Probing Exoplanet Geology by
Observing Hot Rocky Worlds
Andrew Ridden-Harper
[email protected]
Leiden Observatory, Leiden University, Niels Bohrweg 2, 2333 CA, Netherlands
Introduction
A new class of transiting rocky exoplanets has been discovered that presents a unique opportunity to probe the geology of rocky exoplanets. These rocky
exoplanets orbit very close to their hosts stars with typical orbital semi-major axes of ~ 0.015 AU, which is about 25 times smaller than the semi-major axis
of Mercury, which is ~0.39 AU.
This extremely close stellar proximity and resulting intense stellar irradiation can cause material to be released from the surface of the planet and form an
exosphere (like with Mercury) or even a comet-like tail of dust due to sputtering and/or disintegration. By observing planets with exospheres and dust-tails
during transit, properties of the particles like size and composition can be constrained, therefore providing information about the composition and geology
of the planetary surface.
Sputtering
Disintegration
Sputtering is the process of atoms being ejected from a substrate because of
bombardment by intense radiation and/or high-energy particles. If the substrate
is a planetary surface and the ejected atoms have a sufficiently high velocity,
they can form a planetary exosphere. This effect has been well studied on
Mercury with the detection of Na [6] and other species such as Ca [7] in its
exosphere.
If a planet migrates too close to its host star, the intense stellar radiation and tidal
heating can cause the planet to be completely destroyed on a relatively short time
scale [9].
Simulations suggest that elements like Na and Ca may also be present in the
exospheres of exoplanets such as CoRoT-7b and 55 Cancri e [1].
KIC12557548b [2] and KOI2700b [3] have been identified in Kepler photometry
data as showing asymmetric and irregular light curves, which can be explained
by a comet-like cloud of dust and gas trailing behind the planet. This material is
thought to be planetary surface material that was released by the disintegration
of the planet.
An attempt to spectroscopically detect the exosphere of CoRoT-7b has been
carried out, and upper limits were determined [5].
These objects provide the unique and unprecedented possibility of directly
studying material from the surface or even the interior of a rocky exoplanet.
Distance North (R M )
10
5
0
-5
-10
-10
-5
0
5
Distance Tailward (R M )
10
Figure 1: The sodium exosphere of
Mercury observed by the MESSENGER
spacecraft (image credit: NASA)
Figure 3: Artist’s impression of
KIC12557548b (credit NASA)
Figure 4: Transit depth as a function of time from Kepler long cadence data of
KIC 12557548b . The solid line represents a 30 orbit moving average [4].
Figure 2: The simulated column density of the Na exosphere
of CoRoT-7b [1].
Analysis of 55 Cancri e data
The UVES (Ultraviolet and Visual Echelle Spectrograph) instrument on the VLT was used to obtain 138 spectra of 55 Cancri on 4 January 2014 during the transit of the
exoplanet 55 Cancri e. The analysis of these spectra focussed on detecting signals from the exosphere of the exoplanet, particularly from species such as Na, Mg and Ca, which
have been successfully detected on Mercury. The spectra were reduced using the standard UVES reduction recipes. The spectra are dominated by stellar spectral features,
which need to be removed to see spectral lines from the planet.
The steps to remove the stellar spectra are shown below in a series of images (from ‘a’ to ‘d’). Each image is a matrix where each row is an individual 1D spectrum with
wavelength increasing from left to right. The pixel value is the flux of the spectrum at that wavelength. The spectra are stacked vertically in the order that they were observed.
The vertical direction represents time. Presenting the data in this way allows differences between spectra to become more apparent.
a) Initially different continuum levels due to changes in the seeing quality
c) Each spectrum is then divided by the median spectrum to normalise to 1
b) The spectra are normalised to the same continuum level
d) Columns are weighted by noise by dividing by the variance
Future work
• Statistical analysis to search for spectroscopic signal from 55 Cancri e
• Modelling of KIC12557548b to increase understanding and better constrain its properties
• Develop connections between exoplanet science and geophysics
References
[1] A. Mura et al. 2011, Icarus 211 1
[2] Rappaport et al. 2012 ApJ 752 1
[3] Rappaport et al. 2014 ApJ 784 40
[4] van Wekhoven et al. 2014, A&A 561, A3
[5] Guenther et al. 2011, A&A 525 A24
[6] Potter & Morgan, 1985, Science 229, 651
[7] Bida et al. 2000, Nature 404, 159
[8] Croll et al. 2014, ApJ 786 100
[9] Chiang et al. 2013, MNRAS, 431, 3444–3455
Collaborators: Ignas Snellen, Christoph Keller, R. de Kok, M. Fridlund, M. Min, W. van Westrenen, A.P. van den Berg, B. Vermeersen, M. Kenworthy
Background image: Artist’s impression of CoRoT-7b from ESO