The Earth and the other planets of the Solar system turn around the
Sun on orbits all located in about the same plane. This orbital plane
is the same as the equatorial plane of the Sun, determined by
the Sun's rotation. In other words, the rotation axis of the Sun is
very close to the revolution axes of the planets;
all of these axes are aligned, within a few degrees.
It is thought that the planets kept the shape of the original
disc of gas and dust in which it formed, nearly five billions
years ago.
Approximately
300 planets are known today around stars other than our
Sun. About
fifty of these extra-solar planets have by chance an orbit
aligned with the Earth, such that the planet can passes directly in front of
its star when viewed from the Earth, causing a mini-eclipse called
"transit". Transiting planets enable
detailed
studies, and in particular to measure the obliquity of the orbits,
i.e. the angle between the rotation axis of a star and the revolution
axis of a planet around its star. This measurement is done thanks to the
Rossiter-McLaughlin effect, from the name of the two astronomers who
predicted it in the Twenties.
The obliquity of the orbits has been measured for a dozen
extra-solar planets within the last few years, utilizing this method.
For each of them, the orbit of the planet
has been measured to be coplanar with the equator of star. That
confirms that, as for the Solar system, the other planetary systems
were probably formed in flat proto-planetary discs and have
retained their alignment to this day.
However, a team of European astronomers has just revealed a possibly
very-oblique orbit for the first time, in the case of the
transiting extra-solar planet
XO-3b,
discovered in 2007.
Located 850 light-years away in the
constellation of the Giraffe, XO-3b is a planet 12 times more massive
than Jupiter, the largest planet in the Solar system.
Being very close to its star, the XO-3b planet makes a complete
orbit in just three days, causing frequent transits. One of these
transits was observed at
the beginning of this year with the
SOPHIE
instrument, installed on
the
193-cm telescope of the of
Haute-Provence Observatory. This
observation showed the typical signature of an almost polar orbit.
SOPHIE is the new spectrograph that has replaced ELODIE two years ago, the
famous instrument that allowed in 1995 the discovery of the
first
extra-solar planet, by astronomers of the Observatory of
Geneva. SOPHIE allows accurate measurements of stellar radial
velocities. It is in particular used by a team gathering of the
researchers of
Institut d'Astrophysique de Paris (CNRS, University
Pierre & Marie Curie) and of the observatories of
Marseilles-Provence,
Geneva and
Grenoble.
The team has been carrying out a program of research and characterization of extra-solar
planets around different types of stars.
The team measured the obliquity of the orbit of the XO-3b
planet. This result has been published this month in the review
Astronomy &
Astrophysics. The result should be confirmed with further observations,
as the end of the observation
has been made under unfavorable conditions, with the observed star
was low on the horizon. New transits of this planets are planning
to be observed by this team with SOPHIE, and likely by other teams with other
instruments around the world. If the large obliquity of the orbit of XO-3b
were confirmed, it would be the first case of non-aligned orbit for a
planet. This surprising configuration could be the signature of a
particular past event in the life of this planet since its creation in a
proto-planetary disc. For example, it could have undergone the
gravitational interaction of another body (star or planet), which
would have made it leave the original plane of the system. Many theorists
use computer simuations to model such events. This observation put tight
constraints on such models, allowing this type of phenemena
to be better understood.
Note :
This result, announced in mid-2008 (arXiv:0806.0719),
was confirmed in early-2009 by an American team
(arXiv:0902.3461).