KIC 8462852 (dipping again in March 2018)

[stefan r]

Why?

Same reason you see Polaris as the north star.

 

[JMz]

A refinement: The face it presented would change its orientation only very slightly over a few years. (Remember, we have only a few years of observations of this star.)

Because of conservation of angular momentum: the same reason that the Earth's polar axis changes only slightly over a few years, completing a full circuit in 26,000 years ("precession of the equinoxes").

 

[Vanadium 50]

Same reason you see Polaris as the north star.

That didn't clarify anything. Why would a ringed system necessarily show the same face to us every orbit?

 

[stefan r]

That didn't clarify anything. Why would a ringed system necessarily show the same face to us every orbit?

sorry,

If Earth had an ring over the equator then astronomers on Polaris would always see the ring face on. The moon (and earth) would always be near quarter full. The polar ice cap would face Polaris year round but would stop reflecting in the winter when it got dark.

The pole star can be any angle. Uranus has Eta Ophiuchi as a pole star.

 

[Vanadium 50]

Right, but suppose your aliens lived on Regulus rather than Polaris. They would see Earth or Saturn - or better still, Uranus - transit the sun - why is the ring orientation necessarily constant?

 

[JMz]

Because, just like the planet, the rings have angular momentum, and angular momentum is conserved unless acted on by an external torque. What would you propose for the source of that torque?

 

[JMz]

To clarify (I hope): The rings would always appear face-on, whether or not they were between the Sun and that star (though they wouldn't be visible when they weren't occulting the Sun). They don't change orientation relative to the star as they orbit the Sun, just as the Earth's axis doesn't change orientation relative to Polaris or any other star over the course of a year.

 

[stefan r]

To clarify (I hope): The rings would always appear face-on, whether or not they were between the Sun and that star (though they wouldn't be visible when they weren't occulting the Sun). They don't change orientation relative to the star as they orbit the Sun, just as the Earth's axis doesn't change orientation relative to Polaris or any other star over the course of a year.

When Galileo first looked at Saturn he noticed that Saturn had ears. If the rings had been face on it would have looked like another sphere. Rings viewed off of axis might be better for explaining strange light curves.

 

[mfb]

Right, but suppose your aliens lived on Regulus rather than Polaris. They would see Earth or Saturn - or better still, Uranus - transit the sun - why is the ring orientation necessarily constant?

It is not*, but the stellar occultation always happens at the same point in the orbit. And at the same point of the orbit the orientation of a ring system would be constant over short timescales. If precession would be relevant over tens of orbits then the ring system should be extremely short-living.

*edit: I was wrong. It is, see two posts below, the same point in the orbit is not even necessary.

 

[JMz]

When Galileo first looked at Saturn he noticed that Saturn had ears. If the rings had been face on it would have looked like another sphere. Rings viewed off of axis might be better for explaining strange light curves.

The more tilted the rings are, the less they will occult. This hypothesis is already aiming for substantially more coverage than even Saturn's unusually large and dense rings can provide, even if they were oriented like Uranus's.

 

[DaveC426913]

That didn't clarify anything. Why would a ringed system necessarily show the same face to us every orbit?

Am I missing something here? Or are you? (I question myself because I know you're super smart.)

From a distant viewpoint a planet's axial tilt and ring system will always look the same, no matter where it is in its orbit, and no matter what year you look at it.

Planets are gyroscopes!

Saturn's axis and rings are likewise fixed relative to the stellar background. From outside our solar system, it too will always be seen at the same angle.

Like so: (but with rings)

It is only because we are in the solar system that we see Saturn from different angles, and therefore different orientations of its rings.

 

[mfb]

The more tilted the rings are, the less they will occult. This hypothesis is already aiming for substantially more coverage than even Saturn's unusually large and dense rings can provide, even if they were oriented like Uranus's.

We don't know if Saturn's rings are unusually large. J1407b probably has a ring system with 200 times the diameter of Saturn's rings. Easily large enough to obscure the whole star, leading to a massive (>90%) dip in brightness.
The duration and frequency of the dips in KIC 8462852 rule out a similar explanation there.

 

[Vanadium 50]

I now see what you're saying, and my problem is I wrote what I wrote, not what I meant. What I was imaging was a set of irregular rings, darker/thicker in spots, partially obscured by the planet. This would give you a kind of irregular periodicty.

 

[Birrabenzina]

This star is pretty interesting.
Has someone any link to some paper which analyzes this star in detail? Maybe it's a double system with a type Y or T brown dwarf

 

[JMz]

We don't know if Saturn's rings are unusually large. J1407b probably has a ring system with 200 times the diameter of Saturn's rings. Easily large enough to obscure the whole star, leading to a massive (>90%) dip in brightness.
The duration and frequency of the dips in KIC 8462852 rule out a similar explanation there.

A good point. For rings in the Solar System, big planets have rings -- but they're all insignificant (in blocking sunlight for distant observers) except for Saturn. My unstated hypothesis was that large, dense ring systems are very rare, and that the few we know of are known just because of a very strong observational selection effect.

But of course, even if that's true, this star could be one of those few -- after all, it's obviously a rare beast, one way or another.

 

[JMz]

I now see what you're saying, and my problem is I wrote what I wrote, not what I meant. What I was imaging was a set of irregular rings, darker/thicker in spots, partially obscured by the planet. This would give you a kind of irregular periodicty.

Got it -- sort of like Neptune's. My impression is that such rings would not be both large/dense and incomplete, except for a brief interval soon after formation. (And this star is not newly born.) But whether or not that's typical, we are dealing with an atypical system: All explanations so far are either poor fits to the data or improbable scenarios.

 

[stefan r]

This star is pretty interesting.
Has someone any link to some paper which analyzes this star in detail? Maybe it's a double system with a type Y or T brown dwarf

Here is the "official" web site. It has links to peer reviewed papers and also an ongoing blog with current data. The wikipedia entry is fairly good.

A brown dwarf would not eclipse 20% of a type-F main sequence star. It might block 1% or 2%. The light curve of a planet or brown dwarf transiting a star has a flat bottom. The flat bottom light curve is there if you toss a basketball in front of a movie projector. Moths in front of street lights also have flat bottom light curves. You could get a pointy light curve by throwing a basketball partially in front of a street light. The object has to be big.