Planet as large as its star (2009 June 3)

[brian1204]

This APOD raises questions for me: Why didn't the planet become a small star itself as it was forming? Why did the small star become a star instead of a large planet? If they are indeed the same size, why did they develop so differently?

I suppose the planet could have developed in a different area and later captured by the star? Still doesn't answer why one became a planet and the other a star.

Is this another example of what we don't know?

[orin stepanek]

http://apod.nasa.gov/apod/ap090603.html

The difference is in mass. The mass of the star is much greater even though the size is the same. The planet doesn't have enough mass to become a star.

Orin

[aristarchusinexile]

I think it's a wild guess that whatever is causing the fluctuations is a planet.

"Hey, that star is fluctuating!"

"Maybe it has gas."

[BMAONE23]

This APOD raises questions for me: Why didn't the planet become a small star itself as it was forming? Why did the small star become a star instead of a large planet? If they are indeed the same size, why did they develop so differently?

I suppose the planet could have developed in a different area and later captured by the star? Still doesn't answer why one became a planet and the other a star.

Is this another example of what we don't know?

Mass is the key...
The planet has more mass than Jupiter by about 5 times. Though Compression, by Gravity, will keep it only somewhat larger than Jupiter

More mass = more gravity = more compression.

The star would likely have over 20 to 40 times Jupiter's mass and also 20 to 40 times the gravity.

again

More mass = More Gravity = more comperssion = more heat

[bhrobards]

Please explain how this makes sense. Two objects are approximateley the same size. The compositions of the objects is similar. The hot one is ten times the mass of the small cool one. With gravity and the ideal gas law we have a problem.

[Chris Peterson]

Please explain how this makes sense. Two objects are approximateley the same size. The compositions of the objects is similar. The hot one is ten times the mass of the small cool one. With gravity and the ideal gas law we have a problem.

Yes, but you can't simply apply the ideal gas law. The star is undergoing fusion, so its equilibrium state is very different from the planet's. The star contains much more material- hence a greater density. If it weren't fusing, gravity-only equilibrium would produce a body with a very different size.

[neufer]

Explanation: A planet, a white dwarf, and a neutron star orbit each other in the giant globular star cluster M4, some 5,600 light-years away. The most visible member of the trio is the white dwarf star, indicated above in an image from the Hubble Space Telescope, while the neutron star is detected at radio frequencies as a pulsar. A third body was known to be present in the pulsar/white dwarf system and a detailed analysis of the Hubble data has indicated it is indeed a planet with about 2.5 times the mass of Jupiter. In such a system, the planet is likely to be about 13 billion years old. Compared to our solar system's tender 4.5 billion years and other identified planets of nearby stars, this truly ancient world is by far the oldest planet known, almost as old as the Universe itself. Its discovery as part of an evolved cosmic trio suggests that planet formation spans the age of the Universe and that this newly discovered planet is likely only one of many formed in the crowded environs of globular star clusters.

http://science.nasa.gov/headlines/y2003 ... planet.htm

http://hubblesite.org/newscenter/archiv ... rmat/zoom/

[bhrobards]

Fusion is the theoretical result of greater density, not the cause of it. We are considering a red dwarf and a gas giant, not a white dwarf and a neutron star. The presence of fusion at the core should expand the outer layers of the star.

[bystander]

Yet in the system neufer cited, the neutron star is probably the most massive (and most dense), but the smallest in size; while the planet maybe the least massive (and least dense), but the largest in size.

[Qev]

Please explain how this makes sense. Two objects are approximateley the same size. The compositions of the objects is similar. The hot one is ten times the mass of the small cool one. With gravity and the ideal gas law we have a problem.

Bear in mind that, ignoring gravity and assuming both of these objects have the same density, a sphere ten times more massive is going to have only about double the radius. Gravity's effect on density is simply going to reduce that even further. There's even a good example of this sitting in our own solar system; Jupiter is three times more massive than Saturn, and yet is only about 25% larger in terms of radius. Then there's always the extreme cases, like neutron stars, where the greater the mass, the smaller the radius.

Obviously, heating from stellar fusion is going to counteract this trend once the object is massive enough to initiate it. So I would assume that VB10 is in fact larger in diameter than its planet, but probably not by a huge amount. The paper linked to from the APOD page simply states that both objects are ~0.1 times the radius of the Sun, which gives some wiggle-room for one to be larger than the other, while still being 'the same size' on astronomical scales.

[bhrobards]

Please explain how this makes sense. Two objects are approximateley the same size. The compositions of the objects is similar. The hot one is ten times the mass of the small cool one. With gravity and the ideal gas law we have a problem.

Bear in mind that, ignoring gravity and assuming both of these objects have the same density, a sphere ten times more massive is going to have only about double the radius. Gravity's effect on density is simply going to reduce that even further. There's even a good example of this sitting in our own solar system; Jupiter is three times more massive than Saturn, and yet is only about 25% larger in terms of radius. Then there's always the extreme cases, like neutron stars, where the greater the mass, the smaller the radius.

Obviously, heating from stellar fusion is going to counteract this trend once the object is massive enough to initiate it. So I would assume that VB10 is in fact larger in diameter than its planet, but probably not by a huge amount. The paper linked to from the APOD page simply states that both objects are ~0.1 times the radius of the Sun, which gives some wiggle-room for one to be larger than the other, while still being 'the same size' on astronomical scales.

Now I'm really confused, what do you mean "ignoring gravity?" How can there be density and radius without it?

[neufer]

Please explain how this makes sense. Two objects are approximateley the same size. The compositions of the objects is similar. The hot one is ten times the mass of the small cool one. With gravity and the ideal gas law we have a problem.

Bear in mind that, ignoring gravity and assuming both of these objects have the same density, a sphere ten times more massive is going to have only about double the radius. Gravity's effect on density is simply going to reduce that even further. There's even a good example of this sitting in our own solar system; Jupiter is three times more massive than Saturn, and yet is only about 25% larger in terms of radius. Then there's always the extreme cases, like neutron stars, where the greater the mass, the smaller the radius.

Obviously, heating from stellar fusion is going to counteract this trend once the object is massive enough to initiate it. So I would assume that VB10 is in fact larger in diameter than its planet, but probably not by a huge amount. The paper linked to from the APOD page simply states that both objects are ~0.1 times the radius of the Sun, which gives some wiggle-room for one to be larger than the other, while still being 'the same size' on astronomical scales.

Perhaps, some familiarity with "the missing link" here might help:

<<Brown dwarfs are sub-stellar objects with a mass below that necessary to maintain hydrogen-burning nuclear fusion reactions in their cores, as do stars on the main sequence, but which have fully convective surfaces and interiors, with no chemical differentiation by depth. Brown dwarfs occupy the mass range between that of large gas giant planets and the lowest mass stars; this upper limit is between 75 and 80 Jupiter masses (MJ). Currently there is some debate as to what criterion to use to define the separation between a brown dwarf from a giant planet at very low brown dwarf masses (~13 MJ ), and whether brown dwarfs are required to have experienced fusion at some point in their history. In any event, brown dwarfs heavier than 13 MJ do fuse deuterium and those above ~65 MJ also fuse lithium. The only planets known to orbit brown dwarfs are 2M1207b and MOA-2007-BLG-192Lb.

A remarkable property of brown dwarfs is that they are all roughly the same radius as Jupiter. At the high end of their mass range (60-90 Jupiter masses), the volume of a brown dwarf is governed primarily by electron degeneracy pressure, as it is in white dwarfs; at the low end of the range (1-10 Jupiter masses), their volume is governed primarily by Coulomb pressure, as it is in planets. The net result is that the radii of brown dwarfs vary by only 10-15% over the range of possible masses. This can make distinguishing them from planets difficult.

In addition, many brown dwarfs undergo no fusion; those at the low end of the mass range (under 13 Jupiter masses) are never hot enough to fuse even deuterium, and even those at the high end of the mass range (over 60 Jupiter masses) cool quickly enough that they no longer undergo fusion after some time on the order of 10 million years. However, there are other ways to distinguish dwarfs from planets:

Density is a clear giveaway. Brown dwarfs are all about the same radius; so anything that size with over 10 Jupiter masses is unlikely to be a planet.

X-ray and infrared spectra are telltale signs. Some brown dwarfs emit X-rays; and all "warm" dwarfs continue to glow tellingly in the red and infrared spectra until they cool to planet like temperatures (under 1000 K).

Some astronomers believe that there is in fact no actual black-and-white line separating light brown dwarfs from heavy planets, and that rather there is a continuum. For example, Jupiter and Saturn are both made out of primarily hydrogen and helium, like the Sun. Saturn is nearly as large as Jupiter, despite having only 30% the mass. Three of the giants in our solar system (Jupiter, Saturn, and Neptune) emit more heat than they receive from the Sun. And all four giant planets have their own "planetary systems" -- their moons. In addition, it has been found that both planets and brown dwarfs can have eccentric orbits.

Currently, the International Astronomical Union considers objects with masses above the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) to be a brown dwarf, whereas those objects under that mass (and orbiting stars or stellar remnants) are considered planets.>>

[bhrobards]

Thanks for the citation. It will take a while to penetrate.