VLT Detects First Superstorm on HD209458b

Now that is impressive. Measurement of the orbital velocity of the planet, allowing direct determination of the mass of the star. I guess this is the first double-lined eclipsing binary where one component is a planet.

(Incidentally it is nice to see that ESO makes the papers available with the press release.)

Wow that is remarkable.

The mass of the star is quoted as 1.00 ± 0.22 M_sol But the planet mass has a much lower error, 0.64 ± 0.09 M_J. Why is the planet's mass much more constrained than the stars?

Edit: I wonder if they could determine the wind speed as a function of latitude by carefully watching the planet's atmospheric absorption lines red shift during transit ingress and egress. It'd have to be some pretty darned fast measurements as they'd only have a few minutes to do it, but it might allow us to map zones and belts on the planet.

Edit2: Just realised... this suffers from inclination degeneracy effects too. Not knowing the planetary rotation axis, is this only a lower limit to the wind speed?

This is indeed awesome.

Quote :

I guess this is the first double-lined eclipsing binary where one component is a planet.

Nicely put Lazarus

I just think that the public release should be more precise. After all they did not detect a "Storm" but apparently a super-rotating wind.

Sirius_Alpha wrote:

The mass of the star is quoted as 1.00 ± 0.22 M_sol But the planet mass has a much lower error, 0.64 ± 0.09 M_J. Why is the planet's mass much more constrained than the stars?

Good question.

From the equations of motion it is possible to write the expressions for star and planet mass as follows:

m₁ = (P K₂³ (1 + K₁ / K₂)²) / (2π G sin³ i)

swap index 1 and 2 to get planet mass.

Error bars are given for K₁, K₂ and i, so the error on the mass can be expressed:

σ_m² = ((∂m / ∂K₁) σ_K₁)² + ((∂m / ∂K₂) σ_K₂)² + ((∂m / ∂i) σ_i)²

Working through that does give the uncertainties quoted in the paper, so that all checks out.

Uncertainty basically comes down to which of K₂³(1 + K₁/K₂)² (in the expression for star mass) and K₁³(1 + K₂/K₁)² (in the expression for planet mass) has greater uncertainty. Very handwavey way of looking at it, K₂ has significantly higher uncertainty than K₁, and in the expression for star mass the highest order term is in K₂³ as opposed to K₂² in the expression for planet mass.

Ahhh alright. That makes sense. Thanks.

A similar case
http://nexsci.caltech.edu/workshop/2009/pops/Crossfield.pdf

Well it might be a superstorm, or it might just be the effects of a slightly noncircular orbit.

Exoplanets transmission spectroscopy: accounting for eccentricity and longitude of periastron. Superwinds in the upper atmosphere of HD209458b?