Hint of a transiting extended atmosphere on 55 Cancri b

The naked-eye star 55 Cancri hosts a planetary system with five known planets, including a hot super-Earth (55 Cnc e) extremely close to its star and a farther out giant planet (55 Cnc b), found in milder irradiation conditions with respect to other known hot Jupiters. This system raises important questions on the evolution of atmospheres for close-in exoplanets, and the dependence with planetary mass and irradiation. These questions can be addressed by Lyman- transit observations of the extended hydrogen planetary atmospheres, complemented by contemporaneous measurements of the stellar X-ray flux. In fact, planet “e” has been detected in transit, suggesting the system is seen nearly edge-on. Yet, planet “b” has not been observed in transit so far. Here, we report on Hubble Space Telescope STIS Ly and Chandra ACIS-S X-ray observations of 55 Cnc. These simultaneous observations cover two transits of 55 Cnc e and two inferior conjunctions of 55 Cnc b. They reveal the star as a bright Ly target and a variable X-ray source. While no significant signal is detected during the transits of 55 Cnc e, we detect a surprising Ly absorption of 7:5 1:8% (4.2 ) at inferior conjunctions of 55 Cnc b. The absorption is only detected over the range of Doppler velocities where the stellar radiation repels hydrogen atoms towards the observer. We calculate a false-alarm probability of 4.4%, which takes the a-priori unknown transit parameters into account. This result suggests the possibility that 55 Cnc b has an extended upper Hi atmosphere, which undergoes partial transits when the planet grazes the stellar disc. If confirmed, it would show that planets cooler than hot Jupiters can also have extended atmospheres.

[1]  P. J. Wheatley,et al.  Temporal variations in the evaporating atmosphere of the exoplanet HD 189733b , 2012, 1206.6274.

[2]  Tommi T. Koskinen,et al.  A stability limit for the atmospheres of giant extrasolar planets , 2007, Nature.

[3]  Drake Deming,et al.  DETECTION OF THERMAL EMISSION FROM A SUPER-EARTH , 2012, 1205.1766.

[4]  Ch. Helling,et al.  EARLY UV INGRESS IN WASP-12b: MEASURING PLANETARY MAGNETIC FIELDS , 2010, 1009.5947.

[5]  I. Ribas,et al.  Estimation of the XUV radiation onto close planets and their evaporation , 2011, 1105.0550.

[6]  Helmut Lammer,et al.  Atmospheric Escape and Evolution of Terrestrial Planets and Satellites , 2008 .

[7]  Norman Murray,et al.  ATMOSPHERIC ESCAPE FROM HOT JUPITERS , 2008, 0811.0006.

[8]  Kevin France,et al.  OBSERVATIONS OF MASS LOSS FROM THE TRANSITING EXOPLANET HD 209458b , 2010, 1005.1633.

[9]  J. Kasting,et al.  Hydrodynamic planetary thermosphere model: 1. Response of the Earth's thermosphere to extreme solar EUV conditions and the significance of adiabatic cooling , 2008 .

[10]  R. P. Butler,et al.  Detection of a Neptune-Mass Planet in the ρ1 Cancri System Using the Hobby-Eberly Telescope , 2004, astro-ph/0408585.

[11]  H. Lammer,et al.  Energetic neutral atoms as the explanation for the high-velocity hydrogen around HD 209458b , 2008, Nature.

[12]  Lotfi Ben-Jaffel,et al.  Exoplanet HD 209458b: Inflated Hydrogen Atmosphere but No Sign of Evaporation , 2007, 0711.1432.

[13]  D. Ehrenreich,et al.  HST/STIS Lyman-alpha observations of the quiet M dwarf GJ436: Predictions for the exospheric transit signature of the hot neptune GJ436b , 2011, 1103.0009.

[14]  L. Ben-Jaffel Spectral, Spatial, and Time Properties of the Hydrogen Nebula around Exoplanet HD 209458b , 2008, 0807.3796.

[15]  G. H'ebrard,et al.  New observations of the extended hydrogen exosphere of the extrasolar planet HD 209458b , 2008, 0803.1831.

[16]  Daniel C. Fabrycky,et al.  RADIAL VELOCITY PLANETS DE-ALIASED: A NEW, SHORT PERIOD FOR SUPER-EARTH 55 Cnc e , 2010, 1005.4050.

[17]  M. Mayor,et al.  An extended upper atmosphere around the extrasolar planet HD209458b , 2003, Nature.

[18]  M. C. Weisskopf,et al.  An Overview of the Performance and Scientific Results from the Chandra X‐Ray Observatory , 2001, astro-ph/0110308.

[19]  U. Maryland,et al.  Improved precision on the radius of the nearby super-Earth 55 Cnc e , 2011, 1110.4783.

[20]  A. D. Etangs,et al.  A diagram to determine the evaporation status of extrasolar planets , 2006, astro-ph/0609744.

[21]  R. Paul Butler,et al.  Three New “51 Pegasi-Type” Planets , 1997 .

[22]  David Ehrenreich,et al.  Mass-loss rates for transiting exoplanets , 2011, 1103.0011.

[23]  D. Liedahl,et al.  Collisional Plasma Models with APEC/APED: Emission-Line Diagnostics of Hydrogen-like and Helium-like Ions , 2001, astro-ph/0106478.

[24]  M. Langlois,et al.  Society of Photo-Optical Instrumentation Engineers , 2005 .

[25]  E. Agol,et al.  Analytic Light Curves for Planetary Transit Searches , 2002, astro-ph/0210099.

[26]  M. Jura An Observational Signature of Evolved Oceans on Extrasolar Terrestrial Planets , 2004, astro-ph/0402584.

[27]  E. Chiang,et al.  Colliding planetary and stellar winds: charge exchange and transit spectroscopy in neutral hydrogen , 2012, 1206.5003.

[28]  J. Owen,et al.  Planetary evaporation by UV and X‐ray radiation: basic hydrodynamics , 2012, 1206.2367.

[29]  Kento Masuda,et al.  PLANET–PLANET ECLIPSE AND THE ROSSITER–McLAUGHLIN EFFECT OF A MULTIPLE TRANSITING SYSTEM: JOINT ANALYSIS OF THE SUBARU SPECTROSCOPY AND THE KEPLER PHOTOMETRY , 2012, 1209.4362.

[30]  A. Muñoz,et al.  Physical and chemical aeronomy of HD 209458b , 2007 .

[31]  Lee D. Feinberg,et al.  The Space Telescope Imaging Spectrograph Design , 1998 .

[32]  Mark Clampin,et al.  The On-Orbit Performance of the Space Telescope Imaging Spectrograph , 1998 .

[33]  Helmut Lammer,et al.  ENERGETIC NEUTRAL ATOMS AROUND HD 209458b: ESTIMATIONS OF MAGNETOSPHERIC PROPERTIES , 2010 .

[34]  Sona Hosseini,et al.  ON THE EXISTENCE OF ENERGETIC ATOMS IN THE UPPER ATMOSPHERE OF EXOPLANET HD209458b , 2009, 0912.1409.

[35]  Kevin France,et al.  TIME-RESOLVED ULTRAVIOLET SPECTROSCOPY OF THE M-DWARF GJ 876 EXOPLANETARY SYSTEM , 2012, 1204.1976.

[36]  Howard Isaacson,et al.  Five Planets Orbiting 55 Cancri , 2007, 0712.3917.

[37]  N. Gibson,et al.  Hubble Space Telescope transmission spectroscopy of the exoplanet HD 189733b: high‐altitude atmospheric haze in the optical and near‐ultraviolet with STIS , 2011, 1103.0026.

[38]  Canada.,et al.  Atmospheric escape from hot Jupiters , 2004, astro-ph/0403369.

[39]  Jaymie M. Matthews,et al.  A SUPER-EARTH TRANSITING A NAKED-EYE STAR , 2011, 1104.5230.