Giant planet or 'failed star?' Newfound mystery world blurs the lines

[Admin]

By tracing the corkscrew wobble of two stars as they move through the sky, the Gaia space mission has discovered one new giant "planet," plus a new brown dwarf.

Giant planet or 'failed star?' Newfound mystery world blurs the lines : Read more

 

[Helio]

This article makes sense as it holds to the mainstream view, as I understand it, that planets should be assumed to be brown dwarfs when their mass is under about 14x that of Jupiter.

The exoplanet.eu catalog, however, will have many with masses over 25x and some as high as 70x. Mass, however, isn't the key dividing line as the rule since 1999 is whether or not 1/2 of the original deuterium has burned.

The NASA website, IIRC, limits exoplanet masses to about 25x, thus their catalog is notably less in exoplanet number.

I'm curious if I've got the proper handle on this. Do I?

 

[Classical Motion]

I was surprised at the distance the star travels between loops. Sol might have an 11 maybe 22 year wobble. Wonder what it looks like at a distance.

 

[m4n8tpr8b]

This article makes sense as it holds to the mainstream view, as I understand it, that planets should be assumed to be brown dwarfs when their mass is under about 14x that of Jupiter.

The mainstream view is that (1) brown dwarfs have masses between 13 and 80 Jupiter masses, (2) brown dwarfs formed by cloud collapse like stars, rather than coalescing from dust in protoplanetary discs like planets, (3) after formation, brown dwarfs had deuterium fusion in their cores for the short period until that deuterium ran out. The article definitely goes beyond that: it makes the point that whether you use cloud collapse or deuterium fusion as the defining quality, the mass limits are murky and there is no easy way to determine if an object fits either definition.

The part worth to emphasize is that finding signs of either the formation process or the deuterium burning is very difficult, while mass can be determined from spectroscopy, so from a viewpoint of observation and cataloguing, a definition connected to mass is the most useful. (Just for perspective: there is a similar problem in the field of near-Earth objects. From the viewpoint of impact risk, what you would ideally want to know there is the mass of the object. But for most objects, all you have is its total brightness. For such objects, you have to assume an average surface brightness to calculate a diameter, and an average density to calculate mass. So NASA is mandated to find 90% of NEAs larger than 140 m across, but what it does in practice is cataloguing objects brighter than an absolute magnitude of 22.)

The exoplanet.eu catalog, however, will have many with masses over 25x and some as high as 70x. Mass, however, isn't the key dividing line as the rule since 1999 is whether or not 1/2 of the original deuterium has burned.

That site has a page explaining its inclusion criteria. The criterion is a mass of at most 60 Jupiter masses plus the 1-sigma uncertainty in the object's mass; that is, if an object's mass is determined to be 73±13 Jupiter masses, it will be put on the list. For this 60-Jupiter-mass limit, they cite "Hatzes & Rauer, 2015". You can read the abstract of that paper. This was a study of the mass-density distribution of exoplanets, which found no cutoff at 13 Jupiter masses, only at 60, and the authors argue that this cutoff is the only meaningful distinction. What does this mean for us? It means either that whatever the way of formation and whatever the scale of deuterium fusion at under 60 Jupiter masses, the density of the object (its most easily observable material characteristic) will be the same; or (less likely) that either formation or deuterium fusion only happen at 60 or above.

The NASA website, IIRC, limits exoplanet masses to about 25x, thus their catalog is notably less in exoplanet number.

Yes, the NASA exoplanet catalog has a 25 Jupiter masses cut-off. Indeed the European catalog currently has 7,413 entries while NASA's has only 5,832; but the difference could also be down to access to the databases of different research teams.

 

[m4n8tpr8b]

I was surprised at the distance the star travels between loops. Sol might have an 11 maybe 22 year wobble. Wonder what it looks like at a distance.

You are confusing the solar cycle with the motion around the centre of mass.

The 22-year solar cycle is a cycle in the pattern of the Sun's magnetic field, which reverses every 11 years. The most obvious sign of this cycle for us is the number of sunspots, which is highest halfway between the reversals and zero during the reversals, whatever the direction of the magnetic field, thus the cycle in sunspot numbers is half the magnetic cycle, 11 years.

The "wobble" in the article is because the common notion that planets revolve around the Sun is an over-simplification: in truth, all the planets and the Sun revolve around a common centre of gravity, but since the Sun is almost a thousand times heavier than all the planets, that centre of gravity is inside or near the surface of the Sun. So, for example, while Jupiter completes an orbit 778 million kilometres from the centre of mass in 12 years, Sun completes a circle with a 778 thousand km radius, on the opposite side of the centre of mass. Since the planets move at different speeds, the Sun's wobble around the centre of mass is actually a combination of several cycles (of which the Jupiter cycle is by far the largest).

The way astronomers can measure this wobble for other stars is not the motion perpendicular to our line of sight (that's too small to detect) but the one along the line of sight, which can be measured due to the Doppler shift (the shift in the wavelength of lines in the star's spectrum).

 

[rod]

Interesting mass reported for Gaia 4b and 5b. The exoplanet.eu site shows 7413 confirmed exos. Mean Jupiter mass is 12.44 Mjup, and max is 74.6 Mjup listed. 4452 exoplanets show mass Mjup property. The NASA archive site, https://exoplanetarchive.ipac.caltech.edu/index.html has some interesting data too.

5832 confirmed with mean mass 2.326 Mjup with 38090 rows and 6330 rows contain mass Mjup. The max is 80 Mjup in the data. The average or mean is 2.325 Mjup.

 

[Helio]

I was surprised at the distance the star travels between loops. Sol might have an 11 maybe 22 year wobble. Wonder what it looks like at a distance.

Distances are exaggerated for effect. This scaled illustration may show how their mass ratio, though higher than average, causes little movement of the star, but enough for astronomers.

These same mass and distance ratios will balance a seesaw, which is why I like to draw them.

 

[Classical Motion]

Ok, I didn’t realize the “Position in the sky over time” image was an exaggeration. That showed 4 rotations of 570 days. At 244 LY.

I was considering the pitch, not the barycenter. When they said corkscrew, I naturally thought pitch. Not wobble.

 

[Helio]

The mainstream view is that (1) brown dwarfs have masses between 13 and 80 Jupiter masses, (2) brown dwarfs formed by cloud collapse like stars, rather than coalescing from dust in protoplanetary discs like planets, (3) after formation, brown dwarfs had deuterium fusion in their cores for the short period until that deuterium ran out. The article definitely goes beyond that: it makes the point that whether you use cloud collapse or deuterium fusion as the defining quality, the mass limits are murky and there is no easy way to determine if an object fits either definition.

Great explanations, m4!

Yes, I was surprised when I read about the use of deuterium fusion as a determining factor, adding to my confusion.

The part worth to emphasize is that finding signs of either the formation process or the deuterium burning is very difficult, while mass can be determined from spectroscopy, so from a viewpoint of observation and cataloguing, a definition connected to mass is the most useful. (Just for perspective: there is a similar problem in the field of near-Earth objects. From the viewpoint of impact risk, what you would ideally want to know there is the mass of the object. But for most objects, all you have is its total brightness. For such objects, you have to assume an average surface brightness to calculate a diameter, and an average density to calculate mass.

Yes. I did find a useful paper that presents the variation in densities based on exo type (ice; silicate; iron). Using this, I'm able to approximate roughly the expected mass or radius if one variable is known, since the semi-major is given and host's luminosity.

The paper you site really makes your argument stick! Their Fig. 1 I would recommend as one of the best inverts in astronomy graphs! I was surprised, yet with a little thought, it is intuitive, nevertheless.

Yes, the NASA exoplanet catalog has a 25 Jupiter masses cut-off. Indeed the European catalog currently has 7,413 entries while NASA's has only 5,832; but the difference could also be down to access to the databases of different research teams.

I like the NASA approach since brown dwarfs are not likely candidates for habitability. But they may have moons that do offer habitability of some kind, so I prefer using the eu catalog. I also calculate the no. of possible exos that might be candidates in having exomoons of interest. [ Exoplanet Stats ]

Also, my little exoplanet program allows the user to set limits. I use 25 jupiters as the mass limit to better define habitable planets.

Thanks again for your help.

 

[Unclear Engineer]

Being a little picky, here, but would a "brown dwarf" have "moons" or "planets" if it is orbited by something smaller than itself?

And, if brown dwarfs do have satellites of some sort, would they be useful in understanding how protoplanetary nebula actually evolve, considering that their nebula would not have had the same radiation pressure and heating of one that surrounds a star that actually ignited the mainstream hydrogen fusion processes?

 

[Catastrophe]

Google gives:

A "satellite" of a brown dwarf would essentially be a moon orbiting around it, though currently, no confirmed moons have been directly observed around brown dwarfs; however, the concept is theoretically possible and astronomers are actively searching for them, with some evidence suggesting the potential for moons around certain brown dwarfs, like Gliese 229B, which was initially thought to be a single object but was later discovered to be a binary system of brown dwarfs themselves.

Due to their small size and the faintness of brown dwarfs, directly observing a moon around a brown dwarf is extremely challenging with current technology.

Moons orbiting brown dwarfs might experience extreme temperature fluctuations and harsh radiation conditions depending on their proximity to the brown dwarf.

Cat

 

[Unclear Engineer]

If there is a brown dwarf - brown dwarf binary system, it stands to reason that there could be brown dwarf - super Jupiter systems. Which would be a record size for a "moon". Far bigger than the "minor planets " in our asteroid and Kuiper Belts. Sounds like another categorization dispute for astronomers if such systems are ever found.

Is there any reason there should not be brown dwarf systems with "moons" that are equivalent to the planetary systems we are finding around red dwarfs?

If the assumption is that brown dwarfs form from collapse of a cloud of gas and dust, I would think there are all sorts of possibilities for smaller objects to collapse/accrete along with them. But, those smaller objects might maintain gas envelopes (atmospheres) better than those around red dwarfs, due to lack of stellar wind.

Probably hard to find and hard to see with existing technology, unless maybe we can find a really close brown dwarf. But seems like an interesting system to study if we find one.

 

[Helio]

If there is a brown dwarf - brown dwarf binary system, it stands to reason that there could be brown dwarf - super Jupiter systems. Which would be a record size for a "moon". Far bigger than the "minor planets " in our asteroid and Kuiper Belts. Sounds like another categorization dispute for astronomers if such systems are ever found.

I think many consider little Pluto as being a binary system with Charon, so it seems likely larger planets might get this labeling. I would bet they will await example cases before getting in hurry to do so, else why establish a label for something that might not exist, even if very plausible?

If the assumption is that brown dwarfs form from collapse of a cloud of gas and dust, I would think there are all sorts of possibilities for smaller objects to collapse/accrete along with them. But, those smaller objects might maintain gas envelopes (atmospheres) better than those around red dwarfs, due to lack of stellar wind.

It will be interesting to see what is found as these smaller objects become found. Perhaps brown dwarfs due more harm than good by tossing the little guys around.

Red dwarfs may be far more fisty than expected even in stellar winds. They can have monster flares and they are more regular than the more massive stars since they are fully convective outside their core.