Hyper-Earths

[ProtoJeb21]

Some may notice I've been throwing out the phrase "Hyper Earths" several times over the last few weeks. I bet many of you want to know what exactly I'm talking about, so today I will describe a new potential planet type and the multiple confirmed examples of these hellish abominations.

This first began back in May 22nd, 2017 on Exoplanet Explorers, a citizen science project on Zooniverse where users go through processed parts of K2 light curves to try and find transiting planets. After many minutes searching, I came across something that caught my eye. A light curve only known back then as Subject 7673371 showed quite odd dips in starlight every 0.49 days. They were rather small with a depth of about 400 parts per million, but were both clear and noisy - a combination very rarely seen. It looked so odd an so peculiar that I wondered if this was a plant at all, but the folded transit showed some good potential. I felt no other choice but to classify this as a planetary candidate as I went to analyze what I had found. Using ExoFOP data, I discovered that the parent star was a F-class main sequence star known as EPIC 220395236, with a radius of 1.465 times that of the Sun and a temperature of over 6,000 Kelvin. Using the helpful Planetary Calculator and the parameters I already had, I was able to characterize the planet. The results were terrifying. This was a giant planet at 3.18 R_E orbiting so close to the star that temperatures reached an absolutely horrific 2,811 Kelvin, or 4,600*F. This is hotter than TRAPPIST-1 by nearly 300*K! The frightening data that was staring me in the face led me to give this world a proper name I had been waiting to use for a long time: Tartarus, named after the mythological Greek pit of Eternal Damnation, a hellish place where the most horrific beings and evildoers in the Universe were locked up to experience horrifically ghastly tortures for all of eternity. I had no idea how appropriate that name would become.

Some time later (like a few minutes ) I realized something: Tartarus was breaking the laws of the Universe. A planet of its size should most definitely be a gaseous world like Neptune, but with such an extreme temperature it would have to be incredibly puffed up. However, if that were the case, its actual mass would be very similar to that of Earth's. That would be too small to hold onto all those gases in such a hostile environment and would probably evaporate, reducing its radius.This is not what appears to be happening. Only one option remained: Tartarus was an ENORMOUS rocky planet, and I truly mean enormous. In order for it to survive in an environment with thousands of times the stellar flux Earth gets, it would have to be at LEAST 120 M_E, more than that of Saturn and HD 219134 h (Nerrivik). This would lead to a density of 20.576 g/cm³ and around 11.8667 gees of gravity. The conditions on Tartarus would be beyond hellish if the giant mass theory is correct. Most of the planet would be a searing, molten sea of gold, iron, rocks, and most metals in existence. The only land would be continent-sized volcanoes made purely out of Tungsten, the only metal that can survive the conditions here. Volcanic eruptions would be incredibly frequent and far more powerful than anything here on Earth, blowing out huge chunks of semi-molten metals and smothering clouds of toxic plasma. The front side would be scorching with temperatures of at LEAST 5,200*F, hot enough to vaporize iron and tin. This vapor would be pushed through a low but incredibly dense, soupy atmosphere by winds caused as a result of starlight exposure powerful enough to push the planet's atmosphere. Winds would be slow, but pack a punch as hard as getting hit by an asteroid. In "cooler" regions, the metal vapors in the atmosphere would condense into scorching pebbles and globs of molten iron and tin, which would rain SIDEWAYS in a turbulent, superheated atmosphere crackling with violent lightning storms. Overall, quite possibly the most hellish abomination of a planet ever found. This incredible discovery is what led me to make the Hyper-Earth planet class.

So, what exactly is a Hyper-Earth? It would be the next step up from a Mega-Earth, which starts at 10 M_E. For an object to be a Hyper-Earth, it must have at least 50 times the mass of Earth and NOT be a gas planet. It seems unlikely any Hyper-Earths would form with radii of over 4 R_E, about the size of Uranus and Neptune. These giant rocky planets could be similar in mass to Saturn and Jupiter, if not more massive than the latter. Such objects would be incredibly dense and have many times the gravity of Earth. Geologic activity would be very powerful and common on such massive planets, and thick soupy atmospheres would likely form as well. But how many planets are there that would be classified as a Hyper-Earth? The truth: more than you would expect. Here are all the potential Hyper-Earth candidates known to date:

• THANATOS: This is @Cabbink's hellish world, which is very similar to Tartarus. It has a year of 0.52 days, orbits an F-Type star, and has a slightly cooler equilibrium temperature of 4,400*F. However, at about 2.22 R_E, it might actually be a Mega-Earth instead of a Hyper-Earth.
• K2-77b: Another planet within the K2 data, which happens to be just 0.03 R_E larger than Thanatos. It has a much safer orbit, taking about 8 days to circle a 0.76 R_S high-metallicity orange dwarf. However, radial velocity measurements have shown something...odd. They heavily suggest that this planet of 2.25 R_E has a mass of 604 M_E, nearly TWICE that of Jupiter! And there error margins are TINY, both less than one Earth mass. This makes K2-77b the most likely and most massive Hyper-Earth candidate known. It is also the second-densest planet I will list. Put this into perspective: take every object in our solar system that isn't the Sun - all the planets, dwarf planets, asteroids, comets, dank memes, space junk, etc. - and squeeze them together into an object just over twice the radius of Earth. You will not get something as extreme as K2-77b, and that is scary.
• K2-92b: Similar to Tartarus, K2-92b is a world within the gaseous planet size range that is too hot to be a stable Mini-Neptune. This planet is 2.56 R_E and orbits every 0.7 days around a bright F-class star, resulting in temperatures in excess of 2,675*K (around 4,355*F). While larger than Thanatos, it is likely around the lower limit for a Hyper-Earth. Recently determined to be a false positive.
• KEPLER-277b and c: These are a pair of large ice giant-sized planets with absolutely ridiculous masses. Both orbit what might be a G-subdwarf star every 17.32 and 33.00 days. The first, Kepler-277b, is around 88 M_E and 2.9 R_E, giving it a density of 19.89 g/cm³ and 10.464 gees of gravity. Its larger sister, Kepler-277c, is around 3.4 R_E but is less massive at 66 M_E, giving it 5.71 gees of gravity and a density of 9.26 g/cm³. With these values, it may seem like Kepler-277c might have a significant water envelope, maybe between 5 and 10% its total mass. However, both planets could be much more massive, with error margins favoring masses between that of Saturn and Jupiter. 277b and c could be as large as 239 M_E and 167 M_E, making them both huge terrestrial worlds.
• JS 183 b: An exoplanet you probably NEVER heard of. In fact, if not for the Open Exoplanet Catalog, I wouldn't either. This is near the limit of how large a Hyper-Earth can possibly get. At 3.5 R_E, it's pretty close to that 4 Earth radius boundary I mentioned earlier. However, it is far more massive than Jupiter at around 531 M_E, making it the second most massive planet on this list. This gives it over 43 gees of gravity and a density of 67.6 g/cm³. JS 183 b is the coldest planet on this list, orbiting near the habitable zone with an eccentric orbit (0.24) around a 0.44 solar radius, metal-rich red dwarf. 
• K2-33b: This baby of a planet might be a Hyper-Earth, but its mass is so uncertain that I cannot tell for sure. If it is, then it's actually far past the radius limit I set at 4.9 R_E.
• KEPLER-338b: This one is more of an honorable mention, as it is "only" 31 M_E and is therefore not massive enough to be a Hyper-Earth.
• EPIC 22881391b: Here is the densest planet on this list. It was the recently discovered planet orbiting a red dwarf every four hours. However, things are rather odd. Based on radial velocity measurements, it appears to be somewhere around 223 M_E, over TWICE that of Saturn. What makes this even more extraordinary is how this planet is only 0.87 R_E, smaller than Venus. This makes EPIC 22881391b incredibly dense, at a staggering 1,884 g/cm³ with nearly THREE HUNDRED times the gravity of Earth. This would make it the densest non-stellar remnant object in the known Universe. A piece of this planet the size of a sugar cube would weight as much as a small dumbbell! Due to its hostile conditions and incredible gravity, I've nicknamed this abomination Morsaption, which comes from the Latin phrase "Mors Captionem", meaning "Death Trap".
• PSR J1719-1438b: One of the very few pulsar planets known is actually the FIRST Hyper-Earth candidate. This planet (which I will call J1719b for now) has around 330 times the mass of Earth and orbits a tiny, horrifying freak of nature known as a pulsar every 2 hours. It has the shortest year known, and this close proximity to such a deadly object has put significant constraints on J1719b's radius. Calculations show that it cannot be more than 4 R_E, which would place it within the Hyper-Earth range. As many of you have probably heard, J1719b could very well be a Neptune-sized planet made ENTIRELY of diamonds, making it the most exciting planet on this list to visit (SPOILER ALERT: You'll still die there). However, an alternative theory has be proposed, suggesting that J1719b might be a tiny lump of quark matter around 1 km across, created in the merger of two QUARK STARS that created the pulsar PSR J1719-1438.

What do you think of the possibility of Hyper-Earths? Should such a category even exist?

Edited August 9, 2017 by ProtoJeb21
K2-92b was recently determined to be a false positive caused by an eclipsing binary system.

[Bill Phil]

What's the metallicity of the parent star?

[Scotius]

I think we need a better\another name for such planets. Earth - what do they have in common with our homeworld? Fact that they are made mostly of rock and metals? So's Mars and Mercury - but we don't call them Mini Earths.

[Bill Phil]

1 minute ago, Scotius said:

I think we need a better\another name for such planets. Earth - what do they have in common with our homeworld? Fact that they are made mostly of rock and metals? So's Mars and Mercury - but we don't call them Mini Earths.

We kinda do... it's just "sub-Earth." It primarily just means less massive rocky planet.

https://en.m.wikipedia.org/wiki/Sub-Earth

[magnemoe]

is the mass estimates just for how large the planet has to be not to boil away? 
Or because of radial movement of star. 

Giant iron planets sounds a bit unlikely. More plausible that the planet is boiling away. it might well be the core of an gas planet, now the gas is gone even most of the lighter elements and it has an metal atmosphere who is also stripped away over time. 

[UmbralRaptor]

Huh, these are mostly well past the 1.6 R_E rule of thumb. Also I'm somewhat confused as to which ones have both radial velocity and transit measurements.

[Green Baron]

It is a nice habit among scientists to write an abstract ;-)

We should wait for refinements of the methods. Ours are coarse and observations are strongly biased towards large planets. Mega-, hyper-, giant are all prepositions that stimulate the mind but in the long run it'll probably end up with some letter/number classification scheme that is easier to handle and better suited for reasonable comparisons.

For now mass (and maybe estimated equilibrium temp) is the only element for categorization and small planets are a lucky case and grossly underrepresented. Then how about micro-, tiny, and baby preposition :-) ?

We probably can relax until then and use the designations from the catalogues the observations are based upon.

Thinks me ... :-)

Edited August 8, 2017 by Green Baron

[ProtoJeb21]

6 hours ago, magnemoe said:

is the mass estimates just for how large the planet has to be not to boil away? 
Or because of radial movement of star. 

Giant iron planets sounds a bit unlikely. More plausible that the planet is boiling away. it might well be the core of an gas planet, now the gas is gone even most of the lighter elements and it has an metal atmosphere who is also stripped away over time. 

All the planets listed except for Tartarus, Thanatos, and K2-92b have radial velocity measurements. These three planets are inferred to be Hyper-Earths by the fact that any significant water or hydrogen envelope would be blown off into space. I also was thinking that these could be evaporated gas giant cores, maybe from gas giants like Smertrios, which has a core 80 times the mass of Earth. Some of the other planets could have formed naturally (by eating up everything in their system) or, like J1719b, could be the remnants of white dwarfs. The latter seems unlikely for worlds like Morsaption.

[monstah]

I like Emily's take on the subject:

http://www.planetary.org/blogs/guest-blogs/2016/0415-favorite-astro-plots-4-classifying-exoplanets.html

Quote

Looking at data in this way leads to surprising implications. Brown dwarfs are merely high-mass Jupiters. Dwarf planets, like Pluto, are merely low-mass members of the same class containing the Earth. Perhaps the most surprisingly result is that the divide between Neptunian worlds and solid planets like the Earth occurs at just 2 Earth masses. This leaves very little room for Super-Earths and suggests gaseous planets occur at much lower masses than we initially expected. Effectively then, the Earth is the Super-Earth we have been looking for all along [emphasis mine].

 

[Green Baron]

Comparing other planets to our own system may be a natural reaction ("stamp collection" in a Rutherford meaning :-)) but as we have seen in the Trappist-1 system for example (or systems with multiple suns, or gas giants close to their sun) our own solar system is a bad example for comparison.

That is why i think that these prepositions are not exact enough to describe reality which we until now have only a strongly biased view of, due to the methods we can use. Also, there is a lot of anthopo-centricity in these designations ...

[monstah]

4 minutes ago, Green Baron said:

Comparing other planets to our own system may be a natural reaction ("stamp collection" in a Rutherford meaning :-)) but as we have seen in the Trappist-1 system for example (or systems with multiple suns, or gas giants close to their sun) our own solar system is a bad example for comparison.

If you're talking about my post, those are exoplanets... but the blog post I link to is from back a year and half, so naturally newer findings are missing.

[Urses]

We name Earth often as a Pearl, what would be the nickname for such a giant?

I Pledge for "Bowlingball"

[ProtoJeb21]

11 hours ago, Bill Phil said:

What's the metallicity of the parent star?

Not all stellar metallicity values are known. However, most stars with known metal contents are metal-rich. K2-77 is +0.29, JS 183 is +0.2, and EPIC 22881391 is +0.08. What's odd is that Tartarus' host star has a lower metal content of -0.069. The exact metal contents of all other stars are unknown.

Also, I forgot that K2-92b was recently disproved as an eclipsing binary. Dammit.

Edited August 8, 2017 by ProtoJeb21

[Green Baron]

4 minutes ago, monstah said:

If you're talking about my post, those are exoplanets... but the blog post I link to is from back a year and half, so naturally newer findings are missing.

Not specially ...

I make my view more clear.

I mean the classification schemes we have now are basically based on mass, where possible on further derived "features" as denstity, eqilibrium temp. ...

I don't think that a classification based on qualitative, value implying prepositions (super, hyper, jupiter-, neptun-, earth-like, ...) is helpful as it reflects a momentary view ready to be overthrown and is anthropo-cenrtic, which should be avoided ;-)

Hope thaw wasn't too provocative, but it is my opinion.

[monstah]

Just now, Green Baron said:

Hope thaw wasn't too provocative, but it is my opinion.

Oh, no, I quite agree!

[Mitchz95]

18 hours ago, Scotius said:

I think we need a better\another name for such planets. Earth - what do they have in common with our homeworld? Fact that they are made mostly of rock and metals? So's Mars and Mercury - but we don't call them Mini Earths.

Simple, use "terrestrial" instead of Earth, "super-terrestrial" instead of super-Earth, and "hyper-terrestrial" for the OP's planet class.

Edited August 8, 2017 by Mitchz95

[insert_name]

pretty sure something similar already exists

https://en.wikipedia.org/wiki/Chthonian_planet

[YNM]

@OP : IMO we can easily say "terrestrial" for ones with rock surface and "gasseous" for ones with too much gas.

Also, some of the planets you mentioned sounds too gentle if they're to be called "planets". 67 g cm^-3 would imply some form of degeneracy occurs within the object.

If my memory is believeable and they haven't changed it, there exists a core remnant of a star that's the size of planets but it's not a white dwarf or something. Now those would be more interesting...

[ProtoJeb21]

13 hours ago, YNM said:

@OP : IMO we can easily say "terrestrial" for ones with rock surface and "gasseous" for ones with too much gas.

Also, some of the planets you mentioned sounds too gentle if they're to be called "planets". 67 g cm^-3 would imply some form of degeneracy occurs within the object.

If my memory is believeable and they haven't changed it, there exists a core remnant of a star that's the size of planets but it's not a white dwarf or something. Now those would be more interesting...

It's possible they may act like stellar remnants, but there's nothing better to classify them. They're too large to be white dwarfs or neutron stars (radius-wise), too small to be brown dwarfs, and too dense to be gas planets. As @insert_name suggested, several of them, including Tartarus and Thanatos, are likely Chthonian planets that were once gas giants with large cores.

[Green Baron]

Like this one: http://science.sciencemag.org/content/333/6050/1717

Uncategorized ;-)

 

[ProtoJeb21]

I did some composition and orbit models of Tartarus with Universe Sandbox², and they're not pretty.

For this simulation I decided to give Tartarus a rather Earth-like ratio of silicates and iron, which may be present within the cores of gas giants. Tartarus is 24.2% iron and 74.8% silicates, with no amounts of water or hydrogen. With this, the planet is MUCH more massive than I thought. US2 gives Tartarus a mass of 234 M_E, with a density of 39.6 g/cm³ and a gravity of 225 m/s² (about 23 gees). If that wasn't bad enough, when I set Tartarus's orbit it was almost skimming the photosphere of the host star at over 280 km/s. I let the simulation run for a bit, and it produced some both good and bad results. The good news is that the composition I gave for Tartarus and its resulting mass is enough to survive tidal friction from its star. Its orbit is also quite stable, with only tiny variations on the scale of just tens of kilometers. Finally, it also produces a significant wobble on the host star, maxing out at close to 400 m/s. This would make it easy to detect with the radial velocity method...if it exists. Tartarus is still a candidate, and checking for a 0.4905-day radial velocity signature should confirm or disprove its existence. However, there's some bad news: temperatures have been shown to fluctuate quite horrifically, varying between 2,700*C (4,892*F/2,973*K) to 3,400*C (6,152*F/3,673*K), which is MUCH hotter than expected. I'm not too sure how even TUNGSTEN could survive on this planet. Also, mass loss is rather significant, with about 1e+6 to 1.15e+6 kilograms being lost each second. This suggests that Tartarus has only a few million years left of its life before being destroyed by EPIC 220395236.

I'll do some analysis on Thanatos later on.

[Racescort666]

Please excuse my ignorance on this but how do planets achieve such high densities? Is it like how neutron stars are basically star sized atomic neuclei? If the surface has iron and tungsten on it wouldn't the density of the surface materials be 7.8 g/cc and 19.2 g/cc respectively but that is less than the densities discussed? Am I thinking about it wrong because these are their densities at STP and they have different densities and/or assume different forms at absurd pressures like those found at the core of a gas giant? How do they keep those densities and forms with the lighter gas blown away?

[magnemoe]

18 minutes ago, Racescort666 said:

Please excuse my ignorance on this but how do planets achieve such high densities? Is it like how neutron stars are basically star sized atomic neuclei? If the surface has iron and tungsten on it wouldn't the density of the surface materials be 7.8 g/cc and 19.2 g/cc respectively but that is less than the densities discussed? Am I thinking about it wrong because these are their densities at STP and they have different densities and/or assume different forms at absurd pressures like those found at the core of a gas giant? How do they keep those densities and forms with the lighter gas blown away?

My guess if the mass measurements are correct is some sort of low level degenerated matter, not as dense as in white dwarfs but denser than normal matter and its pretty stable. 
We can make degenerated matter, the inward explosion in modern nuclear weapons compress the plutonium degenerated, it not stable as then you use other metals it don't stay degenerated,   it might be that the core stay degenerated even if the pressure is reduced but it still earth core levels. state don't need to be extremely stable just stable enough for us to observe it. 
 

[ProtoJeb21]

6 hours ago, Racescort666 said:

Please excuse my ignorance on this but how do planets achieve such high densities? Is it like how neutron stars are basically star sized atomic neuclei? If the surface has iron and tungsten on it wouldn't the density of the surface materials be 7.8 g/cc and 19.2 g/cc respectively but that is less than the densities discussed? Am I thinking about it wrong because these are their densities at STP and they have different densities and/or assume different forms at absurd pressures like those found at the core of a gas giant? How do they keep those densities and forms with the lighter gas blown away?

The density of a terrestrial object increases with radius due to pressures compacting silicates and iron. That's why a 3 Re planet with an Earth-like composition has a density of at least 30 g/cm^3. However, planets like Tartarus were most likely huge gas giants that wound up too close to their stars and had most of their hydrogen-helium envelopes destroyed by both photoevaporation and tidal forces. Only the heavy elements of the core would have remained.

[Racescort666]

@magnemoe, thanks for pointing me toward degenerated matter. I guess I was assuming that such a concept existed but ultimately unfamiliar with it... until now. 

So, now that I've spent the better part of my evening reading wikipedia articles on things that were totally glossed over in my physics classes, I have more questions. But first, I'm going to cover a bit of what I read to make sure I understand it (people that understand physics better than I do, please correct me where I am wrong):

Degenerate Matter (from the White Dwarf Wikipedia page):

Quote

Such densities are possible because white dwarf material is not composed of atoms joined by chemical bonds, but rather consists of a plasma of unbound nuclei and electrons. There is therefore no obstacle to placing nuclei closer than normally allowed by electron orbitals limited by normal matter.

Is this type of state isn't limited to white dwarf stars? This much makes sense: degenerate matter in white dwarf stars is caused by gravity where there is sufficient gravity to collapse what's left of the star into super-dense matter after the thermal pressure of fusion has ceased. The only thing keeping it from collapsing into a black hole is the degeneracy pressure. Basically: gravity > how-far-atoms-want-to-stay-away-from-each-other-normally.

So, does this condition exist in the cores of typical planets? Or specifically, gas planets?

As the outer layers of a gas planet are blown off wouldn't this condition no longer be present? Hypothetically, it takes thousands/millions of years for the gas part of a gas planet to be blown away right? Wouldn't there be a gradual transition on the core and the extreme pressure relieved? 

Super crazy theory: the metals at the surface (or mantle... I guess... this is definitely not my area of expertise) of the core are strong enough to maintain the pressure to support the degenerate matter under the surface because they've formed some phase/alloy of metal that we've never imagined. 

Am I asking questions that don't have answers? Existential crisis incoming in 5... 4... 3... 2... 1...

9 minutes ago, ProtoJeb21 said:

The density of a terrestrial object increases with radius due to pressures compacting silicates and iron. That's why a 3 Re planet with an Earth-like composition has a density of at least 30 g/cm^3. However, planets like Tartarus were most likely huge gas giants that wound up too close to their stars and had most of their hydrogen-helium envelopes destroyed by both photoevaporation and tidal forces. Only the heavy elements of the core would have remained.

Ninja'd on my own question. 

Is the density determined by the equilibrium between the electron degeneracy pressure and gravity then?

ETA: @ProtoJeb21, I wish I could like your last post more than once because in the last 2-3 hours I spent reading wiki articles, I was wondering exactly that. It's so great to learn new things.

Edited August 10, 2017 by Racescort666