Weird Worlds: Kepler 16

I have talked about the Kepler exoplanet-finding mission in the past (planet spotting, Kepler 11, KOI 730). Today I'm going to look at a planet Kepler has found in a binary star system: Kepler 16(AB)b and next week I'll talk about Kepler 14b which is also in a binary system but in a different configuration.

A binary star system is where two stars are close enough together to be in orbit around each other, instead of individually orbiting the centre of the galaxy. Instead they orbit the centre of the galaxy together, much the same way as the Earth and the moon orbit the sun together. Also sort of like on Tatooine from Star Wars.

Kepler 16b

Artist's impression of the Kepler 16 system.
Credit: NASA/JPL-Caltech/R. Hurt

Basic stats for the Kepler 16 system are somewhat unartistically presented here. I'll go over some of them below.

As far as we know, there are three bodies in the Kepler 16 system: the two stars which are designated Kepler 16A and B (and together Kepler 16(AB). As per usual convention, the more massive star will be A while the smaller will be B. The usual convention, for non-binary systems, is that the star is designated Whatever-a and planets in order of discovery then mass (so mass when several are discovered simultaneously) are designated Whatever-b, Whatever-c etc. The lower-case a for the stars is a bit redundant here, though. Masses and distances for the system are as follows:

• Star A:
• Mass: 0.69 solar masses
• Size: 0.65 solar radii
• Temperature: 4450 Kelvin (about 4180º C or 7600º F)
• Star B:
• Mass: 0.20 solar masses
• Size: 0.23 solar radii
• Stars AB together:
• Orbital period: 41 days
• Orbital separation: 0.224 AU (for comparison, Mercury's orbit is 0.387 AU from the sun)
• Planet Kepler 16(AB)b:
• Mass: 0.33 Jovian masses
• Size: 0.75 Jovian radii
• Orbital period: 229 days
• Orbital radius: 0.70 AU (Venus is 0.72 AU from the sun)

It's not possible to separate out and measure the temperature of star B because it is completely overwhelmed by the light from star A. We can only make guesses based on it's spectral type which is determined from its chemical make up. Also note that the distance measurement for the planet is from the centre of mass of the two stars -- the barycentre.

For a bit of fun, let's work out how big and bright each star would be from the planet, relative to the sun. I should note that this is a gas giant planet and so any possible light would be more likely to exist on one of its moons, not the planet. The suns would appear the same size from a moon, though.

First, how big would the suns appear? (See this post for details on the calculation.) Remember for comparison that the sun (and moon) have an angular diameter of about 0.5º. So, sizes:

• Star A as seen from planet on average: 0.49º, so about the same as the sun
•  Range: 0.43º to 0.59º
• Star B as seen from planet on average: 0.18º, which is about a third of the diameter of the sun
• Range: 0.15º to 0.21º

Now, how much light would reach the planet from the two suns? Or how brightly would the stars illuminate the planet? This is a slightly more complicated calculation which you can read about here (and for comparative purposes, about 1400 Joules of energy hit each square metre of the Earth from the sun each second). Remember, also, that sometimes star B will pass behind star A, sometimes they will appear next to each other and sometimes B will be in front of A, blocking out some of A's light but contributing its own. I'm just going to calculate them separately.

• Star A: 410 Joules per square metre per second, which is a bit less than a third of the light energy the Earth gets from the sun. It is also going to be redder light, thanks to the cooler temperature of the star.
• Star B: assuming its temperature is about 2500 K which is within the plausible range for M type stars (star A is K type, if you're curious), I get about 5 Joules. That's 3-4% of the light from the sun hitting Earth. On the other hand, it's still a lot brighter than the full moon in Earth's sky (and significantly redder). About 1400 times brighter, in fact. And about as bright as 260 100 Watt light bulbs at a distance of 10 m. Except it would probably feel dimmer thanks to the redness.

Finally, let's suppose that there is an Earth-like moon of this gas giant planet Kepler 16b. How warm would the suns make it? The calculations are discussed in this post on the habitable zone. Since most of the warming comes from star A, I'm going to ignore star B for this calculation. The average temperature I get is about 190 K which is about -85º C or -121ºF. So a little bit too cold for life, but not too hot, which is a harder problem to solve. Maybe throw in a bit of greenhouse warming and a lot of snow gear and you have yourself a snowball planet with two suns. Not just cold, but pretty cool.

And if you want to read more, here is a nice New Scientist article about it. And, below, a video from APOD: