Celestia Forums

The first planetary system containing three Neptune-mass planets has been discovered around the star HD 69830, around which an asteroid belt has already been detected. They are all on fairly low eccentricity orbits too.

Extrasolar Planets Encyclopaedia
Nature paper
Space.com news article (beware popups)

This is an exciting find! I've read that they're postulating that the two inner worlds are similar to Mercury, and by that I'm assuming that they think they are terrestrial worlds. But I don't think planets this massive, Jovian or not, could be without an atmosphere.

Could be wrong, of course.

...John...

Some datas are available to create the system for Celestia?

ElChristou wrote:Some datas are available to create the system for Celestia?

Now that would be something extra wouldn't it...
I hope you that that info would be available soon so it could be released in the official Celestia release.

Thanks, Anders

The subject line almost sounds like the start of a joke... "three Neptunes and an asteroid belt walk in to a bar...."

I'm sorry, it's the end of the day on Friday and it's been a long week . But seriously, this does sound rather interesting!

Presumably this system is going to end up in extrasolar.ssc eventually.

I don't know if there's any work being done on rendering circumstellar discs/zodiacal light etc. - what with all the known dust discs, asteroid belts etc, it would be good to have such a feature.

Here's a preliminary code for those planets :

Cham wrote:Here's a preliminary code for those planets :

Code: Select all

"b" "HD 69830"
{
   Texture "neptune.jpg"
   Albedo 0.75  # guess

   Mass       10.5     # M.sin(i) = 0.033 jupiters
   Radius     25000  # guess
   Oblateness 0.01  # guess

   InfoURL "http://exoplanet.eu/star.php?st=HD+69830"

   EllipticalOrbit {
      Period          0.02373
      SemiMajorAxis   0.0785
      Eccentricity    0.1
    Epoch 2453496.8
   }

   RotationPeriod  10  # guess
}

"c" "HD 69830"
{
   Texture "neptune.jpg"
   Albedo 0.75  # guess

   Mass       12.1     # M.sin(i) = 0.038 jupiters
   Radius     25000  # guess
   Oblateness 0.01  # guess

   InfoURL "http://exoplanet.eu/star.php?st=HD+69830"

   EllipticalOrbit {
      Period          0.08641
      SemiMajorAxis   0.186
      Eccentricity    0.13
    Epoch 2453469.6
   }

   RotationPeriod  10  # guess
}

"d" "HD 69830"
{
   Texture "neptune.jpg"
   Albedo 0.75  # guess

   Mass       18.5     # M.sin(i) = 0.058 jupiters
   Radius     25000  # guess
   Oblateness 0.01  # guess

   InfoURL "http://exoplanet.eu/star.php?st=HD+69830"

   EllipticalOrbit {
      Period          0.5394
      SemiMajorAxis   0.63
      Eccentricity    0.07
    Epoch 2453358
   }

   RotationPeriod  10  # guess
}

" Epoch "

These planets have now been added to extrasolar.ssc (go here).

cel://Follow/HIP%2040693/2006-05-21T23: ... 563&lm=174

Ah! I just saw the New Scientist article on this system, and was going to point it out. You guys work quickly!

Grant supplied extrasolar planet updates for HD 69830 as well as transiting planet XO-1b. The parent star of XO-1b is not in Celestia's current star database, so an entry for it was added to extrasolar.stc. You can get the updates from the CVS repository on sourceforge: http://www.sourceforge.net/projects/celestia

Unfortunately, the browse CVS feature seems to be broken right now

--Chris

Chris wrote:You can get the updates from the CVS repository on sourceforge: http://www.sourceforge.net/projects/celestia

Unfortunately, the browse CVS feature seems to be broken right now

Mister Chris, i found estrasolar.stc and extrasolar.ssc file here:

http://celestia.cvs.sourceforge.net/celestia/


Regards..

The Celestia rotation speed for the second planet are likely to be incorrect. It's given as:

Given its distance to the star (0.186 AU, half that of Mercury) and the age of the star (~ 7 (?± 3) Gyr), I believe the planet will be tidally locked.

bdm wrote:The Celestia rotation speed for the second planet are likely to be incorrect. It's given as:

Code: Select all

   RotationPeriod  46 # tidal slowing significant

Given its distance to the star (0.186 AU, half that of Mercury) and the age of the star (~ 7 (?± 3) Gyr), I believe the planet will be tidally locked.

Belief is a dangerous thing

Assuming the same bulk characteristics as Neptune, crunching the numbers for the effects of solar tides indicates that it could be possible that it's not tidally locked - it depends on the initial rotation period of the planet when the system formed (and also assumes it's not moved from its current orbit). Looks like if it started with a rotation period of less than about 12 hours then it should still be rotating independently. A 46 hour rotation period after 7 Ga implies that its original rotation period was about 10.35 hours (assuming a rigidity of 1e9 and moment of inertia of 0.225).

The extrasolar.ssc file is pretty well annotated and referenced these days, so the underlying assumptions for the rotation calculations are laid out at the head of the file:
There are just too many objects for me to hand-calculate spin-down times based on individual stellar age, and it would be pretty futile anyway, given the uncertainties of several other quantities involved. And yet, we've got to have a RotationPeriod for each body, so something has to be done, and this at least produces an appropriate trend based on mass and distance from parent body.
If I had my druthers, such calculations based on very many assumptions would be flagged with scare queries in Celestia's display:
Day Length: 46 hours ???

Figures estimated for consistency with a single other variable might have just a single query:
Radius: 9km ?
for an object the radius of which is deduced from its brightness and an estimate of albedo, for instance.

Grant

Grant Hutchison