What is this gap between hot and cold Jupiters?

What is this gap between hot and cold Jupiters?

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When plotting mass to semi major axis on, there seems to be a gap between Hot Jupiters and Cold Jupiters. A lot of Jupiter-mass planets at less than 0.1 AU have been discovered, and a lot at more than 1 AU, but there aren't so many in between.

Since many planets closer and further away from their star have been discovered for the same mass, it seems unlikely that this gap would be due to observational bias.

Giant planets like Jupiter form through accretion in the outer part of the disk, beyond the frost line where the material is cool enough for volatile icy compounds to remain solid. These planets cannot form closer to their host star, since there is there is not enough matter in the protoplanetary disk at smaller radii.

Due to interactions with other planetesimals and the protoplanetary disk itself, the orbit of the young planets changes. This is planetary migration. Interactions with the protoplanetary disk cause planets to migrate inwards. This migration occurs rather fast, so we don't see many Jupiter mass planets at intermediate distance from their star.

The protoplanetary disk is somewhat doughnut shaped. When a migrating planet reaches the inner edge of the doughnut, the sum of forces exerted on the planet change which halts the migration.

This is why there are two clumps in the diagram, for Jupiter-mass planets. One clump (i.e. cold Jupiters) where these planets form, and one clump (i.e hot Jupiters) where they end up after migrating, but nothing in between.

How Hot is Jupiter?

Given how far Jupiter is from the Sun, you might think that “how cold is Jupiter?” would be a more relevant question and you would be partially right. “How hot is Jupiter?” becomes more relevant the deeper into the planet’s atmosphere and core that you travel. Near the very center of the planet, scientists believe that temperatures can reach 35,500 C.

The outer edges of Jupiter’s atmosphere are much cooler than the core region. Temperatures in the atmosphere are thought to be as cold as -145 degrees C. The intense atmospheric pressure on Jupiter contributes to temperature increases as you descend. Not far into the atmosphere the pressure can be ten times what it is here on Earth and scientists speculate that the temperature is 20 degrees C(average room temperature on Earth). A few hundred km deeper into the planet and hydrogen becomes hot enough to turn into a liquid. The temperature at this point is believed to be over 9,700 C. The layer of dense molten hydrogen metal extends to the 78th percentile of the planet’s radius. Between the cold clouds and the molten lower regions is an interior atmosphere of hydrogen. The hydrogen in this region is at a temperature where there are no distinct liquid and gas phases, so the hydrogen is said to be in a supercritical fluid state.

The molten inner regions of the planet serve to heat the rest of the planet through convection, so Jupiter gives off more heat than it receives from the Sun. This heating prevents it from being an ice giant instead of a gas giant, but wreaks havoc in the atmosphere. Storms and high winds are generated by cool air and warm air mixing here on Earth. Scientist think that the same holds true on Jupiter. The Galileo spacecraft observed winds in excess of 600 kph. One difference is that the jet streams that drive storms and winds on Earth are caused by the Sun heating the atmosphere. On Jupiter it seems that the jet streams are driven by the planets’ own heat. Storms on Jupiter are as out-sized as the planet. The Great Red Spot is a single storm that has been raging for hundreds of years. Other storms have been observed to grow to more than 2,000 km in diameter in a single day.

“How hot is Jupiter?” is more relevant than you may have thought. The planet’s inner heat seems to be the basis for its identity as a stormy world. The actual temperatures of the different areas of the planet may not be a mystery much longer. Hopefully, the recently launched JUNO space mission will clear up many of the Jovian theories that scientists currently have.

We’ve written many articles about the temperature of planets for Universe Today. Here’s an article about how hot Mercury is, and here’s an article about how hot Venus is.

We’ve also recorded an episode of Astronomy Cast just about Jupiter. Listen here, Episode 56: Jupiter.

Examples of empathy gaps

Examples of empathy gaps appear in a variety of situations and take various forms.

When it comes to misjudging our own emotions and behaviors, the empathy gap can, for example, cause us to:

  • Overestimate our ability to stay composed in an upcoming stressful event, if we’re currently calm.
  • Overestimate the likelihood that we will be able to stop consuming an addictive substance, such as coffee, if we just consumed it so we’re not feeling cravings at the moment.
  • Underestimate how much our feelings for someone affected our judgment in the past, if we no longer have feelings for that person.

When it comes to misjudging the emotions and behaviors of others, the empathy gap can, for example, cause us to:

  • Struggle to understand why someone who is nervous about something acted the way that they did, if we don’t share their feelings on the topic.
  • Struggle to see that someone doesn’t necessarily have the same feelings toward us as we have toward them.
  • Struggle to predict how a person will act when they’re angry about something, if we’re currently calm.

A notable example of how the empathy gap can influence people is that it can cause them to underestimate the influence of visceral drives on their decision-making, when it comes to factors such as hunger, desire, fear, or pain. This happens primarily when people believe that they will act in a rational and controlled manner in a certain situation, but end up failing to do so due to the influence of their visceral drives, which they weren’t actively experiencing when they were thinking about the future.

Accordingly, the empathy gap can cause people to be unprepared for situations where they are affected by various visceral drives that cause them to do things that satisfy their instincts, urges, and cravings in the short-term, but that also fail to help them accomplish their long-term goals, or to act in the way that they would ideally prefer. The influence of the empathy gap in such cases can be so powerful that people often continue to assume that they will be able to handle a certain type of situation properly, even if they have been repeatedly proven to be wrong about this in the past.

When plotting exoplanet discoveries with x being semi-major axis and y being planet mass, they form three distinct groups. Why is this?

I created the following plot when I was messing about with the exoplanet data from It seems to me to form three distinct groups of data. Why are there gaps between the groups in which we don't seem to have found many exoplanets? Is this due to the instruments used or discovery techniques or are we focussing on finding those with a specific mass and semi major axis?

2 2

This is basically part of my area of research so I will try and begin to scratch the surface of this problem!

The exoplanet community would also like to know! First I will say these gaps are absolutely NOT due to observational problems. Our observational issues are mostly towards the bottom right of the plot. Gaps such as the hot neptune desert are well within our region of observations.

The gap at sub 10 day orbit of Jupiter mass planets (on your plot that is <0.05AU and 10-100 Mearth) is known as the Hot Neptune desert (actually most gaps in populations of astrophysical bodies get called deserts). We have no idea why this exists.

One theory is that unlike their Jupiter mass counterparts, the hot Jupiters, they lack the mass to keep hold of their atmosphere from being stripped by stellar activity. This means they would travel down your plot to become hot super Earths. There are problems with this idea in that this process should take hundreds of millions to billions of years so we should actually observe a lot more of these than we do. Further the desert transition is quite sharp. I do not think this is likely to be the primary cause.

A second theory is that this highlights a difference in formation mechanism between hot super earths (mentioned in this paper linked before) and hot jupiters. This also has a problem that it assumes there is a single formation mechanism for HJ planets. People are finally starting to believe there may actually be more than one formation mechanism for HJs. So this gap would need to be explained by all valid formation mechanism (the various mechanisms are reviewed here but its a long read!). In particular in situ formation and disc migration mechanisms have a hard time explaining this gap (as well as the gap between hot and cold jovian planets at the top of your plot).

If (and I think this is unlikely due to observations of very young HJs, 1 and 2) the formation mechanism for HJs is high eccentricity migration then this gap is obtained for free as it could be explained by roche lobe overflow. This is that when a giant planet is in a highly eccentric orbit and passes its pericenter (closest to the star) the atmosphere breaches the roche limit of the star and experiences atmospheric stripping. As the planet continues to circularise it would rapidly lose atmosphere and become a hot super earth.

So the bottom line here is that this one gap (which I believe is the most well studied) is not fully understood. A proper explanation (of all gaps?) will come once we have reevaluated planetary formation and migration mechanisms. We kind of had to throw the book of what we knew on this out the window once we started getting exoplanet observations! If I was to make an educated guess (I sure as hell wouldnt put money on this guess though as our understanding of formation and migration still has a lot of work) I would say it may actually be a combination of ideas 2 and 3 as they both can end up doing similar things (or be responsible for the upper and lower boundaries of the desert).

What is this gap between hot and cold Jupiters? - Astronomy

Context. Ultra-hot Jupiters are the hottest exoplanets that have been discovered so far. They present a unique possibility to explore hot and cold chemistry on one object. The tidally locked ultra-hot Jupiter HAT-P-7b has a day-to-night temperature difference of ≃2500 K, confining cloud formation to the nightside and efficient ionisation to the dayside. Both have distinct observational signatures.
Aims: We analyse plasma and magnetic processes in the atmosphere of the ultra-hot Jupiter HAT-P-7b to investigate the formation of an ionosphere and the possibility of magnetically coupling the atmospheric gas as the base for an extended exosphere. We show which ions and atoms may be used as spectral tracers, and if and where conditions for lightning may occur within the clouds of HAT-P-7b.
Methods: We used 3D modelling results as input for a kinetic cloud formation code and evaluated characteristic plasma and magnetic coupling parameters. A local thermodynamical equilibrium radiative transfer was solved for the ionised gas phase. This study is confined to thermal ionisation only.
Results: The ionisation throughout HAT-P-7b's atmosphere varies drastically between day- and nightside. The dayside has high levels of thermal ionisation and long-range electromagnetic interactions dominate over kinetic electron-neutral interactions, suggesting a day-night difference in magnetic coupling. K + , Na + , Li + , Ca + , and Al + are more abundant than their atomic counterparts on the dayside. The minimum magnetic flux density for electrons for magnetic coupling is B < 0.5 G for all regions of HAT-P-7b's atmosphere.
Conclusions: HAT-P-7b's dayside has an asymmetric ionosphere that extends deep into the atmosphere, the nightside has no thermally driven ionosphere. A corresponding asymmetry is imprinted in the ion and neutral composition at the terminators. The ionosphere on HAT-P-7b may be directly traced by the Ca + H&K lines if the local temperature is ≥5000 K. The whole atmosphere may couple to a global, large-scale magnetic field, and lightning may occur on the nightside.

What is a hot Jupiter?

Could these worlds be the extreme cousins of gas giant Jupiter?

Hot Jupiters are huge worlds made of gas that are heated to high temperatures by their star

Asked by Elizabeth Perry

Hot Jupiters are exactly what their name suggests. These alien worlds are made of gas (just like their prototype) and are often found orbiting extremely closely to their star – much closer in than Mercury is to our Sun. Some get so hot that their surfaces are often found to reach temperatures of a thousand to a several thousand degrees Celsius. In short, their star roasts their gases.

Hot Jupiters are fairly exotic, at least by the standards of our Solar System, with some of these sweltering worlds possibly having clouds of molten rock scudding through their skies. They are also likely to be tidally locked, with one side of these worlds facing their star and the other pointing to freezing cold space – such a temperature difference between the two hemispheres whips up plenty of extreme weather.

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What is this gap between hot and cold Jupiters? - Astronomy

The subjects treated in this dissertation refer to Jupiter's hot and cold plasma tori and to the atmosphere of Io, a jovian moon located in a nearly circular orbit at a distance of 5.9 jovian radii from the center of Jupiter. The hot and cold plasma tori are centered at 5.7 and 5.3 jovian radii from Jupiter and are so named because of their electron temperatures of 5 and 1.5 eV respectively. For the hot torus we model the ion partitioning and energy balance by assuming that there are independent sources of neutral sulfur and oxygen atoms, that the thermal electrons have a density of 2000cm^ < -3>and a temperature of 5 eV. We also include a small percentage of hot (

1 keV) electrons. In our model of energy and mass balance of the cold torus we consider that its primary source of plasma is radial diffusion inward from the hot torus. The time scale for this process is assumed to be identical to that required for energetic electrons to supply the energy necessary to power the synchrotron radiation. The primary source of energy is the rotational energy of Jupiter. Charge exchange between thermal ions and an extended neutral cloud of sulfur and oxygen produces fresh ions which are accelerated to corotation by the magnetic field of Jupiter. These fresh accelerated ions are the source of energy which powers the cold torus emissions. The main ion loss mechanism is a novel two-step process whereby charge exchange between ions and neutral molecules transforms ions into fast neutrals. Since it is generally agreed that the source of neutrals to the hot and cold tori is the atmosphere of Io, then in order to better understand torus processes we need to understand the behavior of Io's atmosphere. Thus, we proceeded to develop numerical gasdynamic SO_2 models of sublimation and volcanic atmospheres of Io by means of computer simulations. Using a fine computational grid and the computational capabilities of the Cray supercomputers, we are able to find all the relevant atmospheric properties such as pressure, density, temperature and velocity. We investigate day side and night side atmospheres. We find that volcanoes form an extended atmosphere on Io. We also investigate a sublimation H_2S atmosphere.

Jupiter's Icy Moon Europa Has a Really Weird Cold Spot

Just because Jupiter's moon Europa is coated in ice doesn't mean all that ice is the same temperature.

And now, scientists have mapped the hot and cold spots on the moon's surface using data gathered from Earth, with accuracy down to 125 miles (200 kilometers). While most of the temperature variations they measured can be explained by sunlight's influence on the ice, there's one unusually cold spot that is stumping the scientists behind the new research.

That spot, which falls on the moon's northern hemisphere, stood out in images taken at different times of the day, which surprised the scientists. They weren't sure what might be causing the local coldness and didn't know of any geologic features there that could be responsible.

Probably unrelated but nevertheless intriguing is a coincidence on the opposite side of the moon: an unusually warm area at Pwyll crater, which is one of the youngest impacts on the moon. That made more sense to the team, they wrote in their paper about the research, because scientists know that craters on other solar system bodies tend to retain heat compared to their surroundings.

The measurements are based on data gathered by the Atacama Large Millimeter/submillimeter Array (ALMA), in Chile. Then, they compared those measurements with the temperatures predicted by a thermal model of the moon, which considers how much sunlight hits the world and how the icy surface reflects that light, based in part on observations made by the Voyager 2 spacecraft.

Over most of the moon's surface, the data and the model lined up well, except for Pwyll crater and the cold spot in the northern hemisphere. The scientists were also surprised to find that there didn't seem to be much connection between local geologic features and the temperature.

The team wants to keep using ALMA to study surface temperatures on the moon in the hope of making their estimates more robust. But there may not be much more they can do from Earth — so they're also pinning their hopes on NASA's Europa Clipper mission due to launch in the 2020s.

What is this gap between hot and cold Jupiters? - Astronomy

Movies always depict meteors as flaming balls of fire, streaking across the sky, and igniting anything they touch after they reach the earth. Is this true? I know that they get hot as they enter our atmosphere, but I also know that it is pretty cold in space, so they start out with quite a chill. Do meteors really get hot enough to keep them flaming all the way to the ground? If a meteor fell at my feet, could I touch it? Would it be cold or hot?

This is a good question, and one that we really don't have a great answer for. It's true that the chunks of rock and/or ice that form meteorites have been travelling through space for at least millions of years, and are therefore very cold when they begin their descent through our atmosphere. As they hit the atmosphere, the outside of the rock begins to heat up (forming the "fusion crust" on the meteorite). The hot outside begins to ablate (or be stripped off), which removes heat. The meteorite falls through the atmosphere in seconds, so for larger rocks, only the outside part has time to be heated. So the question is: When they hit, are they still cold because the hot parts of the rock were removed via ablation, or does the outside manage to get hot enough to burn things?

Unfortunately, there really aren't very many meteors that are picked up directly after they've fallen, so it's hard to do good statistics on which ones are hot or cold. So far it seems that some of each have been found. For example, this FAQ lists reports of meteorites (compiled by Don Blakeslee of Wichita State University) that have been touched soon after they fell, and some people reported that the rock was hot, some that it was warm, and some that there was frost on the outside! These reports are all of a qualitative nature, usually based on the testimony of a small number of people.

We certainly don't have big fires starting when meteorites hit Earth, so although they may singe grass or burn someone, they definitly don't hit the ground as a flaming fireball, the way you sometimes see it depicted in movies.

Many astronomers believe that small rocks hitting the ground should not be hot. In a [email protected] article about the recent fireball over Pennsylvania, written by Tony Phillips, the planetary scientist Don Yeomans is quoted as saying,"Rocky asteroids are poor conductors of heat. Their central regions remain cool even as the hot outer layers are ablated away. Small rocky meteorites found immediately after landing will not be hot to the touch."

In their meteorite FAQ, the American Meteor Society says "The ablation process, which occurs over the majority of the meteorite's path, is a very efficient heat removal method, and was effectively copied for use during the early manned space flights for re-entry into the atmosphere. During the final free-fall portion of their flight, meteorites undergo very little frictional heating, and probably reach the ground at only slightly above ambient temperature." However, they point out that there really aren't many reports, and those we have are often "prone to hearsay".

So, in summary, we don't really know what temperature meteorites are when they fall. The problem is that there really isn't much quantitative data to base an answer on! However, many astronomers believe that small meteorites should be barely warm, or even cool when they hit the ground. The temperature probably varies depending on the size and composition of the original rock. For example, some materials might ablate more efficiently than others, or conduct the heat better. It's an interesting question, though, and one I wish we had a better answer to!

25 Replies


I enjoy a gap if you have the space for it. Gives me somewhere new to get my arm stuck in while trying to fish out a cable :)

I would agree. if you have speace. We leave a 1U gap where we can. And blank of the gap to make sure the airflow is optimal.

Ah sorry I dont mean gaps inside the racks, I mean outside the racks, so a space between rack to rack.

Yea, I agree its best to leave a gap if you can even though for some reason I like the way it looks when its all tight. That's just me though lol.

Yea that's what everyone thought..

What about cold aisle vs hot aisle. Leaving a gap would mess up airflow right?

Gary D Williams

Hype1099 wrote:

What about cold aisle vs hot aisle. Leaving a gap would mess up airflow right?

No, as long as the cold aisle is at the front and the hot aisle is at the back the airflow would be fine but all servers and racks are designed to be bolted together. There are more stable like that.


Airflow is restricted to within / through the server so space between the racks doesn't mean much in that regard.. If on the other hand, you have the extra room that would allow you to move in between the racks, that might be helpful at some point.

For myself - I prefer to keep everything neat and tidy so I would have no gap between the racks IF i had free access behind them. In other words, if the racks are pushed up against a wall, then I would like a gap between them for access to the rear, but if the racks are positioned so that you can easily get behind them, then I would keep them close together.


If I only had a couple I'd probably leave about 3ft between racks so I can fit between them. On more than one occasion I've dropped a bolt or something and side access makes it so much easier. If you got more than 2 or 3 I'd seriously consider bolting them together for space and stability.

So at 3 I'd say if you aren't going to leave liberal space for a person to fit between them, you might as well bolt them together. Probably better use of the space in your area too.

Avoid both spaces between and within racks. All the thermal airflow is designed to move front to back. In fact, leaving a gap either in the rack or between can allow heated air to recirculate from back to front.

I've never seen a situation where side panel access let me do anything I couldn't accomplish from the front or back.

We have a cable management channel bolted between the racks. No "open" space, but a nice, clean, covered area for running cables between the racks. The channel bolts to both racks, so it adds to the stability of the structure too.

if there are no gaps between servers, (or worse, servers are placed on top of each other) then this will serve as one large heat conductor radiating heat on themselves thus rendering cooling inefficient.

I've always connected my racks into one large pod. It makes for a stable configuration. And you should use a method to cable from rack-to-rack other than going through the racks, such as under a raised floor or with a trough on top of the racks. This keeps everything neat and organized and prevents airflow issues.


Racks? No gap, but at least one vertical cable trough between them.


Two schools of thought here:

If the room has suitable cooling and is clean, remove the sides from the middle cabinet, as well as the corresponding ones from the external ones and get a baying kit for sturdiness, this allow for inter cabinet patching.

If cooling is an issue, and for further security, place them immediately next to each other with their sides still on and have different locks. Place any items you think you'll need extra help getting too in the outside cabinets, make sure you have switches in all the cabinets, and inter cabinet patching for the network equipment. Only allow the server guys access to the cabinet with the servers in!

Think I'm going to get a baying kit for them and put them in a pod of 3 then. Its a new room with sufficent cooling and patching between racks (each rack will have its own patch panel anyway at top of rack), but like any new build IT is getting the short straw so the room is a small as they want (big as I can get) so space is a premium already.


Yep, if I ever have the room I will leave a gap.

I haven't bayed mine, but there's no gap, mostly for space reasons. Has not been an issue working with them.

Adept Communications is an IT service provider.

+1 for the vertical wire manager.

If you have ample cooling then I would not gap the racks externally. First off you are just opening yourself up to trying to fish stuff from between the racks. Why do that if you don't have to.

I would gap your servers at least 1U inside the racks to maximize the airflow in the rack.


NetAdminWorld is an IT service provider.

Since others have mentioned hot / cold sides but no one has posted an image, here is one.

Thanks all. Racks have been consolidated together without a gap between so the central panels removed. Pretty neat solution especially considering I got these 800mm racks due to the ample cable management which our previous had none, so this should make it better. Have left enough room to get around the outside of the racks if needed, and pushing them up close gives me the room I need to be able to add another rack later on. If we need to go past that I'll be pushing for them to go co-lo anyway by that point.

Room will be 4.2m x 4.6m and the new AC going in will provide more than enough cooling, while also leaving space to grown this if needed. The version of hot/cold we have is similar to the photo except the AC is mounted behind and above the racks (not directly above) so it throws cold over the front, sucked into the front of servers and hot vents back up into the AC. Works exceptionally well in our current room so am confident of it here.


I've worked in school where the biggest closet had the racks side by side with no central panels. It made running inter-rack cabling very easy and kept a consistent rack temp throughout the entire section. It also made working with KVM, temp senors, and various 'stuff' that gets tossed into a rack much easier. It contains the wiring clutter, too.

I'm currently with a different company and our server room has 2 racks side by side without space, but they kept the side panels in. Rack to rack route sucks and makes for a disgusting mess of cables on top of the racks. Each rack has it's own temp, but stays pretty regulated. I want to redesign the racks, but I haven't been around long enough to get that through, yet.