Astronomers have discovered a rare, warm, massive Jupiter-like planet
orbiting a star that is rotating extremely quickly. The discovery raises
puzzling questions about planet formation – neither the planet's
comparatively small mass nor its large distance from its host star are
expected according to current models. The observations that led to the
discovery were made using the SPHERE instrument at ESO's very large
telescope. The article describing the results has been accepted for
publication in the journal Astronomy & Astrophysics.
Paraphrasing Isaac Asimov, scientific progress is announced not so
much by “Eureka!” than by “Hm, this is odd!” The newly discovered
planetary system HIP 65426 is a case in point: With a central star in
ultrafast rotation, the absence of a gas disk one would have expected
for a system 14 million years old and a comparatively light, distant
planet, the system doesn't quite fit the existing models for how
planetary systems come into being.
Planets are formed in gigantic disks of gas and dust that surround
young stars. In the young planetary systems that have been found so far,
including all of those observed with the SPHERE instrument, remnants of
the disk are usually still visible. There is some degree of correlation
in mass: massive stars tend to have more massive disks, forming more
massive planets.
Enter HIP 65426b, a planet newly discovered by a group of astronomers
that includes researchers from the Max Planck Institute for Astronomy
(MPIA), and its host system. HIP 65426b was discovered with the SPHERE
instrument at the Very Large Telescope at ESO's Paranal Observatory in
Chile, which took a direct image of the planet. The central star, HIP
65426, is part of what might be termed a stellar kindergarten: the
Scorpius-Centaurus association which contains between 3000 and 5000
stars that formed at approximately the same time, at a distance of
almost 400 light-years from Earth. Applying common astronomical
techniques for dating stars both to HIP 65426 individually and to its
stellar neighbors, it follows that HIP 65426 is only about 14 million
years old.
Gael Chauvin of the University of Grenoble and the University of
Chile, the lead author of the study, says: "We would expect a planetary
system this young to still have a disk of dust, which could show up in
observations. HIP 65426 does not have such a disk known for the moment –
a first indication that this system doesn't quite fit our classical
models of planetary formation."
An unusual planet
There is, however, the planet HIP 65426b. Comparing the direct
observations with suitable models, HIP 65426b is a warm Jupiter-like
planet, with a temperature of about 1300-1600 Kelvin (1000-1300 degrees
Celsius), about 1.5 times the radius of Jupiter, and between 6 and 12
times Jupiter's mass. This would make HIP 65426b a gas giant, like
Jupiter, with a solid core and thick layers of (mostly hydrogen) gas.
Indeed, spectral examinations using SPHERE's spectrograph indicate the
presence of water vapor and reddish clouds, similar to Jupiter's. The
planet is far out, orbiting its host star at 100 astronomical units (100
times the average Earth-Sun distance, and more than four times
Neptune's distance from the Sun).
Again, this represents various levels of oddness: Stars of the type
of HIP 65426 (spectral class A2V) are expected to have about twice the
mass of the Sun; it has long been assumed that such a star would have
much more massive giant planets than the 6-12 Jupiter masses of HIP
65426b. On the other hand, such giant planets would not be expected as
far out as HIP 65426b.
Last but not least, the host star HIP 65426 is special, as well:
According to spectra taken with ESO's HARPS spectrograph, it rotates
about 150 times as fast as the Sun. There is only one other star of
similar type that is rotating as fast, and that one is part of a binary
star system. In such a system, matter transfer from one star to the
other can spin up the receiving star. How a single star could have sped
up that much requires an explanation.
The origin of HIP 654426b: a system-wide drama?
So far, the astronomers can only speculate about the origin of the
newly discovered system's peculiar properties. A possible scenario
involves a regular planetary-scale drama: Initially, HIP 65426b would
have formed much closer to the star (explaining its comparatively low
mass), and at least one other massive body would have formed as well. At
some point, HIP 65426b and that other body would have come close enough
for HIP 65426b to be catapulted outwards (up to its current great
distance) and the other body moving inwards and merging with the star
(causing the star's rapid rotation). The planets traversing the system
could also have destabilized the disk, explaining why it did not survive
long enough to be observed.
An alternative explanation would involve particular dynamics of the
protoplanetary disk, with both the star and the planet forming by
collapse at the same time by fragmentation – which would still require
an explanation for why the disk was so short-lived to have vanished by
now.
More definite explanations will have to wait for additional
observations and simulations. They could have an impact on our
understanding of how gas giants form, evolve, and possibly migrate, in
general. This, in turn, is crucial for understanding the formation of
planetary systems as a whole: the mass of the host star aside, most of
the mass in a planetary system is carried by such giant planets, and the
presence and properties of such planets has a decisive influence on the
formation of their smaller cousins, such as Earth-like planets or
Super-Earths.
For the SPHERE team, the discovery holds an additional special
significance. This is the first planet discovered using the SPHERE
instrument. MPIA director Thomas Henning, who is one of the fathers of
the SPHERE instrument and a co-author of the present study, adds:
"Direct images of exoplanets are still very rare, but they contain a
wealth of information about planets such as HIP 65426b. The analysis of
the direct light of the planet allows us to constrain the composition of
the planet's atmosphere with great confidence. " Images exist for less
than 20 of the currently known 3600 exoplanets; the common methods of
detection are all indirect, relying as they do on how the presence of a
planet influences the host star's light. Direct imaging is very
difficult, given that stars are so bright their light drowns out any
light from surrounding planets. SPHERE has been designed to optimally
suppress the stars' light, allowing for images and spectra of
surrounding planets. So far, direct imaging is the only way to detect
planets whose distance from their host star is large – planets such as
the unusual HIP 65426b.
Back Ground information
The research described here was published as G. Chauvin et al.,
"Discovery of a warm, dusty giant planet around HIP 65426" in the
journal Astronomy and Astrophysics.
E-print of the article at arXiv
The MPIA researchers involved are Markus Feldt, Beth Biller (also
University of Edinburgh), A.-L. Maire, J. Olofsson (also Universidad de
Valparaíso), M. Samland, M. Janson (also Stockholm University), M.
Keppler, G. D. Marleau (also University of Bern), Paul Mollière,
Christoph Mordasini (also University of Bern), A. Müller, Thomas
Henning, O. Möller-Nilsson, A. Pavlov, J. Ramos, Wolfgang Brandner,
Taisyia Kopytova (also University of Arizona), J. Schlieder (also NASA
Goddard Space Flight Center).
The SPHERE consortium is composed of 12 major European institutions
which designed and built the SPHERE planet imager for the ESO's Very
Large Telescope (eso1417): Institut de Planétologie et d'Astrophysique
de Grenoble; Max-Planck-Institut für Astronomie in Heidelberg;
Laboratoire d’Astrophysique de Marseille; Laboratoire d’Etudes Spatiales
et d’Instrumentation en Astrophysique de l’Observatoire de Paris;
Laboratoire Lagrange in Nice; ONERA; Observatoire de Genève; Italian
National Institute for Astrophysics coordinated by the Osservatorio
Astronomico di Padova; Institute for Astronomy, ETH Zurich; Astronomical
Institute of the University of Amsterdam; Netherlands Research School
for Astronomy (NOVA-ASTRON) and ESO.
Science Contact
Dr. Markus Feldt
(SPHERE CO-PI)
Phone:+49 6221 528-262
Email: feldt@mpia.de
Max-Planck-Institute for Astronomy, Heidelberg, Germany
Prof. Dr. Thomas Henning
Director - Max Planck Institute for Astronomy; Professor at the University of Heidelberg
Phone:+49 6221 528-200
Email: henning@mpia-hd.mpg.de
Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg
Personal homepage
MPIA Heidelberg
Public Information Officer
Markus Pössel
Public Information Officer
Phone:(+49|0) 6221 528-261
Email: pr@mpia.de
Science Contact
Dr. Markus Feldt
(SPHERE CO-PI)
Phone:+49 6221 528-262
Email: feldt@mpia.de
Max-Planck-Institute for Astronomy, Heidelberg, Germany
Prof. Dr. Thomas Henning
Director - Max Planck Institute for Astronomy; Professor at the University of Heidelberg
Phone:+49 6221 528-200
Email: henning@mpia-hd.mpg.de
Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg
Personal homepage
MPIA Heidelberg
Public Information Officer
Markus Pössel
Public Information Officer
Phone:(+49|0) 6221 528-261
Email: pr@mpia.de
