## Kepler Spots "Perfectly Aligned" Alien Worlds 73

astroengine writes

*"When NASA's Kepler space telescope started finding planets at odd angles to their parent stars, scientists wondered if our solar system's tidy geometry, with the planets neatly orbiting around the sun's equator, was an exception to the rule. That idea can be laid to rest thanks to an innovative use of the Kepler data which aligned three planets circling the sun-like star Kepler-30 with a giant spot on the star's surface. 'The planets themselves are not all that remarkable — two giant Jupiters and one super-Earth — but what is remarkable is that they aligned so perfectly,' astronomer Drake Deming, with the University of Maryland, told Discovery News."*
## Re:I'd have assumed... (Score:5, Informative)

I'm a software engineer so my knowledge is rather limited, but I'd have assumed that the orbits of planets would tend to be in the same plane as the spinning of the galaxy, so if you look at a mostly flat galaxy, you'll find mostly aligned orbits, and if a galaxy was more... "chaotic", the orbits would be likewise less aligned in respect with each other.

Well, each solar system has an invariable plane [wikipedia.org] perpendicular to its aggregate angular momentum vector. The rotation of the star and the orbits of the planets tend to be quite close to this plane. The exception is when the orbits have been perturbed by passage of an object of significant mass at an angle to the plane. Note that the invariable plane of any particular solar system in a galaxy is not necessarily similar to that of the galaxy as a whole, although aggregated over all solar systems in the galaxy they are the same.

## Re:I'd have assumed... (Score:5, Informative)

I'd have assumed that the orbits of planets would tend to be in the same plane as the spinning of the galaxy

If you consider that the Milky Way is not aligned with the ecliptic [wikipedia.org] then you should see that the assumption is erroneous with respect to the only solar system we understand well.

## Re:I'd have assumed... (Score:5, Informative)

Disclaimer: IAAAA (I am an amateur astronomer).

The moon orbits at a mean inclination of 5.something degrees to the ecliptic. If it were synchronised to the equatorial plane we wouldn't have nearly as many eclipses (lunar or solar) as we do. In fact, we'd have about 1/50 the number.

## Re:mind-job, anyone? (Score:5, Informative)

HOWEVER, and here's the paradox: if not all stars have orbiting planets, then the number of stars with orbiting planets is less than infinite, ie, finite.

Bzzzt! Fail - thanks for playing.

Once you start playing with infinite numbers you have to be very careful with concepts like "less than" or "more than". Just because you can demonstrate that one set is in some way smaller than another infinite set does *not* demonstrate that the smaller one is finite.

Consider the set of natural numbers - 1,2,3,4 etc. This is infinite. Then consider the set of even natural numbers - 2,4,6,8 etc. Clearly there are members of the first set which are not members of the second set, and so one might be tempted to conclude that the second set is in some way smaller, and therefore by your argument, finite.

In fact one can set up a one-to-one mapping between the two sets:

12

24

36

etc. and thus both sets have precisely the same (infinite) number of members.

## Re:mind-job, anyone? (Score:4, Informative)

Once you start playing with infinite numbers you have to be very careful with concepts like "less than" or "more than".

Numberphile has a nice video about different types of infinity: http://www.numberphile.com/videos/countable_infinity.html [numberphile.com]

## Re:I'd have assumed... (Score:5, Informative)

Actually, I can.

Even assuming that the orbital inclination of both planets were identical, those orbits undergo what's known as apsidal precession. This is the movement of apoapsis (the furthest point of the orbit) around the primary over time - the orbit orbits. For Earth this varies between 100,000-400,000 years, and is tidally dependent. Ergo, the relative angle of inclination between planets changes and while Earth is below the ecliptic, the orbit of Venus at that point, even if the planet was in the same orbital position (ie between Earth and Sol), may be above the ecliptic hence no transit occurs.

This also happens with the Moon. Fortunately the systems are stable enough that we can predict when transits and eclipses will occur by concentrating only on dates and times where ascending and descending nodes of their orbits intersect the ecliptic. The great thing about the Moon is that it crosses the ecliptic exactly twenty six times per year, so we can predict with reasonable certainty from the off that solar and lunar eclipse patterns will repeat with fair precision every 6585 days, or every 223 Lunar orbits. (approximately 18 years).

## Re:I'd have assumed... (Score:5, Informative)

> those toy 2d simulators of gravity of stars and planets: start with a chaotic configuration, let it evolve for some time, and see how the stuff often evolves in one big central blob with possibly some circular orbits and short lived elliptical ones

The technical term is called

N-Bodysimulation/problem and is impossible to solve an exact solution with our current mathematics and computers where N > 3.http://en.wikipedia.org/wiki/N-body_simulation [wikipedia.org]

http://en.wikipedia.org/wiki/N-body_problem [wikipedia.org]

I found one demo here ...

http://michael.peopleofhonoronly.com/dev/nbody.html [peopleofhonoronly.com]

In order for the simulator to work properly, every simulator has to use a hack: gravity has infinite speed, which contradicts the mainstream assumption that gravity is limited to the speed of light.

Another anomaly is that the the galaxy "seems" to rotate in lock-step.

http://en.wikipedia.org/wiki/Galaxy_rotation_curve [wikipedia.org]

It "appears" the orbits of the planets also seem to be quantized.

Celestial Mechanics is not going to be "solved" anytime soon.

## Re:I'd have assumed... (Score:4, Informative)

Q for a physic-y person - The earth orbits around the sun's equator, but its own equator is at an angle to the sun-planet plane (hence, seasons). Does the moon, then orbit around earth's equator (at an angle to the sun), or in the same plane as the sun's equator (or some other plane entirely)?

First things first. There is no actual link between the equator of a body, and the orbits of the satellites of that body. The rotation of the planet, its orbital plane, the rotation of any moons or satellites of that planet, and the orbital planes of those moons are all independent from each other.

Now, having said that, if a satellite formed by being spun off of a quickly rotating object, or if both the satellite and the planet both coalesced from the same rotating blob of matter, then there will be a tendency for the satellite to be in the equatorial plane. However, once the satellite is no longer physically connected, the orbital plane or the planetary axis can both shift independently of each other (through perturbations from other bodies). If a satellite was formed by an asteroid collision knocking the planet apart, then there may be some relationship between the planet's original spin and the satellite's orbit. However, it also greatly depends on the speed and angle that the asteroid hit the planet with, and of course, the collision could drastically change the axis of rotation of the planet. If a satellite was captured by the planet's gravity, it could be in any orbital plane, and could even orbit completely opposite to the direction of rotation of the planet.

It takes a lot of energy to alter an orbital plane. So, just like the Earth spins and its axis is fixed in space, (pointing towards Polaris), so too the Earth's orbit is fixed in space. We call this the ecliptic. The ecliptic is inclined roughly 23.5 degrees from the plane of the Earth's equator. The equatorial plane of the sun is inclined roughly 7 degrees from the ecliptic. The plane of the lunar orbit is inclined at about 5 degrees from the ecliptic. Saturn's rings are along it's equator, and they are tipped 26.7 degrees. Uranus has its axis almost in the plane of the ecliptic at an inclination of 97 degrees. Venus's north pole is pointing south relative to ours (177.4 degrees).

Of course, having said all that, there may be tidal effects and relativistic effects that a spinning gravity well may impart on a satellite to coax it into the equatorial plane.