Distant Planet Brings Astronomers Closer To Home

Using a network of telescopes scattered across the globe, including the
Danish 1.54m telescope at ESO La Silla,(Chile), astronomers [1]
discovered a new extrasolar planet significantly more Earth-like than
any other planet found so far. The planet, which is only about 5 times
as massive as the Earth, circles its parent star in about 10 years. It
is the least massive exoplanet around an ordinary star detected so far
and also the coolest [2]. The planet most certainly has a rocky/icy
surface. Its discovery marks a groundbreaking result in the search for
planets that support life.

The new planet, designated by the unglamorous identifier of
OGLE-2005-BLG-390Lb, orbits a red star five times less massive than the
Sun and located at a distance of about 20,000 light years, not far from
the centre of our Milky Way galaxy.

Its relatively cool parent star and large orbit implies that the likely
surface temperature of the planet is 220 degrees Centigrade below zero,
too cold for liquid water. It is likely to have a thin atmosphere, like
the Earth, but its rocky surface is probably deeply buried beneath
frozen oceans. It may therefore more closely resemble a more massive
version of Pluto, rather than the rocky inner planets like Earth and Venus.

“This planet is actually the first and only planet that has been
discovered so far that is in agreement with the theories for how our
Solar System formed”, said Uffe Gråe Jørgensen (Niels Bohr Institute,
Copenhagen, Denmark), member of the team.

The favoured theoretical explanation for the formation of planetary
systems proposes that solid `planetesimals’ accumulate to build up
planetary cores, which then accrete nebular gas – to form giant planets
- if they are sufficiently massive. Around red dwarfs, the most common
stars of our Galaxy, this model favours the formation of Earth- to
Neptune-mass planets being between 1 and 10 times the Earth-Sun distance
away from their host.

“OGLE-2005-BLG-390Lb is only the third extra-solar planet discovered so
far through microlensing searches”, said Jean-Philippe Beaulieu
(Institut d’Astrophysique de Paris, France), the lead author. “While the
other two microlensing planets have masses of a few times that of
Jupiter, the discovery of a 5 Earth mass planet – though much harder to
detect than more massive ones – is a strong hint that these lower-mass
objects are very common.”

Contrary to most exoplanets discovered, it was found using the
“microlensing” technique, based on an effect noted by Albert Einstein in
1912.

“With this method, we let the gravity of a dim, intervening star act as
a giant natural telescope for us, magnifying a more distant star, which
then temporarily looks brighter”, explained team member Andrew Williams
(Perth Observatory, Australia). “A small `defect’ in the brightening
reveals the existence of a planet around the lens star. We don’t see the
planet, or even the star that it’s orbiting, we just see the effect of
their gravity.”

Such an intervening star causes a characteristic brightening that lasts
about a month. Any planets orbiting this star can produce an additional
signal, lasting days for giant planets down to hours for Earth-mass planets.

In order to be able to catch and characterize these planets,
nearly-continuous round-the-clock high-precision monitoring of ongoing
microlensing events is required. This is achieved by the PLANET network
of 1m-class telescopes consisting of the ESO 1.54m Danish at La Silla
(Chile), the Canopus Observatory 1.0m (Hobart, Tasmania, Australia), the
Perth 0.6m (Bickley, Western Australia), the Boyden 1.5m (South Africa),
and the SAAO 1.0m (Sutherland, South Africa). Since 2005, PLANET
operates a common campaign with RoboNet, a UK operated network of 2m
fully robotic telescopes currently comprising the Liverpool Telescope
(Roque de Los Muchachos, La Palma, Spain) and the Faulkes Telescope
North (Haleakala, Hawaii, USA).

The OGLE (Optical Gravitational Lensing Experiment) search team (led by
A. Udalski, Warsaw University Observatory, Poland) discovered the event
OGLE-2005-BLG-390 on 11 July 2005, triggering the PLANET telescopes to
start taking data. A light curve consistent with a single lens star
peaking at an amplification of about 3 on 31 July 2005 was observed,
until 10 August when PLANET member Pascal Fouqué, observing at the
Danish 1.54m at ESO La Silla, noticed a planetary deviation. An OGLE
point from the same night showed the same trend, while the last half of
the planetary deviation, lasting about a day, had been covered by images
from Perth Observatory. The MOA (Microlensing Observations in
Astrophysics) collaboration was later able to identify the source star
on its images and confirmed the deviation.

No other interpretation than the presented sub-Neptune mass planet with
its quoted parameters appeared to fit the extensive data set. This
discovery brings a fresh look at the field of planetary science.

In particular, astronomers now think that such frozen worlds are much
more common than their larger, Jupiter-like brethren. “Indeed if
Jupiter-like planets were as widespread, the microlensing method should
have found dozens of them by now”, said David Bennett (University of
Notre Dame, USA), another PLANET team member.

The microlensing technique is most probably the only method currently
capable of detecting planets similar to Earth. “The search for a second
Earth is the driving force behind our research and this discovery
constitutes a major leap forward since it is the most Earth-like planet
we know of so far”, said co-author Daniel Kubas, ESO.

A report has been published in the January 26 edition of the leading
journal Nature (“Discovery of a cool planet of 5.5 Earth masses through
gravitational microlensing” by J.-P. Beaulieu, D. P. Bennett, P. Fouqué,
A. Williams, M. Dominik, U. G. Jorgensen, D. Kubas et al.).