If the exoplanet HD 100-546 is larger than some stars, how come it hasn’t collapsed into a low-mass star?
In: Physics
On Earth, if an object gains more material, it gets bigger.
But with GAS planets and stars, if the planet gets more gas material, gravity gets stronger, so it pulls the gas more, so the planet or star gets slightly smaller.
Basically, mass shrinks the planet, and creates more pressures and heat in the core, until the pressure and temperature are sufficient for nuclear reactions. At which point the heat from the nuclear may create extra pressures and inflate the star again.
Anyway, big (volume) does not necessarily mean heavy (mass), so you CAN have a visually big planet that is lightweight, by gas giant standards, and not have enough mass to have nuclear reactions and be a star.
These small stars like white dwarfs are extremely dense (like thousands and millions of times more dense than “normal matter” with which we are familiar through our senses and which makes up planets). They have already burned up their thermonuclear fuel and collapsed in on their own insides catastrophically, having no energy to support their outside layers. Gas giants as planets don’t have enough mass for a thermonuclear reaction to start in the first place.
HD 100-548 is far from being large enough in terms of mass to turn into a star. It is not even big enough to be counted as a ‘Brown Dwarf’ which are basically larger versions of HD 100-546b.
HD 100-546b maybe larger in terms of volume than say a red dwarf (which is a star) but it has no way close to enough mass to turn the fusion on.
We have just started to research the HD 100546 star system. It is a fairly young system and still have its acreation disk which planets form from. So it is very hard to see things in it. Adding to this there are not may star systems of this type so we do not know these very well. Whe do have some indications of one planet or maybe even two planets in the star system. And one of these is large enough that it could be a brown dwarf but it could also be a regular planet. There is some overlap in masses between brown dwarfs and gas giants because they can have different densities, composition, temperature, core material, etc. which does affect its ability to burn deuterium or not. We do not know if HD 100546 b does burn deuterium which would make it into a brown dwarf but from what we can tell now it does not look like this process have started making it a gas giant.
This is a gross oversimplification, but I’m only patching this together after a couple minutes of research.
That’s because it *isn’t* more massive than some stars. It’s much less.
I saw an estimate somewhere that this planet has, give or take, around the mass of 20 Jupiters. This is, and I’m being very optimistic here, barely enough to form an extremely “cold” (still pretty hot compared to a summer day) brown dwarf. It takes hundreds, if not thousands, of Jupiter masses to form even the smallest Red Dwarf star.
Further, depending on who you ask, brown dwarves aren’t stars at all: they’re just huge gas giants with **some** stellar properties.
Again, this is a MASSIVE oversimplification, but hope that clears something up.
So when you pile on some more gas to something like Jupiter, it’ll get bigger… at first. Keep on piling on gas and it’ll start compressing those lower layers.
Imagine you have a stack of pillows and you keep adding a pillow on top. The pillow on bottom gets more and more squished as you add more on top.
Anyway, the point is that you could have something more massive than Jupiter that’s actually a little bit smaller than it; or, in other words, you can have planets larger than stars because the gas wasn’t squished down enough. Once you compress the gas enough, and it gets hot enough, then it’ll start fusion and become a star.
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