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Fast spin of the young extrasolar planet β Pictoris b



The spin of a planet arises from the accretion of angular momentum during its formation1,2,3, but the details of this process are still unclear. In the Solar System, the equatorial rotation velocities and, consequently, spin angular momenta of most of the planets increase with planetary mass4; the exceptions to this trend are Mercury and Venus, which, since formation, have significantly spun down because of tidal interactions5,6. Here we report near-infrared spectroscopic observations, at a resolving power of 100,000, of the young extrasolar gas giant planet β Pictoris b (refs 7, 8). The absorption signal from carbon monoxide in the planet’s thermal spectrum is found to be blueshifted with respect to that from the parent star by approximately 15 kilometres per second, consistent with a circular orbit9. The combined line profile exhibits a rotational broadening of about 25 kilometres per second, meaning that β Pictoris b spins significantly faster than any planet in the Solar System, in line with the extrapolation of the known trend in spin velocity with planet mass.

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Figure 1: Broadened cross-correlation signal of β Pictoris b.
Figure 2: Spin of β Pictoris b.


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We thank T. de Zeeuw for granting Director’s Discretionary Time on the VLT to perform these observations (292.C-5017(A)). I.A.G.S. acknowledges support from an NWO VICI grant. R.J.d.K. acknowledges the NWO PEPSci programme.

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Authors and Affiliations



I.A.G.S. designed the project with help from B.R.B., R.J.d.K., M.B. and J.B. The analysis was led by I.A.G.S. and he wrote the first version of the manuscript. I.A.G.S. and B.R.B. made the connection with the European Extremely Large Telescope. R.J.d.K. constructed the planet atmosphere models. B.J.B., R.J.d.K., M.B., J.B. and H.S. discussed the analyses and results, and commented on the manuscript.

Corresponding author

Correspondence to Ignas A. G. Snellen.

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Extended data figures and tables

Extended Data Figure 1 Model spectra and cross-correlation signals.

The model spectral templates (left panels) and the resulting cross-correlation signals (right panels) for (from top to bottom) a high-VMR CO model, a low-VMR CO model, the previous model with an added high VMR of H2O, an H2O-only mode and a CH4 model. Atmospheric pressures are in units of bars.

Extended Data Figure 2 Simulated E-ELT observations.

We simulated observations of a rotating spot on β Pictoris b as would be made by the future 39-m European Extremely Large Telescope. Such observations could be conducted with the planned METIS40. The three panels on the left show the position of the spot at times approximately 1 h apart. The spot was given a surface brightness twice that of the rest of the planet’s atmosphere. The right-hand panel shows the difference between three cross-correlation signals with respect to the average cross-correlation profile as indicated by the dashed curve (scaled down by a factor of 25), with the spot signature moving from −15 to +5 km s−1.

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Snellen, I., Brandl, B., de Kok, R. et al. Fast spin of the young extrasolar planet β Pictoris b. Nature 509, 63–65 (2014).

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