Andy Lloyd's Dark Star Blog
Blog 59 (February 2018)
Planets vs Brown Dwarfs
Question: When is a planet not a planet?
Answer: When it's a brown dwarf.
No one could accuse mighty Jupiter of being a brown dwarf, but if Jupiter were to start piling on the pounds in a big way, then after a while it would start to look more and more like its darker, denser cousin - the slightly diminutive sub-brown dwarf.
Now, terrestrial planets which are ten times the mass of Earth have become known as super-Earths. Our solar system lacks one of these objects (unless the painfully shy Planet Nine turns out to be one (1)), but they seem common enough elsewhere. Similarly, then, we might like to use the term super-Jupiters. If a super-Jupiter has, say, ten Jupiter masses (10Mj), then is it still a planet... or is it instead a brown dwarf?
Classically (and I use this term advisedly, because there is nothing particularly 'classic' about the crazy new world of brown dwarfs), the lowest mass for a brown dwarf is about 13 Jupiter masses (13Mj). This weighing-in limit marks the point where the planet's internal pressure-cooker begins to 'burn' deuterium, or undergo deuterium fusion whereby a deuterium nucleus and a proton combine to form a helium-3 nucleus. This limit is known as the 'DBMM' and marks the lowest theoretical mass for a brown dwarf, at least in terms of the point where a planet lights up (2). The bright flames of young Brown dwarfs quickly die out, having 'burned' all their available nuclear fuels in no time. They are the James Deans of the stellar spectrum.
Cold brown dwarfs
But the DBMM is only one possible definition of the transition point between planets and brown dwarfs. This seemingly straightforward definition may be an over-simplification: The primordial waters may be greatly muddied - by rocks. Brown dwarfs, and super-Jupiters, may form under differing circumstances:
Brown dwarfs may form like stars and condense under their own weight from dense dust and gas nebulae. They may do this completely on their own, or within a striking distance of a companion star. The point is that they spontaneously and rapidly condense from a nebula of gas and dust, in process known a dynamical instability.
Giant planets, on the other hand, accrete out of a star's rotating disk of cosmic after-birth - a process known as core accretion. It is thought that planets like Jupiter begin with a substantial rocky core (which accumulate from a multitude of collisions between grains and dust and ice). This growing rocky mass attracts to it a very significant shroud of gas, that then coalesces into a gas giant. However, if these planets really pile the weight on, and go over the 13Mj deuterium burning limit, they will light up, in much the same way as their much larger 'stellar' brown dwarf cousins.
The problem, then, is distinguishing between these two sets of brown dwarf object. The rocky core component of the burgeoning super-Jupiter planet may marginally affect the deuterium-burning limit in comparison to the condensed stellar gas ball version. But even if this was significant during the brown dwarf's early life, the effect wears off within a couple of hundred million years (3), and the two classes of object become essentially indistinguishable from one another. So, the defining point distinguishing planets from brown dwarfs remains rather fuzzy. Essentially, mass is not sufficient on its own to tell the difference between the planetary brown dwarfs, and the stellar brown dwarfs.
Hot brown dwarfs
A new definition is now being offered by Kevin Schlaufman of John Hopkin's University (4). Rather than focussing solely upon the deuterium-burning limit, this instead addresses the issue of where the brown dwarf or super-Jupiter planet formed. In particular, it looks at whether the object formed around an iron-rich star. The thinking goes that if the parent star is iron-rich, like our Sun, then there will be a greater potential for the formation of rocky worlds in its midst, and therefore a greater potential for super-Jupiter planets.
"In Schlaufman’s new argument, the missing property is the chemical makeup of a solar system’s own sun. He says you can know your suspect, a planet, not just by his size, but also by the company he keeps. Giant planets such as Jupiter are almost always found orbiting stars that have more iron than our sun. Brown dwarfs are not so discriminating. That’s where his argument engages the idea of planet formation. Planets like Jupiter are formed from the bottom-up by first building-up a rocky core that is subsequently enshrouded in a massive gaseous envelope. It stands to reason that they would be found near stars heavy with elements that make rocks, as those elements provide the seed material for planet formation.
"Not so with brown dwarfs. Brown dwarfs and stars form from the top-down as clouds of gas collapse under their own weight. Schlaufman’s idea was to find the mass at which point objects stop caring about the composition of the star they orbit. He found that objects more massive than about 10 times the mass of Jupiter do not prefer stars with lots of elements that make rocks and therefore are unlikely to form like planets." (5)
So, according to Schlaufman's population analysis, the lower limit for brown dwarfs needs to shift downwards from 13Mj to 10Mj. Arguably, then, the objects whose masses were in the 10 - 13Mj range, and which might have been classified as sub-brown dwarfs, should now be reclassified as brown dwarfs - even though they are incapable of actually lighting up at birth into deuterium burning failed stars. The argument goes, I suppose, that tiny fledgling brown dwarf 'stars' are bright for such a short period of time that it makes little sense to classify them just on the basis of that brief, but bright, period of deuterium fusion. Instead, it is more important to look at how, and where, the object formed in the first place.
Dark Star disk
Personally, I suspect this argument will quietly rage on for a while yet in astronomical circles. For me, this is relevant insofar as any purported 'Nemesis'-like object in our own solar system might be properly classified as a brown dwarf. If it was born within the same stellar nursery as the Sun, but at a considerable distance (the most likely scenario, I would say), then it would properly be considered a brown dwarf under this new framework, even if its mass were below the 13Mj DBMM limit. That's because it would have formed independently from the Sun, like a star.
That's quite a relief, in a way, because when I first wrote about this 'Dark Star', I called it a "brown dwarf" (6,7). Only later did I realise that it more properly should be termed a "sub-brown dwarf", due entirely to its mass being between Jupiter and 13Mj. Now, I might have to go back to the original classification!
Ice Nine
Having said all of that, it should be pointed out that the interiors of gas giants and brown dwarfs are, really, an unknown quantities. The way planets form remains largely theoretical, although there is supportive observational evidence of extrasolar planets in the midst of protoplanetary disks, creating swirling, tell-tale bands in the dust (8). Nonetheless, it's quite impossible to probe the depths of this giant worlds - even when you can put space-probes in orbit around them. The so-called ice giants, Uranus and Neptune, are little better. All of these giant planets/sub-brown dwarfs share a common feature: extremely high internal pressure. Many scientists speculate about what bizarre chemistry may be going in within these immense worlds. Atoms and molecules are ground together by the pressure, creating conditions which are incredibly difficult to replicate on our light-weight world.
Interior of the Ice Giants
Until recently I had failed to realise the significance of the title of Joe Satriani's awesome guitar track entitled 'Ice Nine'. Having now read an article about super-condensed ice in the interior of Uranus (yes, really - and this was not in a medical journal!), I now appreciate the source of the maestro's inspiration. A laboratory experiment may have replicated conditions within the ice giants, allowing scientists to speculate that these massive worlds are rich in 'Ice VII':
"Scientists [at Lawrence Livermore National Laboratory]
... began by squeezing water molecules in between diamonds to make ice VII, a crystal form of ice produced under incredibly high pressure shown at the left. Using a laser, they then sent shocks through the ice with pressures millions of times greater than the pressure of air in our atmosphere.... heir results provided evidence for the existence of this superionic ice, in which hydrogen ions move freely in an oxygen crystal lattice. In simplest terms, scientists have evidence that they’ve made a special kind of incredibly high-pressure ice in a lab that conducts electricity really well."The temperature and pressures of this ice imply that it could make up a “large fraction” of Uranus and Neptune’s interiors. The researchers propose that it may even be able to explain some of the behavior of these planets’ magnetic fields." (9)
So, the super-compressed interior 'ice' within these distant worlds may generate 'superionic conduction in water ice at planetary interior conditions' (10). Theoretical explorations of phase transitions of ice at high pressures predicted these kinds of superconductive effects thirty years ago (11). This property may help to explain the unusually displaced - and tilted - magnetic field structures of the ice giants. Other theories tend towards the idea that the magnetic fields are "probably made primarily by degenerate metallic fluid hydrogen" (12), which would be more in keeping with the interiors of gas giants and brown dwarfs.
Written by Andy Lloyd, 8th February 2018
References:
1) K. Batygin & M. Brown "Evidence for a Distant Giant Planet in the Solar System" The Astronomical Journal, 151:2, 20 January 2016,
2) Eduardo Martin "The Birth and Evolution of Brown Dwarfs"
3) P. Mollière & C. Mordasini "Deuterium burning in objects forming via the core accretion scenario" Astronomy & Astrophysics, 7 November 2012, 547: A105.
4) Kevin Schlaufman "Evidence of an Upper Bound on the Masses of Planets and its Implications for Giant Planet Formation" publication ApJ, 853:37, 18 January 2018,
5) Phil Sneiderman "Johns Hopkins Scientist Proposes New Definition of a Planet" John Hopkins Press release, 22 January 2018, with thanks to Mark
6) Andy Lloyd "The Dark Star Theory" 1999
www.andylloyd.org/darkstar01.htm
7) Andy Lloyd "Dark Star: The Planet X Evidence" Timeless Voyager Press, 2005
8) Max Planck Institute for Astronomy "Disks and Extrasolar Planets"
9) Ryan Mandelbaum "Scientists Observe Incredible New Kind of Ice Thought to Exist in Uranus' Center" 6 February 2018
10) Marius Millot et al. "Experimental evidence for superionic water ice using shock compression" Nature Physics, 5th February 2018
11) P. Demontis, R. LeSar & M. Klein "New high-pressure phases of ice", Phys. Rev. Lett. 60, 2284–2287 (1988)
12) W. Nellis "Unusual Magnetic Fields of Uranus and Neptune: Metallic Fluid Hydrogen" Modern Physics Letters B (Condensed Matter), 2015, 29:(1)
An Elliptical sub-Brown Dwarf
Far away in the Flying Fish constellation known as Volans (who knew?) is an old star system containing a rather unusual flying object of its own. The exoplanet HD76920b is four times heavier than Jupiter (4Mj), making it a planetary sub-brown dwarf. Although the star system is more than six billion years old, and is approaching the end of its life (he parent star is a red giant), the planetary orbit played out by HD76920b is far from settled. Instead, Australian astronomers have discovered that its orbit is highly elliptical, bringing it very closer to its star at perihelion (1). 'Hot Jupiters' have become a well-known exoplanet phenomenon, but they're usually young objects whizzing around the parent star.
This object is behaving more like a comet than a regular gas giant planet. Indeed, this is the most eccentric orbit yet observed in a fully evolved system. Something has perturbed it into this odd orbital path - its current path is not sustainable over the long-term, as the gravitational attraction of the star will deform its shape during perihelion, buffering and smoothing its orbit into a more circular shape over time. This highly elliptical orbit strongly suggests something has taken place to shake this old system up, and this might tell us something new about how planetary systems form and evolve over time (2).
But it also creates a remarkable precedent for a sub-brown dwarf's potential to be found in an elliptical, even comet-like orbit. Remember, this system is not young. This seemingly chaotic arrangement cannot be attributed to the jostling of young bucks in the primordial planetary nest. How did HD76920b end up in this dynamically unstable orbit? The orbit is roughly equivalent to those of comet 2P/Encke or asteroid 3200 Phaethon, both of which are responsible for meteor streams in our solar system. These shifted inwards from more distant, circular orbits as a result of a lengthy series of gravitational perturbations.
The Pan-Pacific Planet Search (PPPS) team who discovered this object think that some kind of scattering has taken place within the system, which is similar to the mechanisms which drew Encke and Phaethon into their dynamically unstable orbits:
"There is no indication that HD 76920 has an unseen binary companion, suggesting a scattering event rather than Kozai oscillations as a probable culprit for the observed eccentricity... Given the tidal interactions that are likely occurring between HD 76920b and its host star, there is clearly the potential for a similar process to be occurring in the HD 76920 system - with the newly discovered planet having tidally decoupled from a distant perturber and then injected to its current highly eccentric orbit." (3)
As expressed above, the alternative mechanism is through the transfer of eccentricity and inclination from a highly inclined binary companion object to the comet/asteroid/planet in question. In some cases, the clear presence of a binary object in the system provides a ready solution to the eccentric orbit of an exoplanet through this so-called Kozai-Lidov effect. But, in the case of HD76920, no such companion object can be seen. Missing a prime suspect, the researchers seem to be putting this incredible orbit down to tidal interactions between the sub-brown dwarf and the burgeoning red giant star.
Is this realistic, given that these same tidal forces are also wheeled in to explain the opposing effects which tend towards circularisation of orbits? I'm not convinced, and I'm not sure the research team is, either. Something else is going on here, surely. They suggest other, more improbable alternatives, one of which is the capture of a free-floating planet into an elliptical, eccentric orbit (3). But they caution that generally these types of interaction place the captive object into a far more distant orbit (for instance, this may be a possible solution for how the putative Planet Nine object came to be where it is (4)).
There is so much we have yet to understand about the formation and dynamics of planetary systems. Each new bit of 'weirdness' astronomers discover only goes to show that the remarkable diversity on show can only serve to challenge our fundamental assumptions.
Written by Andy Lloyd, 9th February 2018
References:
1) Jonti Horner et al. "We've found an exo-planet with an extraordinarily eccentric orbit" 16 November 2017
2) University of Southern Queensland Press Release "USQ astrophysicists make surprising find of exoplanet with extraordinarily eccentric orbit" 17 November 2017 (No longer available) - Instead see:
3) Robert Wittenmyer et al. "The Pan-Pacific Planet Search. VII. The Most Eccentric Planet Orbiting a Giant Star", The Astronomical Journal, 4 Dec. 2017, Vol, 154, (6),
https://arxiv.org/abs/1711.05378
4) James Vesper & Paul Mason "Simulation of Rogue Planet Encounters with the Solar System: Is Planet 9 a Captured Rogue?" January 2017, American Astronomical Society, AAS Meeting #229, id.424.05,
SpaceX
As readers will appreciate, this Dark Star blog is predominantly about Planet X, and all its works. So, there's just that one letter to provide a tenuous link to SpaceX. But it's enough! What an incredible triumph this week, as Elon Musk and his private sector team kicked celestial butt! The powerful triple rocket that shot the Falcon Heavy up into space reminded me of a blanched version of Thunderbird 3. Then there was the 2008 Tesla Roadster un-manned by the dummy test pilot Starman, who is now heading towards a Mars-crossing orbit (1).
SpaceX boosters synchronised return
But most magnificent of all were the synchronised booster rockets returning to the landing pad. As I joked with my class, it was like watching two Olympic gymnasts executing synchronised routines and landing perfectly together - only, this time, they managed to accelerate to 20,000 km/hr during the main somersault. No mean feat. A triumph of engineering and awesomeness. One can only begin to imagine the computational power at work to help those two babies descend so perfectly.
Thunderbird 3
As many other people interested in space exploration and science, I've been sceptical about how the private sector would compare to the government-funded space programmes of old. But, clearly, SpaceX has shown its capacity to amaze, and enthral. Arguably, after 30 years in the doldrums, American innovation and space kudos have come together to challenge the pretenders to NASA's throne. Personally, I can't wait to see what SpaceX do next. If nothing else, they've already doubtlessly inspired a whole new generation of budding scientists.
Written by Andy Lloyd, 8th February 2018
References:
1) Marina Koren "Where Is Elon Musk's Space Tesla Actually Going?" 8 February 2018
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