Found: Planets skimming a star's surface

Oct 11, 2013

A new planet-hunting survey has revealed planetary candidates with orbital periods as short as four hours and so close to their host stars that they are nearly skimming the stellar surface. If confirmed, these candidates would be among the closest planets to their stars discovered so far. Brian Jackson of the Carnegie Institution for Science's Department of Terrestrial Magnetism presented his team's findings, which are based on data from NASA's Kepler mission, at the American Astronomical Society's Division for Planetary Sciences meeting.

Most gas giant exoplanets with orbital periods less than or equal to a few days are unstable. This is due to decay in their orbits caused by the effects of their star's proximity. For rocky or icy planets, this disruption could bring them close enough to the star that the force of their own gravity can no longer hold them together in the face of the star's gravity.

Motivated by these considerations, Jackson's team conducted a search for very short-period transiting objects in the publicly available Kepler dataset. Their preliminary survey revealed about a half dozen planetary candidates, all with periods less than 12 hours. Even with masses of only a few times that of Earth, the short periods mean they might be detectable by currently operating ground-based facilities.

If confirmed, these planets would be among the shortest-period planets ever discovered, and if common, such planets would be particularly amenable to discovery by the planned TESS mission, which will look for, among other things, short-period rocky .

In his presentation, Jackson described the survey, what has been learned about the candidates from Kepler data, and the team's plans for follow-up observations.

The team includes Carnegie's Christopher Stark and Alan Boss, as well as Elisabeth Adams of the Planetary Science Institute, Michael Endl of the McDonald Observatory, Phil Arras of the University of Virginia, and Drake Deming of the University of Maryland at College Park.

Explore further: Spectacular supernova's mysteries revealed

Related Stories

Kepler mission discovers 461 new planet candidates

Jan 08, 2013

(Phys.org)—NASA's Kepler mission Monday announced the discovery of 461 new planet candidates. Four of the potential new planets are less than twice the size of Earth and orbit in their sun's "habitable ...

41 new transiting planets in Kepler field of view

Aug 27, 2012

(Phys.org)—Two newly submitted studies verify 41 new transiting planets in 20 star systems. These results may increase the number of Kepler's confirmed planets by more than 50 percent: to 116 planets hosted ...

How common are earths around small stars?

Jun 03, 2013

(Phys.org) —The Kepler mission has revolutionized the study of exoplanet statistics by increasing the number of known extrasolar planets and planet candidates by a factor of five, and by discovering systems ...

Recommended for you

Spectacular supernova's mysteries revealed

Aug 22, 2014

(Phys.org) —New research by a team of UK and European-based astronomers is helping to solve the mystery of what caused a spectacular supernova in a galaxy 11 million light years away, seen earlier this ...

Supernova seen in two lights

Aug 22, 2014

(Phys.org) —The destructive results of a mighty supernova explosion reveal themselves in a delicate blend of infrared and X-ray light, as seen in this image from NASA's Spitzer Space Telescope and Chandra ...

Toothpaste fluorine formed in stars

Aug 21, 2014

The fluorine that is found in products such as toothpaste was likely formed billions of years ago in now dead stars of the same type as our sun. This has been shown by astronomers at Lund University in Sweden, ...

Swirling electrons in the whirlpool galaxy

Aug 20, 2014

The whirlpool galaxy Messier 51 (M51) is seen from a distance of approximately 30 million light years. This galaxy appears almost face-on and displays a beautiful system of spiral arms.

User comments : 52

Adjust slider to filter visible comments by rank

Display comments: newest first

katesisco
1 / 5 (13) Oct 11, 2013
Looking for Earth?
Rocky bodies around a star inside the gas giants?
Perhaps like Mercury?
GSwift7
4 / 5 (4) Oct 11, 2013
with orbital periods as short as four hours and so close to their host stars that they are nearly skimming the stellar surface


Especially if the orbits are significantly eliptical.

Imagine standing on the dark side of such a planet. You would probably be able to feel the tide change throughout the orbital elipse.
GSwift7
4.5 / 5 (8) Oct 11, 2013
Looking for Earth?


No, they were looking for planets of any size or type that orbit VERY close to their star.

Rocky bodies around a star inside the gas giants?


No, this has nothing to do with what is inside a gas giant.

Perhaps like Mercury?


No, they are looking for planets that orbit MUCH closer than Mercury.

The planets they are looking for are either inside the Roche limit or very close to it. The Roche limit is the point at which the gravity of the larger object will slowly tear apart a smaller object. This happens becuase the star will pull more on the near side than the far side of the planet. It starts as an egg-shaped bulge in the planet, on the side facing the star (the planet will be tidally locked). The egg-shaped bulge will actually feed the process and make itself even worse. The more it bulges toward the star, the more the star pulls on that side. Until it comes apart. Finding a planet in this process, or its remains, would be awesome
Q-Star
4.6 / 5 (9) Oct 11, 2013
Finding a planet in this process, or its remains, would be awesome


Truly bizarre, eh? And a grand testimony to the technology and techniques developed these last few years.
HannesAlfven
1 / 5 (15) Oct 11, 2013
It's interesting that mainstream astrophysicists have yet to latch onto the very appealing notion of rocky bodies rotating around particularly cool brown dwarfs within the illuminated, electrical envelope. Note that the planet's surface would always be lit on all sides; there would be no day/night dichotomy. It appears a bit striking to a number of us that this would be the ideal location for the origin of life in the universe, if it can be shown that the temperature is sometimes at the right point and stable. One wonders if Wal Thornhill will be given any credit for the idea when they finally come to realize it, as it's a topic which he discusses in his book, "The Electric Universe".
GSwift7
4.2 / 5 (5) Oct 11, 2013
You know what a mess that would make in a solar system?

If a planet were torn apart, orbiting very close to the star, and moving really fast, parts of the planet would be flung into the star and other parts would be hurled out into the system. Conservation of angular momentum allows the outbound debris to attain crazy velocity. You would have a 'spray' of material looping out into the solar system along the ecliptic, right where all the planets are, in all kinds of crazy elipses. Every planet in the system would get massive bombardments. I would bet that every planet in the system would get sterilized by thousands of years of large impacts, and secondary impacts from the debris of those impacts.

Truly bizarre, eh?


Yeah, and one of a very limited number of ways to actually see what the inside of a planet is made of.
rkolter
5 / 5 (5) Oct 11, 2013
It's interesting that mainstream astrophysicists have yet to latch onto the very appealing notion of rocky bodies rotating around particularly cool brown dwarfs within the illuminated, electrical envelope.


You do realize that a planet revolving (not rotating) around inside or that excessively near a star (even a brown dwarf) would heat due to friction, have it's atmosphere stripped away and in any case have a quickly degrading orbit?
antialias_physorg
2 / 5 (3) Oct 11, 2013
and other parts would be hurled out into the system

Not sure. What force did you have in mind that would fling anything outwards? Especially very close to the star I'd suspect all of the debris would eventually fall int oit.
Jonseer
1.4 / 5 (21) Oct 12, 2013
This research produces nothing of value to humans, and it's results no matter how profound are of little consequence.

What a waste of brainpower and resources.

It's high time societies demand astronomers get funding only for research that has some clear benefit.

Imagine if these geniuses were forced to build on that tech. and create things that actually improve the lives of lowly humans what great things would be accomplished.

Clearly even the great minds need boundaries. Left to their own devices they've created a field that produces nothing of value, but does allow them to call playing full time a career.

Oh and they don't get credit for the tech. to build the equipment either.

That is a product of the useful fields of science that are dedicated to improving the lot of life on planet Earth.

In all ways astronomical research like this is parasitic drawing resources away from fields that do good things for all for use by astrophysicists to do good for themselves.
antialias_physorg
4 / 5 (13) Oct 12, 2013
This research produces nothing of value to humans

You mean survival of humanity is not of value to humans.
You have a very different idea about what is valuable than everyone else on the planet.


It's high time societies demand astronomers get funding only for research that has some clear benefit.

No. It is high time that the education system gets more funding so that it is capable of teaching to people like you that there are things of benefit in astronomy. A task at which it has clearly failed.


Clearly even the great minds need boundaries.

No. Great minds need to be protected from people like you (i.e dumbshit 'managers' who would try to 'steer' intelligence). The world needs intelligent people. The world doesn't need people like you.
What have you ever discovered? Diddly squat. If people were like you we'sd still be living in trees.
Jonseer
1.4 / 5 (20) Oct 12, 2013
antialias_physorg

I'd say a reply like yours proves my point beyond a doubt.

You obviously consider yourself a strong supporter and a huge fan of astronomy.

Yet you can't make one valid, clear statement spelling out any benefit to humanity from this sort of pointless research that really is nothing more than genius imaginations interpreting data for fun.

The best you can do is a vague inference that people who do enjoy astronomy know of some benefit, hmmm.

The world needs it's most intelligent individuals to do things that help everyone.

The world doesn't need its most intelligent people walling themselves off so they can play 24/7 speculating about data the reality of which we will never come closer to than #s on a page.

By the way, what have you discovered lately?
alfie_null
3.4 / 5 (5) Oct 12, 2013
Imagine if these geniuses were forced to build on that tech. and create things that actually improve the lives of lowly humans what great things would be accomplished.

That's a steep slippery slope you've slid down. I suggest you don't not remember the past - perhaps history being another "useless" area of endeavor in your opinion.
Clearly even the great minds need boundaries. Left to their own devices they've created a field that produces nothing of value, but does allow them to call playing full time a career.

Oh and they don't get credit for the tech. to build the equipment either.

In all ways astronomical research like this is parasitic drawing resources away from fields that do good things for all for use by astrophysicists to do good for themselves.

You really have it in for astronomers, don't you?. You must have been burned badly sometime in the past.
TheGhostofOtto1923
2.6 / 5 (10) Oct 12, 2013
Yet you can't make one valid, clear statement spelling out any benefit to humanity from this sort of pointless research that really is nothing more than genius imaginations interpreting data for fun
The compulsion of any species is to explore its environment to the limits of its senses in order to ascertain any threats or benefits to its existence. Our senses now extend without limit. We have determined that the universe is an extremely dangerous place. We can see stars exploding and planets being devoured. Is there anything out there which could cause this to happen here?

We observe the record of violence within our own system. An asteroid recently exploded over russia, did you miss that? We dont know how variable our own star is. We dont know if ice ages or global warming events are caused by solar fluctuations.

Sciences like astronomy arent just prudent - they are a biological imperative. We need to know what is going on around us. Not exploring is not an option.
antialias_physorg
3 / 5 (4) Oct 13, 2013
You obviously consider yourself a strong supporter and a huge fan of astronomy.

I'm merely an opponent of willful ignorance.

Yet you can't make one valid, clear statement spelling out any benefit to humanity

As with much basic research the immediate benefit may not be apparent. Even Einstein thought the laser effect was merely a curiosity.
Ideas how stuff may be used come about in a certain way: You collect data in all kinds of fields and then, at some point, someone has an "a-ha" moment and makes something off it. And the ideas often come from widely diverging fields. You don't get innovation unless there is knowledge.

By the way, what have you discovered lately?

A better way to semi-automatically categorize osteoarthritic damage in the knee. It's not a world-shaking discovery, but it utilizes a structure in the knee that no one has been able to segment before.

On secondthoughtthinkagain
1 / 5 (14) Oct 13, 2013
In all ways astronomical research like this is parasitic drawing resources away from fields that do good things for all for use by astrophysicists to do good for themselves.


This may be partially true but extends to all scientific endevours not just astronomers. But the fact is that a wealthy society supports more and more parasitic past-times.

We start off just surviving then with enough leisure we support a parasitic doctor and/or leader then with more leisure time we support tool makers and cooks then weavers and designers and eventually people that just sit around and observe things.

These observers eventually became scientists and some of these astronomers.
On secondthoughtthinkagain
1 / 5 (11) Oct 13, 2013
As with all science they are parasitic to an extent on society as a whole - at least for a time. Eventually at some future time something beneficial may come of it and then they are no longer classed as a parasite. There are some groups of society that can never lead to any benefit and so remain a parasite on the host society.

I could name a whole class of people that never contribute anything of worth and yet consume enormous resources and they are not scientists. Historically though the class of people I am thinking of were quite useful in providing a framework within which scientists and others could work. After-all not all members of an otherwise useless class of people were content to just bludge off their society forever justifying their existence by maintaining the status quo.
IMP-9
5 / 5 (4) Oct 14, 2013
Yet you can't make one valid, clear statement spelling out any benefit to humanity from this sort of pointless research that really is nothing more than genius imaginations interpreting data for fun.


Never heard of spin off? The X-ray mirrors from XMM-Newton went to better medical imaging for small targets, Chandra's detectors had similar impact. Millimeter detectors designed for CMBR work were the basis for new airport scanners and new mammogram technology. Adaptive Optics leads to better diagnostics of the eye and surgery. I for one know a plasma astronomer (a real one not an EU one) who developed devices for both medical use and commercial application... The examples are there if you take the time to look. Astronomy drives industry which drives innovation leading to growth. Attempting to ignore equipment is totally and completely false, without the researcher the mission wouldn't fly.
GSwift7
3 / 5 (2) Oct 14, 2013
anti_alias:

Not sure. What force did you have in mind that would fling anything outwards? Especially very close to the star I'd suspect all of the debris would eventually fall int oit


Just simple transfer of momentum. The planet wouldn't be encountering friction from the star's atmosphere yet, so anything that falls into the star must trade angular momentum with something else in the debris. There should be a lot of pieces of debris spinning around each other gravitationally in the debris cloud. It's kinda like when you throw a partially full bottle of water into the air, spinning end over end. It will wobble. A pair of objects (it could be many objects, but I'm using a pair as a simple example), bound only by mutual gravity would trade momentum as they 'wobble' like the water bottle. Any change in the orbit of the heavier object would result in an equal but opposit change to the smaller, in proportion to their relative masses.
GSwift7
3 / 5 (2) Oct 14, 2013
continued:

This is exatly like using a planet for a gravitational slingshot to change the orbits of our spacecraft.

The spacecraft can either gain or lose angular momentum from the planet (yes they can use them to slow down as well as speed up). There is actually an equal but opposite effect on the planet too, but due to the enormous difference in mass, the effect on the planet is unmeasurably small.

Now image a plant-sized debris field, with countless numbers of objects circling eachother, and giving each other gravitational assists, just like our spacecraft. Some slow down and fall into the star, while their counterparts speed up by a proportional amount and get flung outward. There's no other way to explain any objects falling into the star, since there wouldn't be any friction. This trade-off of angular momentum is part of the mechanism that rips the planet apart in the first place, as the outside half of the planet's mass tries to outrun the inside half, there's a shear force.
antialias_physorg
1 / 5 (1) Oct 14, 2013
Just simple transfer of momentum.

If you're thinking slingshot effects: That doesn't work at a star (no matter what star trek says). It only works with objects orbiting a star since you're taking away orbital velocity to boost your craft's speed. The star of a solar system doesn't have an orbital velocity (unless it is a binary, of course).

The planet itself has a potential energy and that's it. No part of that planet will be able to get further out. Anything being ripped away will be on the side facing the star (as it is there where the gradient is largest) and it's not moving relative to one another - so no slingshot effect there, either.

Eventually you'll get a debris ring - and as close as this one is to the star there's already 'solar atmsophere' friction to contend with. In the end the stuff will just drop in.
Mr_Science
1 / 5 (10) Oct 14, 2013
Would adding such mass to the star cause it to go supernova? If not, how many more planets would the start need to eat? Is it even conceivably possible? I have not looked up the answers to these questions and I don't really expect an answer. Just food for thought.
antialias_physorg
3.7 / 5 (3) Oct 14, 2013
Would adding such mass to the star cause it to go supernova?

No
If not, how many more planets would the start need to eat?

Quite a few. Consider that our sun comprises about 99.8% of the mass of our solar system. You could throw everything else into it and it wouldn't change noticeably.
GSwift7
1 / 5 (1) Oct 14, 2013
That doesn't work at a star (no matter what star trek says). It only works with objects orbiting a star since you're taking away orbital velocity to boost your craft's speed. The star of a solar system doesn't have an orbital velocity


I'm guessing that my post was too long, so you didn't read it?

Nothing is using the star for a slingshot. As you correctly pointed out, that that's impossible. The pieces of debris will slingshot each other, especially right after the planet breaks apart, while the debris cloud is still bunched up together.

and as close as this one is to the star there's already 'solar atmsophere' friction to contend with. In the end the stuff will just drop in.


A miniscule amount of friction compared to the inertia of the mass involved. Gravitational interaction betwen the debris pieces themselves will happen long before friction slows them down significantly.

Our asteroid belt shows this, where there's not enough friction to do the job.
GSwift7
1 / 5 (1) Oct 14, 2013
Would adding such mass to the star cause it to go supernova? If not, how many more planets would the start need to eat? Is it even conceivably possible?


Adding mass isn't what causes a supernova. They only go supernova when they run out of fusion fuel, and that's only for stars several times more massive than ours. To get Sol to supernova, you would need to add several more solar masses, and then wait for its fusion reactions to become too weak to fight off the pull of gravity. Then it can collapse and blow up.
Q-Star
4.3 / 5 (6) Oct 14, 2013
Would adding such mass to the star cause it to go supernova?


Stars go supernova because they can no longer fuse lighter elements into heavier elements, producing thermal energy to counter the gravitational collapsing.

If not, how many more planets would the start need to eat? Is it even conceivably possible? I have not looked up the answers to these questions and I don't really expect an answer. Just food for thought.


White dwarfs can go supernova by accrediting material from a nearby object, the amount of material necessary is roughly the same for all type I a supernovae, and since all white dwarfs are found in a narrow size range, they make for very good standard candles for determining distances.

Q-Star
4 / 5 (4) Oct 14, 2013
Adding mass isn't what causes a supernova. They only go supernova when they run out of fusion fuel, and that's only for stars several times more massive than ours.


Not so for the supernovae we love the most, the Sn Ia, they are made from already exhausted stars, the white dwarfs.
TheGhostofOtto1923
1.8 / 5 (5) Oct 14, 2013
What force did you have in mind that would fling anything outwards?
Perhaps it would work something like this:

"In this visualization, some of the murdered star's debris falls into the void, but some is spun up and shot out at high velocities. Credit: NASA, S. Gezari (The Johns Hopkins University), and J. Guillochon (University of California, Santa Cruz)"
http://www.youtub...Hf6I9P5A
Q-Star
4.4 / 5 (7) Oct 14, 2013
What force did you have in mind that would fling anything outwards?
Perhaps it would work something like this:

"In this visualization, some of the murdered star's debris falls into the void,


Oh the humanity,,,, what has it come to that we allow stars to be wantonly murdered, how can we stand by while such depravity is occurring?
TheGhostofOtto1923
1 / 5 (3) Oct 14, 2013
What force did you have in mind that would fling anything outwards?
Perhaps it would work something like this:

"In this visualization, some of the murdered star's debris falls into the void,


Oh the humanity,,,, what has it come to that we allow stars to be wantonly murdered, how can we stand by while such depravity is occurring?
That wordage is right from the space.com website. Perhaps you would prefer devoured? Masticated? Splooshed? Chomped up and crapped out?

Which?

The point being, that some sort of mechanism is responsible for ejecting material by 'spinning it up', and I doubt that it is specific to black hole/star collisions. You all can figure out what that is.
Q-Star
4.3 / 5 (6) Oct 14, 2013
The point being, that some sort of mechanism is responsible for ejecting material by 'spinning it up', and I doubt that it is specific to black hole/star collisions. You all can figure out what that is.


It was a very good point,,,,,, But I was laughing at the NASA guy's choice of the word murder describing it. I would have used "destroyed" or "shredded" before I used the word "murdered". I would think that murder should be reserved for things involving malice and forethought.
antialias_physorg
2.3 / 5 (3) Oct 15, 2013
The pieces of debris will slingshot each other

As I said: that only works if the pieces have relative orbital velocities to each other (which they don't, since they are all part of the same planet initially).
For this to work they would have to:
a) move away from the main body and then
b) move back to the main body to get slingshotted around

the a) part does happen (as the stuff gets drawn closer to the star it goes into a deeper orbit -even less potential energy- and then you get a debris ring) - but the b) part doesn't happen.

There's no big danger for the solar system. It's a bit comparable with the rings of Saturn. There's really not a lot being ejected out into space by the motion of the moons close to it.
TheGhostofOtto1923
1 / 5 (4) Oct 15, 2013
Ah geez AA is trying to do engineering again. Did you watch the simulation AA? What is your ass-born explanation for all that matter flung out into the Weltall when, by your special brand of reasoning, none of it should escape?

I'm sure the answer can be found on the Internet or in a formal education in celestial mechanics.
no fate
3 / 5 (6) Oct 15, 2013
IMP-9: I couldn't have said it better. Spin offs are the basis for modern society's technical foundation. I also know a Plasma physicist who has developed some really mind blowing medical tech, it seems the fields support each other nicely.

Jonseer: Targeting scientists as the entity who is comsuming money better spent elsewhere? I can think of several completely frivolous expenditures that humanity could do without, which would put alot more money into the worlds problems than mothballing any scientific research.
GSwift7
1 / 5 (1) Oct 15, 2013
antialias:

I think you're missing part of the dynamic that happens when an object is torn apart by gravity.

Your points a) and b) are both satisfied in this situation. As a planet begins to break apart the debris chunks will begin a spiral retrograde rotation. The inside lanes of material will attempt to move ahead of the outside lanes of material. However, that material is still gravitationally attracted. So, as if you had runners on circular track, with each runner trying to stay in their own lane, running at the same speed. The inside runners would move ahead due to their shorter path. But gravity is like being tied together with rubber bands, so the inside runners begin to be pulled back and out, while the outside runners are getting pulled forward and in. Individual pieces will whip around each other like rolerskaters holding hands around a corner.

The wiki on Roche limit has a nice diagram of how this starts but they stop before showing the rotation that follows.
Fleetfoot
5 / 5 (1) Oct 15, 2013
As a planet begins to break apart the debris chunks will begin a spiral retrograde rotation.


Typo? That should be prograde, your next line is right:

The inside lanes of material will attempt to move ahead of the outside lanes of material.


That's the correct direction but the lost material moves ahead of the decaying planet feeding a growing ring system. Friction or viscosity in the ring reduces internal velocities so minimal amounts will get ejected by slingshots so the question is how much does the passage of the slower planet clear material from the outer edge of the ring while also feeding it.

The wiki on Roche limit has a nice diagram of how this starts but they stop before showing the rotation that follows.


It shows material bleeding off the sun-side edge moving ahead as you say (though it also incorrectly shows the rear side crumbling). The next image correctly shows the final resulting debris ring.
TheGhostofOtto1923
1 / 5 (4) Oct 15, 2013
@G

Again, speculation is great (no it's not) but the star in the simulation as well as the atmospheres of gas giants are not comprised of chunks.

A description of the sim might be on the net somewhere or maybe an email to it's creators will give you your answer. Maybe tidal forces or rotation of the doomed object.
GSwift7
1 / 5 (1) Oct 16, 2013
as well as the atmospheres of gas giants are not comprised of chunks


I was talking about what happens when a rocky planet has this happen, despite the fact that the story above is talking about a gas giant. The whole process is different with a gas giant because the planet acts as a fluid. There's a completely different set of equations for a fluid.

Typo? That should be prograde


no, retrograde. The debris will spin in the opposite direction of the orbit.

but the lost material moves ahead of the decaying planet feeding a growing ring system


It rotates around itself in an elipse that slowly elongates up and down the path of the original orbit (and flattens). Until the debris is scattered sifficiently, it still has a common center of gravity. You're oversimplifying. This is a many-body system.

though it also incorrectly shows the rear side crumbling


It should be stretched in both directions, similar to tides here on Earth, but more complicated.
GSwift7
3 / 5 (2) Oct 16, 2013
When an object enters the Roche limit and gets torn apart it's almost the exact opposite of what happens when material accretes to form an object. This is why all the planets rotate in the same direction as their orbits. Conservation of angular momentum causes the rotation as the material is pulled in from the near and far side. When an object comes apart, the opposite happens, and the debris will spin opposite of the direction a planet would normally spin. Additionally, the pieces don't leave directly. Just like the opposite of objects falling in to form a planet, where they get pulled into tightening elipses, pieces leaving will travel in widening elipses until they break free of the gravitational influence of the debris field's center of mass.
Fleetfoot
not rated yet Oct 16, 2013
I was talking about what happens when a rocky planet has this happen, .. The whole process is different with a gas giant because the planet acts as a fluid.


You are forgetting how close the planet is to the star, the surface will be molten and some will boil replenishing an "atmosphere".

The debris will spin in the opposite direction of the orbit.


Ah right, but it orbits prograde faster than the planet. However, friction with the atmosphere and ring material will dissipate the spin.

It rotates around itself in an elipse that slowly elongates up and down the path of the original orbit (and flattens). Until the debris is scattered sifficiently, it still has a common center of gravity. You're oversimplifying. This is a many-body system.


You are thinking the material comes off in chunks, I see it as a steady but variable flow which slowly adds all round the disc. The dynamics will be complex but overall will be a gradual disintegration into a ring syste
Fleetfoot
not rated yet Oct 16, 2013
though it also incorrectly shows the rear side crumbling


It should be stretched in both directions, similar to tides here on Earth, but more complicated.


Tidal forces are inverse cube so the bulge on the far side will be lower than that on the near side. There will be atmospheric leakage but the surface will most likely be sound. The shape will be more like a pear with maximum loss at the stalk.

Additionally, the pieces don't leave directly. Just like the opposite of objects falling in to form a planet, where they get pulled into tightening elipses, pieces leaving will travel in widening elipses until they break free of the gravitational influence of the debris field's center of mass.


I understand your thinking but that won't happen. When material falls in, it has to lose angular (orbital) momentum. When leaving, nothing accelerates it, the planet will be tidally locked, so it can only flow smoothly off the star-facing side at the top of the atmosphere.
GSwift7
1 / 5 (1) Oct 16, 2013
You are thinking the material comes off in chunks, I see it as a steady but variable flow which slowly adds all round the disc. The dynamics will be complex but overall will be a gradual disintegration into a ring syste


I've done more reading, and it seems that we are both right, in regard to whether it comes apart in chunks or just disolves away, depending on the composition of the body. If the body has enough rigidity or viscosity, then it can hold itself together longer, which allows a sudden catastrophic failure.

In the case of objects with no mechanical resistance, the tidal force will exactly cancel the surface gravity on both sides and loose material like atmosphere will drift away.

For objects in between those two examples, it should ooze and flow apart like wax in a lava lamp.

Here's a cool paper on the subject:

http://keith.aa.w...2001.pdf

Make sure to read section 6, which uses Phobos as an example. Spin rate even matters for locked body
GSwift7
1 / 5 (1) Oct 16, 2013
When leaving, nothing accelerates it, the planet will be tidally locked, so it can only flow smoothly off the star-facing side at the top of the atmosphere


Nope, that's completely wrong.

Tidal force pulls in both directions along the axis of force, not just toward the center of gravity. Read the paper I linked to, or check out a wiki on tides. It's counter-intuitive, but a stong tide canceles the gravity on both sides of an affected object, in opposite directions. Our own oceans have a high tide both on the side facing the moon and opposite to it.

Notice the section of the linked paper which specifically says that the surface of Phobos will lose loose material from both sides when it reaches the point where the tide cancels surface gravity.

It's interesting that tides don't pull directly toward the center of gravity either. The linked paper says 30 degrees is a typical angle for the shearing force which is perpendicular to tidal force.
GSwift7
1 / 5 (1) Oct 16, 2013
P.S. I've been really enjoying this topic. This is a really complicated subject, and there are several long-standing unanswered questions about this dynamic.
Fleetfoot
not rated yet Oct 16, 2013
If the body has enough rigidity or viscosity, then it can hold itself together longer, which allows a sudden catastrophic failure.


Yes, that sounds reasonable but it also depends on the size, small rocks have more relative strength than a planet. However I was thinking about this particular case where it is "skimming the surface" of the star.

For objects in between those two examples, it should ooze and flow apart like wax in a lava lamp.


Exactly, no matter how rocky, it will be lava.

Here's a cool paper on the subject:

http://keith.aa.w...2001.pdf

Make sure to read section 6, which uses Phobos as an example. Spin rate even matters for locked body


Too late tonight but it's downloaded to go on the Kindle, thanks for the link, it looks interesting at first glance.
TheGhostofOtto1923
1.7 / 5 (3) Oct 16, 2013
Here is one example of scientific speculation, in this case the formation of 2 earth-sized planets from the tidal destruction of a gas giant:

"The massive planet reached the tidal destruction radius (eq. 1). The gaseous mass of the
planet was lost and part of it formed a temporary accretion disk around the core, that now
is the EHB star. The metallic core of the massive planet was tidally destructed into several
Earth-like bodies immediately after the gaseous envelope was removed. Different bodies
had different energy per unit mass. Some of them spiral-in and were further destructed
by the core, while other survived at orbital separations of & 1R⊙ within the gaseous disk.
The bodies interacted with the disk and among themselves and migrated, as planets around
young stars do. Two of the bodies survived and reached an almost resonance. These are the
observed Earth-like planets."
http://arxiv.org/...68v1.pdf

-The core didnt have time to change state before fragmentation.
Fleetfoot
5 / 5 (1) Oct 16, 2013
When leaving, nothing accelerates it, the planet will be tidally locked, so it can only flow smoothly off the star-facing side at the top of the atmosphere


Nope, that's completely wrong.

.. Our own oceans have a high tide both on the side facing the moon and opposite to it.


Don't worry, I'm well aware of that but I was thinking of the unusual case where the diameter of the smaller body is within an order of magnitude of the separation. On further thought, I guess there would be a slight imbalance but there would certainly be loss on both sides.

It was the "smooth flow" that was the key point though.

It's interesting that tides don't pull directly toward the center of gravity either. The linked paper says 30 degrees is a typical angle for the shearing force which is perpendicular to tidal force.


Yes, the diagram shows that, the near side is closer to the star so orbital velocity will be higher. The material will be drawn prograde as we said before.
TheGhostofOtto1923
2.5 / 5 (4) Oct 16, 2013
Here is more detail on the black hole/star interaction simulation I posted above:
http://astrobites...k-holes/

"the collective N-body interaction of the many stars in the nuclear star cluster will tend to, every once in a while, deposit a star on a rather unfortunate orbit on which it will either by tidally disrupted or swallowed whole by the black hole."

-And fling some of itself free as in the simulation. This is true as well with planets and stars.
GSwift7
3 / 5 (2) Oct 17, 2013
And fling some of itself free as in the simulation. This is true as well with planets and stars


Not sure why people down-voted you. I don't see anything wrong with your comment. Though supermassive black holes eating stars is a special case, it is the same mechanism, just with some of the math terms set to infinity or zero.

I have always found it interesting that the Roche limit of a supermassive black hole is actually 'inside' the event horizon (whatever that means). I don't agree with the statement in the work you linked to where he says that a planet might cross the event horizon in one piece. Once you cross the event horizon, the atomic bonds can only work in one direction (since no EM can go outward), so they should fail. Even atoms should come apart, since the subatomic parts can only orbit and vibrate in one direction.

Wow, that's really off topic, but fun to imagine.
Fleetfoot
not rated yet Oct 17, 2013
And fling some of itself free as in the simulation. This is true as well with planets and stars


Not sure why people down-voted you. I don't see anything wrong with your comment.


Stars "evaporate" from clusters but that is very different from dust grains being ejected from say Saturn's rings. In the former case there is no dominant body, in the latter the whole ring is well down in Saturn's gravity well.

I don't agree with the statement in the work you linked to where he says that a planet might cross the event horizon in one piece. Once you cross the event horizon, the atomic bonds can only work in one direction (since no EM can go outward), ..


Local to a freefalling object, spacetime is described by the Minkowski metric outside, crossing or inside the radius that a distant observer describes as the event horizon. For a SMBH, the tidal forces at the EH can be no worse than at the surface of the Earth.
GSwift7
not rated yet Oct 17, 2013
For a SMBH, the tidal forces at the EH can be no worse than at the surface of the Earth


It wouldn't be tidal forces that destroy the object, but if escape velocity is C, then no solid matter can be stable.

Here's a question: If the escape velocity is C at the event horizon, then what is the escape velocity half way from the event horizon to the center of the BH?
Mr_Science
1 / 5 (9) Oct 17, 2013
Oddly enough....C
Fleetfoot
not rated yet Oct 17, 2013
For a SMBH, the tidal forces at the EH can be no worse than at the surface of the Earth


It wouldn't be tidal forces that destroy the object, but if escape velocity is C, then no solid matter can be stable.


As long as it is in freefall, it is no different to sitting in deep space, remember the equivalence principle. Only tidal forces affect an object in freefall.

Here's a question: If the escape velocity is C at the event horizon, then what is the escape velocity half way from the event horizon to the center of the BH?


The value would become imaginary, or equivalently the spatial and temporal axes swap. More obviously, there is no escape velocity because nothing escapes.