A lot of people are already blogging this but here we have it: an optical image of a planet orbiting around another star. The star is Fomalhaut and the planet is excitingly named Fomalhaut b. Name suggestions anyone? It orbits roughly 115 AU from the star and is approximately 3 Jupiter masses in size. The inset image shows the different captures of the planet by Hubble, taken two years apart.
Of course, since this object has not cleared its orbit, I suppose it must actually be an exodwarfplanet.
I see that Ryan at The Martian Chronicles has already likened the photo to the Eye of Suaron, so Frodo might be a good name for something small sneaking by. Or Gollum, come to think of it.
So, here’s a question. How are planet sizes actually measured? I mean, you refer to this as “3 Jupiter masses in size” – is this how we do it when we have no proper viable measurements, refer to them in terms of scale? What about when we do have measurements, or can measure them? How do we gauge their size, and how should we refer to it? Do we just refer to a surface mileage, or is there a sort-of radius measurement? Also: Is there a tonnage for weight, or equivalent?
Thanks to the genius of Isaac Newton, we have fairly simple equations which can tell us the masses of planets very easily – so long as something orbits that planet. If a planet has a moon orbiting it then it becomes almost eerily easy to derive the mass of the planet if we know the period and radius of the moon’s orbit.
Mercury and Venus’ masses were only accurately derived when we sent spacecraft to orbit them – before that Venus’s orbit had to be derived by measuring the deformation of the Earth’s orbit when Venus passed by.
In the case of exoplanets (planets around other stars) it is easy to know their mass if we know the mass of the parent star – which we usually do. We can just take the radius and period of the exoplanet’s orbit and it again becomes trivial.
You may ask how we know the mass of stars. Binary stars are easy since again we have orbital mechanics to fall back on. Single stars have their masses derived by other characteristics. The evolution and structure of stars is very well understood and so in general we can know their ages if we can see their luminosity and temperature. This is nowhere near as precise as the way in which we know planetary masses but it is pretty reliable.