voked — “cold start” and “hot
start” — lead to very different
observable properties. For instance, in the cold start scenario,
gas accretion reaches a runaway
stage, producing shocks that radiate away all the incoming energy, resulting in a low temperature, low luminosity planet. In
the hot start scenario, the shocks
are radiatively ineffective, resulting in a planet with both high luminosity and high temperature.
Of course, intermediate models
are possible with initial entropy
varying between these two extreme cases. The only way to determine the initial conditions is,
therefore, to study young planetary systems.
Previous directly-imaged exoplanets had luminosities only compatible with those predicted by the hot start scenario, whereas 51
Eri b is faint enough to be reproduced by
both scenarios. The cold start scenario is usually associated with the core accretion formation mechanism, in which a core is built
from planetesimal agglomerations followed
by rapid gas capture. This mechanism is the
adopted hypothesis to explain the formation
of the gas giants in the Solar System. Therefore, 51 Eri b might have formed like Jupiter,
with modest extensions to the classical core
accretion model; like the pebble accretion
which facilitates planet formation at larger
distances than the typical 1-5 AU from the
central star.
Tip of the Iceberg?
51 Eri b is the first exoplanet found by GPI.
It belongs to a low mass, low temperature,
methane dominated, close-in category to
which earlier instruments were not sensitive enough to detect in previous exoplanet
searches. We hope it represents the tip of
October 2015
the iceberg of extrasolar planets that will be
directly imaged in the next few years — especially by instruments such as GPI, and its
European cousin: the Spectro-Polarimetric
High-contrast Exoplanet Research (SPHERE)
at the Very Large Telescope.
The direct exoplanet imaging community is
undertaking large-scale campaigns, which
will hopefully lead to additional discoveries
that will place our own Solar System in the
context of other extrasolar systems. Studies
such as these are the key to understanding
the formation of giant planets, their evolution, and, ultimately, how they interact with
potentially life-bearing terrestrial planets,
which will undoubtedly be discovered with
future instruments.
Figure 5.
An artist’s visualization
of the Jupiter-like
exoplanet, 51 Eri b, seen
in the near-infrared
light that shows the
hot layers deep in its
atmosphere glowing
through clouds.
Because of its young
age, this young cousin
of our own Jupiter is
still hot and carries
information on the
way it was formed 20
million years ago.
Credit: Danielle Futselaar
& Franck Marchis (SETI
Institute).
Julien Rameau is a postdoctoral fellow at the Institut de Recherche sur les Exoplanètes, Université
de Montréal. He can be contacted at:
[email protected]
Robert De Rosa is a postdoctoral fellow at the University of California Berkeley. He can be contacted
at: [email protected]
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