Spitzer Space Telescope - Directors Discretionary Time Proposal #541


The First Exoplanet Smaller than the Earth

Principal Investigator: Sarah Ballard
           Institution: Harvard University

     Technical Contact: Sarah Ballard, Harvard University

Co-Investigators: 
David Charbonneau, Harvard University
Jessie Christiansen, Harvard University
Drake Deming, NASA Goddard Space Flight Center
Daniel Fabrycky, Harvard University
Matthew Holman, Smithsonian Astrophysical Observatory
Heather Knutson, University of California, Berkeley
Sara Seager, Massachusetts Institute of Technology

Science Category: extrasolar planets
 Observing Modes: IRAC Post-Cryo Mapping
  Hours Approved: 18.0

Abstract:
Our team has collected good evidence for a planet smaller than the Earth 
transiting the nearby M-dwarf star GJ 436, which is already known to host 
a Neptune-sized planet.  We first identified the signal in a 3-week-long 
photometric monitoring campaign by the NASA EPOXI Mission.  Based on the 
EPOXI data, we predicted a transit event in an extant Spitzer 8-micron data 
set of this star.  Our subsequent analysis of those Spitzer data confirmed 
the signal of the predicted depth and at the predicted time.  However, the 
existing EPOXI and Spitzer data are not sufficient to support such an 
extraordinary claim.  We propose to confirm the existence of this planet 
by observing the star for 18 hours spanning a predicted time of transit 
and at a wavelength where the signal-to-noise would provide an ironclad 
detection.  With a radius only 75% that of the Earth, the new planet GJ 436c 
would be by far the smallest exoplanet yet discovered and indeed the first 
exoplanet akin to the terrestrial planets of the Solar system.  Moreover, 
the star's low mass and nearby distance imply that the planetary mass could 
be estimated from a dedicated radial-velocity campaign.  When combined 
with the radius estimate we will obtain from our proposed Spitzer data, 
this would provide the first constraint on the average density and hence 
bulk composition of an Earth-like exoplanet.  We argue for Director's 
Discretionary Time based both on the compelling scientific opportunity 
to study an Earth-like exoplanet, and the fact that our prediction for 
the transit times degrades rapidly with time, making it imperative to 
obtain these observations in the 2010 Jan 14 - Feb 25 visibility window.  
If successful, our program would provide a dramatic example of the value 
of an extended Warm Mission during the coming years when humanity uncovers 
the population of rock and ice exoplanets orbiting nearby stars.