The universal relation between mass and rotation speed of planets
Map of the sky around Beta Pictoris
Around Beta Pictoris
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Videos
VLT measures the spin of Beta Pictoris b
Observations from ESO’s Very Large
Telescope (VLT) have, for the first time, determined the rotation rate
of an exoplanet. Beta Pictoris b has been found to have a day that lasts
only eight hours. This is much quicker than any planet in the Solar
System — its equator is moving at almost 100 000 kilometres per hour.
This new result extends the relation between mass and rotation seen in
the Solar System to exoplanets. Similar techniques will allow
astronomers to map exoplanets in detail in the future with the European
Extremely Large Telescope (E-ELT).
Exoplanet Beta Pictoris b orbits the naked-eye star Beta Pictoris [1], [2], which lies about 63 light-years from Earth in the southern constellation of Pictor (The Painter’s Easel). This planet was discovered nearly six years ago and was one of the first exoplanets to be directly imaged. It orbits its host star at a distance of only eight times the Earth-Sun distance (eso1024) — making it the closest exoplanet to its star ever to be directly imaged [3].
Using the CRIRES instrument on the VLT, a team of Dutch astronomers from Leiden University and the Netherlands Institute for Space Research
(SRON) have now found that the equatorial rotation velocity of
exoplanet Beta Pictoris b is almost 100 000 kilometres per hour. By
comparison, Jupiter’s equator has a velocity of about 47 000 km per hour
[4], while the Earth’s travels at only 1700 km per hour [5].
Beta Pictoris b is more than 16 times larger and 3000 times more
massive than the Earth, yet a day on the planet only lasts 8 hours.
“It is not known why some planets spin fast and others more slowly,” says co-author Remco de Kok, “but
this first measurement of an exoplanet’s rotation shows that the trend
seen in the Solar System, where the more massive planets spin faster,
also holds true for exoplanets. This must be some universal consequence
of the way planets form.”
Beta Pictoris b is a very young planet, only about 20 million years old (compared to 4.5 billion years for the Earth) [6]. Over time, the exoplanet is expected to cool and shrink, which will make it spin even faster [7]. On the other hand, other processes might be at play that change the spin of the planet. For instance, the spin of the Earth is slowing down over time due to the tidal interactions with our Moon.
The astronomers made use of a precise technique called high-dispersion spectroscopy to split light into its constituent colours — different wavelengths in the spectrum. The principle of the Doppler effect
(or Doppler shift) allowed them to use the change in wavelength to
detect that different parts of the planet were moving at different
speeds and in opposite directions relative to the observer. By very
carefully removing the effects of the much brighter parent star they
were able to extract the rotation signal from the planet.
“We have measured the wavelengths of radiation emitted by the
planet to a precision of one part in a hundred thousand, which makes the
measurements sensitive to the Doppler effects that can reveal the
velocity of emitting objects,” says lead author Ignas Snellen. “Using
this technique we find that different parts of the planet’s surface are
moving towards or away from us at different speeds, which can only mean
that the planet is rotating around its axis“.
This technique is closely related to Doppler imaging, which has been
used for several decades to map the surfaces of stars, and recently that
of a brown dwarf [8] — Luhman 16B (eso1404).
The fast spin of Beta Pictoris b means that in the future it will be
possible to make a global map of the planet, showing possible cloud
patterns and large storms.
“This technique can be used on a much larger sample of exoplanets
with the superb resolution and sensitivity of the E-ELT and an imaging
high-dispersion spectrograph. With the planned Mid-infrared E-ELT
Imager and Spectrograph (METIS)
we will be able to make global maps of exoplanets and characterise much
smaller planets than Beta Pictoris b with this technique”, says METIS principal investigator and co-author of the new paper, Bernhard Brandl.
Notes
[1] Beta Pictoris has many other names, e.g. HD 39060, SAO 234134 and HIP 27321.
[2] Beta Pictoris is one of the best-known examples
of a star surrounded by a dusty debris disc. This disc is now known to
extend out to about 1000 times the distance between the Earth and the
Sun. Earlier observations of Beta Pictoris’s planet were reported in eso0842, eso1024 and eso1408.
[3] The observations made use of the adaptive optics
technique compensating for the Earth’s atmospheric turbulence which can
distort images obtained at even the best sites in the world for
astronomy. It allows astronomers to create super-sharp images, almost as
good as those that could be seen from space.
[4] Since Jupiter has no solid surface from which to
determine the planet’s rotation rate, we take the rotation speed of its
equatorial atmosphere, which is 47 000 km per hour.
[5] The Earth’s rotation speed at the equator is 1674.4 km per hour.
[6] Earlier measurements suggested that the system was younger.
[7] This is a consequence of the conservation of
angular momentum and is the same effect that makes a spinning ice skater
turn more rapidly when they bring their arms closer to their body.
[8] Brown dwarfs are often dubbed “failed stars” as, unlike stars such as the Sun, they are not massive enough to sustain nuclear fusion reactions.
More information
This research was presented in a paper “Fast
spin of a young extrasolar planet”, by I. Snellen et al., to appear in
the to appear in the journal Nature on 1 May 2014.
The team is composed of Ignas A. G. Snellen (Leiden
Observatory, Leiden University, Leiden, the Netherlands),
Bernhard Brandl (Leiden Observatory), Remco J. de Kok (Leiden
Observatory, SRON Netherlands Institute for Space Research,
Utrecht, the Netherlands), Matteo Brogi (Leiden Observatory),
Jayne Birkby (Leiden Observatory) and Henriette Schwarz (Leiden
Observatory).
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partner of a revolutionary astronomical telescope ALMA, the largest
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E-ELT, which will become “the world’s biggest eye on the sky”.
Links
- Research paper
- Photos of the VLT
- Photos of CRIRES
- The CRIRES instrument on the VLT
- More about METIS: A Mid-Infrared E-ELT Imager and Spectrograph
Contacts
Ignas SnellenLeiden Observatory
Leiden, The Netherlands
Tel: +31 71 52 75 838
Cell: +31 63 00 31 983
Email: snellen@strw.leidenuniv.nl
Richard Hook
ESO Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591
Email: rhook@eso.org
