by far the smallest known eclipsing M-dwarf. The derived mass and radius for OGLE-TR-122b are in agreement with the theoretical expectations. OGLE-TR-122b is the first observational evidence that stars can indeed have radii comparable or even smaller than giant planets. In such cases, the photometric signal is exactly that of a transiting planet and the true nature of the companion can only be determined with high-resolution spectroscopy.

Key words: stars: low-mass, brown-dwarfs -- stars: fundamental parameters -- stars: binaries -- binaries: eclipsing

SIMBAD Objects

© ESO 2005