# Orbit of a double planet

In a system where 2 planets of similar mass are orbiting around a common barycenter, would the following configuration considered stable on the long run?

• They orbit at 1,6 AU from their star
• The star is a F8 and is 19% more massive than the Sun
• One planet is the same size and mass than Earth, but the other is 10% more massive
• They are located roughly at 90 000 km form their common barycenter
• They are tidally locked to each other
• Their double planet system is tilted at 25 degrees, so they can have seasons

Is it flawless?

Note that the concept is not mine (source) but I want to know if it works.

• It seems fine, at a glance. The star stats are typical, the orbital radius is fine, the masses aren't unnatural, the semi-major axis is okay, the tilt is plausible, and tidal locking is possible in this setup. I think the answer is a simple "yes". I can't speak for whether or not some of the calculated numbers in the link work out, but all this seems fine. – HDE 226868 Aug 2 '15 at 19:23

Okay, let's look at this:

We have a yellow-white F8 star somewhat larger than our sun, with the planets orbiting at 1.6 AU. This is in the correct ballpark for the worlds to be in the habitable zone of such a star. However, please note the somewhat higher UV output of the star.

A point to consider is that during the spring and autumn equinoxes, there could be repeated solar eclipses over the course of a number of local days - which, given the mass of the larger world as 7.1768*10^24kg (being 1.2 times that of Earth at 5.98×10^24kg) and the distance of 180,000km, would be 8d 0h 37m 5.59s. If I'm wrong, and the orbital separation is 90,000, the orbital period/day length would be 2d 20h 6m 2.97s. The latter would be more hospitable, though either way, there would be a large difference between day-time and night-time temperatures either way. Given the details given in http://www.cartographersguild.com/showthread.php?t=28951, it would appear that a day length of 62h and an orbital separation of 90,000km is the correct figure. Using the stated planetary mass from that page and the larger world's geosynchronous orbital radius of 89,238km, we get an orbital period/day length of 2d 14h 6m 6.86s. This is around 6h different from the value obtained using the figures in the OP's question so please note the importance of exact figures.

For the planets to be tidally locked at 180000km separation is also plausible, and even more so at 90,000km, given their mass. That their plane of common orbit is tilted 25° could be a problem - the sun 's gravity would be trying to pull this back into the plane of the ecliptic, but it isn't beyond the bounds of possibility.

• True but the more rapid the orbit, I think the more rapid the orbit the more of an effect the solar tide will have. On the other hand, the brighter star means they'll be further away and tides vary by $\frac{1}{r^3}$. – Jim2B Aug 3 '15 at 18:17