# If there was air between the Sun and Earth, how warm would we get?

Based on this question: How loud would the Sun be?

If instead of a vacuum we had air between the sun and earth (let's say of the same composition we have here on earth) how warm would our planet get?

This is a really hard question to answer as Carl Witthoft alludes to. The main problem is that there is no way you can finagle putting at atmosphere around the Sun and have it extend to the Earth. It would be wildly unstable and no matter what happens, the general result will be an explosion of energy that kills the Earth. And you can't really say you're going to wave some magic wand and prevent all of that because you're then fundamentally altering physics and you won't end up answering the question you wanted to ask to begin with.

Your only way of answering this question is to look at the current atmosphere of the Sun and try to extrapolate. Take a look at the figure below. This represents both the temperature (solid line) and density (dashed line) of the Sun vs height from the surface (what the "surface" of the Sun means is a whole other question). The temperature at the surface of the Sun is a measly $5800:mathrm{Kelvin}$. As you increase in height, this temperature (i.e., the temperature of the plasma atmosphere above the "surface" of the Sun) doesn't change much. This region of the atmosphere is known as the chromosphere. But then, at a particular height known as the transition region, there's a drastic increase in the temperature. It goes from a few thousand degrees to a couple million degrees. This area of the atmosphere is the Corona. This graph cuts off at $10,000:km$ but the Sun's atmosphere extends far out beyond Pluto.

Hopefully you can fully appreciate the point I'm getting at which is that Earth actually is inside the atmosphere of the Sun and that atmosphere is really hot! The reason we of course aren't boiled to death is that the atmosphere is incredibly sparse. Only a handful of particles per cubic centimeter. So what you're really proposing is to increase the density of this atmosphere by an insane amount. You want to make it approximately $10^{20}$ times more dense! If we just extrapolate properties, that's going to turn the already hot atmosphere of the Sun (around the Earth) into something incredibly hot. And the huge density means we'll really feel the effects. The Earth will likely be vaporized in a flash, before the cataclysmic explosion which I alluded to above can even happen.

Sorta depends -- you can't have a uniform density atmosphere, so to reach something close to one standard atm. near Earth's orbit (which, BTW would wreak havoc with orbital stability in the first place due to drag), the density would have to be something horrible nearer to the Sun.

This would be an excellent xkcd what-if question if phrased more along the lines of "what density profile" ; my careful, detailed calculation (i.e. pulled out of an impolite orifice) suggests that such an atmosphere would rapidly collapse into the Sun proper due to gravitational attraction. I hesitate to speculate on just how hot the outer reaches would get due to optical absorption -- supposing that this atmosphere suddenly popped into place at an overall temperature in the 200-300 K range and immediately started both heating and collapsing.

## How Does the Sun Heat the Earth?

The sun heats the Earth by shining light on it and radiating heat toward it. The atmosphere absorbs the heat, keeping it close to the Earth's surface where it can sustain life. Without heat and energy from the sun, life on Earth would not exist. Light energy not only warms the planet, it supplies the calories that are consumed by every living thing on the planet.

Several types of light shine on the Earth thanks to the sun, including infrared light, ultraviolet light and visible light. Because it is simple energy, light is transformed into heat once it reaches the surface of the Earth. This is why the temperature is warmer when the sun is shining.

In addition to light rays, the sun also emits radiation in the form of X-rays, microwaves and radio waves. When radiation reaches the atmosphere, the molecules of the atmosphere heat up, causing the molecules around them to become warm as well. The process that causes the atmosphere to warm up is called conduction, according to UCSB ScienceLine. Because there are no atmospheric particles for radiation to interact with in space, it remains quite cold even though the energy that warms the Earth passes right through it.

## If there was air between the Sun and Earth, how warm would we get? - Astronomy

To further explore the causes and effects of global warming and to predict future warming, scientists build climate models&mdashcomputer simulations of the climate system. Climate models are designed to simulate the responses and interactions of the oceans and atmosphere, and to account for changes to the land surface, both natural and human-induced. They comply with fundamental laws of physics&mdashconservation of energy, mass, and momentum&mdashand account for dozens of factors that influence Earth&rsquos climate.

Though the models are complicated, rigorous tests with real-world data hone them into powerful tools that allow scientists to explore our understanding of climate in ways not otherwise possible. By experimenting with the models&mdashremoving greenhouse gases emitted by the burning of fossil fuels or changing the intensity of the Sun to see how each influences the climate&mdashscientists use the models to better understand Earth&rsquos current climate and to predict future climate.

The models predict that as the world consumes ever more fossil fuel, greenhouse gas concentrations will continue to rise, and Earth&rsquos average surface temperature will rise with them. Based on a range of plausible emission scenarios, average surface temperatures could rise between 2°C and 6°C by the end of the 21st century.

Model simulations by the Intergovernmental Panel on Climate Change estimate that Earth will warm between two and six degrees Celsius over the next century, depending on how fast carbon dioxide emissions grow. Scenarios that assume that people will burn more and more fossil fuel provide the estimates in the top end of the temperature range, while scenarios that assume that greenhouse gas emissions will grow slowly give lower temperature predictions. The orange line provides an estimate of global temperatures if greenhouse gases stayed at year 2000 levels. (©2007 IPCC WG1 AR-4.)

### Climate Feedbacks

Greenhouse gases are only part of the story when it comes to global warming. Changes to one part of the climate system can cause additional changes to the way the planet absorbs or reflects energy. These secondary changes are called climate feedbacks, and they could more than double the amount of warming caused by carbon dioxide alone. The primary feedbacks are due to snow and ice, water vapor, clouds, and the carbon cycle.

### Snow and ice

Perhaps the most well known feedback comes from melting snow and ice in the Northern Hemisphere. Warming temperatures are already melting a growing percentage of Arctic sea ice, exposing dark ocean water during the perpetual sunlight of summer. Snow cover on land is also dwindling in many areas. In the absence of snow and ice, these areas go from having bright, sunlight-reflecting surfaces that cool the planet to having dark, sunlight-absorbing surfaces that bring more energy into the Earth system and cause more warming.

Canada&rsquos Athabasca Glacier has been shrinking by about 15 meters per year. In the past 125 years, the glacier has lost half its volume and has retreated more than 1.5 kilometers. As glaciers retreat, sea ice disappears, and snow melts earlier in the spring, the Earth absorbs more sunlight than it would if the reflective snow and ice remained. (Photograph ©2005 Hugh Saxby.)

### Water Vapor

The largest feedback is water vapor. Water vapor is a strong greenhouse gas. In fact, because of its abundance in the atmosphere, water vapor causes about two-thirds of greenhouse warming, a key factor in keeping temperatures in the habitable range on Earth. But as temperatures warm, more water vapor evaporates from the surface into the atmosphere, where it can cause temperatures to climb further.

The question that scientists ask is, how much water vapor will be in the atmosphere in a warming world? The atmosphere currently has an average equilibrium or balance between water vapor concentration and temperature. As temperatures warm, the atmosphere becomes capable of containing more water vapor, and so water vapor concentrations go up to regain equilibrium. Will that trend hold as temperatures continue to warm?

The amount of water vapor that enters the atmosphere ultimately determines how much additional warming will occur due to the water vapor feedback. The atmosphere responds quickly to the water vapor feedback. So far, most of the atmosphere has maintained a near constant balance between temperature and water vapor concentration as temperatures have gone up in recent decades. If this trend continues, and many models say that it will, water vapor has the capacity to double the warming caused by carbon dioxide alone.

### Clouds

Closely related to the water vapor feedback is the cloud feedback. Clouds cause cooling by reflecting solar energy, but they also cause warming by absorbing infrared energy (like greenhouse gases) from the surface when they are over areas that are warmer than they are. In our current climate, clouds have a cooling effect overall, but that could change in a warmer environment.

Clouds can both cool the planet (by reflecting visible light from the sun) and warm the planet (by absorbing heat radiation emitted by the surface). On balance, clouds slightly cool the Earth. (NASA Astronaut Photograph STS31-E-9552 courtesy Johnson space Center Earth Observations Lab.)

If clouds become brighter, or the geographical extent of bright clouds expands, they will tend to cool Earth&rsquos surface. Clouds can become brighter if more moisture converges in a particular region or if more fine particles (aerosols) enter the air. If fewer bright clouds form, it will contribute to warming from the cloud feedback.

See Ship Tracks South of Alaska to learn how aerosols can make clouds brighter.

Clouds, like greenhouse gases, also absorb and re-emit infrared energy. Low, warm clouds emit more energy than high, cold clouds. However, in many parts of the world, energy emitted by low clouds can be absorbed by the abundant water vapor above them. Further, low clouds often have nearly the same temperatures as the Earth&rsquos surface, and so emit similar amounts of infrared energy. In a world without low clouds, the amount of emitted infrared energy escaping to space would not be too different from a world with low clouds.

Clouds emit thermal infrared (heat) radiation in proportion to their temperature, which is related to altitude. This image shows the Western Hemisphere in the thermal infrared. Warm ocean and land surface areas are white and light gray cool, low-level clouds are medium gray and cold, high-altitude clouds are dark gray and black. (NASA image courtesy GOES Project Science.)

High cold clouds, however, form in a part of the atmosphere where energy-absorbing water vapor is scarce. These clouds trap (absorb) energy coming from the lower atmosphere, and emit little energy to space because of their frigid temperatures. In a world with high clouds, a significant amount of energy that would otherwise escape to space is captured in the atmosphere. As a result, global temperatures are higher than in a world without high clouds.

If warmer temperatures result in a greater amount of high clouds, then less infrared energy will be emitted to space. In other words, more high clouds would enhance the greenhouse effect, reducing the Earth&rsquos capability to cool and causing temperatures to warm.

See Clouds and Radiation for a more complete description.

Scientists aren&rsquot entirely sure where and to what degree clouds will end up amplifying or moderating warming, but most climate models predict a slight overall positive feedback or amplification of warming due to a reduction in low cloud cover. A recent observational study found that fewer low, dense clouds formed over a region in the Pacific Ocean when temperatures warmed, suggesting a positive cloud feedback in this region as the models predicted. Such direct observational evidence is limited, however, and clouds remain the biggest source of uncertainty--apart from human choices to control greenhouse gases&mdashin predicting how much the climate will change.

### The Carbon Cycle

Increased atmospheric carbon dioxide concentrations and warming temperatures are causing changes in the Earth&rsquos natural carbon cycle that also can feedback on atmospheric carbon dioxide concentration. For now, primarily ocean water, and to some extent ecosystems on land, are taking up about half of our fossil fuel and biomass burning emissions. This behavior slows global warming by decreasing the rate of atmospheric carbon dioxide increase, but that trend may not continue. Warmer ocean waters will hold less dissolved carbon, leaving more in the atmosphere.

About half the carbon dioxide emitted into the air from burning fossil fuels dissolves in the ocean. This map shows the total amount of human-made carbon dioxide in ocean water from the surface to the sea floor. Blue areas have low amounts, while yellow regions are rich in anthropogenic carbon dioxide. High amounts occur where currents carry the carbon-dioxide-rich surface water into the ocean depths. (Map adapted from Sabine et al., 2004.)

On land, changes in the carbon cycle are more complicated. Under a warmer climate, soils, especially thawing Arctic tundra, could release trapped carbon dioxide or methane to the atmosphere. Increased fire frequency and insect infestations also release more carbon as trees burn or die and decay.

On the other hand, extra carbon dioxide can stimulate plant growth in some ecosystems, allowing these plants to take additional carbon out of the atmosphere. However, this effect may be reduced when plant growth is limited by water, nitrogen, and temperature. This effect may also diminish as carbon dioxide increases to levels that become saturating for photosynthesis. Because of these complications, it is not clear how much additional carbon dioxide plants can take out of the atmosphere and how long they could continue to do so.

The impact of climate change on the land carbon cycle is extremely complex, but on balance, land carbon sinks will become less efficient as plants reach saturation, where they can no longer take up additional carbon dioxide, and other limitations on growth occur, and as land starts to add more carbon to the atmosphere from warming soil, fires, and insect infestations. This will result in a faster increase in atmospheric carbon dioxide and more rapid global warming. In some climate models, carbon cycle feedbacks from both land and ocean add more than a degree Celsius to global temperatures by 2100.

### Emission Scenarios

Scientists predict the range of likely temperature increase by running many possible future scenarios through climate models. Although some of the uncertainty in climate forecasts comes from imperfect knowledge of climate feedbacks, the most significant source of uncertainty in these predictions is that scientists don&rsquot know what choices people will make to control greenhouse gas emissions.

The higher estimates are made on the assumption that the entire world will continue using more and more fossil fuel per capita, a scenario scientists call &ldquobusiness-as-usual.&rdquo More modest estimates come from scenarios in which environmentally friendly technologies such as fuel cells, solar panels, and wind energy replace much of today&rsquos fossil fuel combustion.

It takes decades to centuries for Earth to fully react to increases in greenhouse gases. Carbon dioxide, among other greenhouse gases, will remain in the atmosphere long after emissions are reduced, contributing to continuing warming. In addition, as Earth has warmed, much of the excess energy has gone into heating the upper layers of the ocean. Like a hot water bottle on a cold night, the heated ocean will continue warming the lower atmosphere well after greenhouse gases have stopped increasing.

These considerations mean that people won&rsquot immediately see the impact of reduced greenhouse gas emissions. Even if greenhouse gas concentrations stabilized today, the planet would continue to warm by about 0.6°C over the next century because of greenhouses gases already in the atmosphere.

Chris - If you have a gas, which is, say, in an aerosol you're going to spray into your armpit, like your deodorant, there's a gas under pressure in there.

When you spray it in your armpit, it feels very, very cold. What's happened? Well the gas has expanded. Put simply, if you imagine there's some kind of piston inside the aerosol can, when the gas expanded it effectively pushed on the piston, it's done some work, let's say.

If something has does some work, it must have less energy after it's done the work than before it did the work.

Since temperature is proportional to the energy in the particles, if something's got less energy, it's therefore going to be at a lower the temperature so the temperature must fall and that's why we think that when a gas expands, the temperature goes down.

Dave - OK. And now this is actually very related to why mountains are cold. The temperature of things on Earth is sort of in balance between the amount of heat which is arriving either from the sun or from heat moving around the world, and the amount of heat it can lose by radiating into space.

The only things which can absorb sunlight very well tend to be on the ground. The atmosphere is transparent, so the heat is going into the ground and heating it up and then that heats up the air above it.

The tops of mountains are very, very small. So, basically, what's the temperature of the atmosphere at 30,000 feet? The reason why that's very cold is because if you have pockets of air which is being warmed up on the surface of the Earth and then it lifts up by convection, it's moved upwards, the pressure drops to about half the pressure it was before, which means that gas expands. As gases expand, they get cold, so the air gets very, very cold.

So, the air around the mountain is very, very cold and also anything which is pointy like a mountain has got lots - can emit infrared light in lots more directions and the flat thing has got more surface area compared to mountain sunlight which hits it. So it cools down better during the night and emits light into the space very much better so it tends to be very cold.

## How the Temperature Varies During the Day and Night

When I was in school, I never seemed to have the right coat on. If I walked to school at 7:30 a.m. (0730) in my heavy coat, I would often be too hot on the way home at 3:30 p.m. (1530) On the other hand, it would be too cold in the morning to wear a lighter coat.

Now, as a trained meteorologist, I know the reasons why. Do you know what time of day it is the coldest? Or when it is the warmest?

Fortunately, it’s fairly easy to find some data to answer this question. On the GLOBE web site, you can find GLOBE ONE under “projects” and find data for 10 automatic weather stations from Black Hawk County, Iowa. Figure 1 shows how the temperature varied during five fair-weather days in April 2002, at Station 4.

Figure 1. Air temperature Tavg (red) and dew point Tdavg (blue) at a site in Black Hawk County, Iowa. Height: 1.5 m above the surface. The data are averages of five days with clear skies in April, 2004.

Looking at the graph, the highest temperature is at around 2230 UTC or 4:30 (1630) in the afternoon, local standard time. The lowest temperature is around 7 in the morning local standard time.

Did you expect the temperature to be warmest at noon, when the sun is highest in the sky? Many people do. Why doesn’t that happen?

Let’s start by considering the energy coming from the Sun. Between sunrise and sunset, the radiation from the Sun is continuously adding more energy to Earth’s surface. If this energy didn’t escape somehow, the temperature would be warmest at sunset.

We know this doesn’t happen. So, let’s take a closer look at what does happen. I’ll use data from southeastern Kansas.

Figure 2. For two clear-sky days at a grassland site in southeastern Kansas, ground surface temperature and air temperature (top), downwelling (downward) solar radiation and net radiation (bottom). Notice how the net radiation goes to zero at about 19 hours past midnight and stays negative until about 5 hours past midnight. All times are local time.

In Figure 2, like Figure 1, the air temperature peaks late in the afternoon at 16 hours past midnight (1600 or 4 p.m. local standard time) on May 30, and 16 hours past midnight on May 31 (40 minus 24 hours = 16 hours, 1600 or 4 p.m.).

We know the air at 1.5 meters is heated by radiation and convection.

The bottom of Figure 2 shows what is happening with the radiation. There is still energy coming in from the Sun at 4 p.m. (1600) and afterwards (until about 19.5 hours past midnight). However, the downwelling solar radiation isn’t the whole story.

Some of the solar energy is reflected back upward.

Also the air (greenhouse gases), clouds, and Earth’s surface radiate energy in the infrared. On the days represented in Figures 1 and 2, clouds of course are not a factor. Typically, the infrared radiation from the ground is greater than that from the air. The surface infrared radiation is what is measured by the instrument used in the GLOBE Surface Temperature Protocol: the instrument converts the infrared radiation from a surface (the grass, or asphalt, or bare ground) into a temperature. (For further information about the Surface Temperature see “Teacher’s Guide/Protocols” under “Teachers” in the drop-down menu.)

The net radiation in Figure 2 is the incoming radiation (downwelling solar and infrared) minus the outgoing radiation (reflected solar and upwelling infrared). That is, the net radiation is downward between five hours past midnight (0500) and 19 hours past midnight (1900 or 7 p.m.).

I think I’ve convinced you (and myself) why the warmest air temperature isn’t when the sunlight is strongest. But why isn’t the warmest temperature at around 19 hours past midnight when the net radiation stops heating the ground and starts to go negative?

The reason is that heat is lost through convection.

Air currents carry heat away from the surface. Apparently, at 4 p.m. (1600) local time on both days in Figure 2, the incoming energy from the net radiation just balances the net outgoing energy from convection (convection brings up heat from the ground to 1.5 meters, but it also carries heat from 1.5 meters upward), and the air temperature reaches its maximum. Before 1600 (4 p.m.), the net radiation brings in more energy than convection currents remove, and the air temperature increases. After 1600 (4 p.m.), convection carries away more heat than the radiation is bringing in, and the temperature decreases.

Sometimes we call the adding up of incoming and outgoing heat a “heat budget,” because of the similarity to money. When you save more money than you spend, the amount of money in your bank account — or in your piggy bank — increases. If you spend more money than you save, the amount of money in your bank account or piggy bank gets smaller. When you spending as much as you are putting in, the amount of money stays the same.

What about the surface temperature? This is a little more complicated, because the ground is not only losing energy through convection currents, but it is also losing energy through evaporation and heating up the cooler soil below. These extra losses lead to the surface temperature dropping earlier in the day than the air temperature, around 14 hours past midnight.

At night, things are in some ways simpler. There is no sunlight. On clear nights with little wind, such as those illustrated in Figures 1 and 2, the air and ground keep cooling off by giving off infrared radiation (note that the net radiation at the bottom of Figure 2 is negative throughout the night). Since this continues all night, the coolest temperatures are in the early morning, near the time of sunrise.

Heat transport by air (convection) occurs when winds stir up the air near the surface. This complicates the situation. On average, convection tends to slow the temperature drop at 1.5 meters, with the minimum near sunrise.

## If there was air between the Sun and Earth, how warm would we get? - Astronomy

(Disclaimer: This Will Not Happen.)

Aside from widespread panic and confusion, not much. Earth would cool as it does after sunset, and we would be kept warm by the heat retained in the atmosphere, oceans, and land as we are every night.

This is with the assumption that the Sun is simply "frozen" for an hour -- say, a giant bushel is put over it. If you actually turned off all fusion in the Sun, it would collapse and then explode, and then we'd have other fish to fry (although it would take longer than an hour for the explosion to happen). Don't get confused -- the Sun doesn't have enough mass to become a supernova, this would be a different process, which won't happen because you can't turn off all fusion in the Sun. Though there is a small but finite probability that it can stop right now on its own. But it won't. Very probably. Very. But it could.

If the Sun failed to turn back on in an hour (going back to the bushel case now), we would have serious problems. Certainly within a week, the temperature on Earth would have dropped below freezing. People on the coasts might survive longer than the rest, because of the heat the oceans would release on the other hand I could imagine some intense weather along the coasts due to the temperature gradients. People with large energy reserves would also last longer.

I suppose the place to be would be at the bottom of the ocean, near a geothermal vent (not recommended if you breathe air), where many species may survive for quite some time, perhaps indefinitely?! Some microscopic, one-celled bacteria thrive in the dark, hot and toxic environment of these vents. With no sunlight, these 'archaea' bacteria convert sulphur and other chemicals coming from these vents into energy, much like plants use the sun for photosynthesis. So if the sun didn't turn back on, life on the surface may not like it very much, but bacteria and other extremophiles may not even notice the difference!

#### Sara Slater

Sara is a former Cornell undergraduate and now a physics graduate student at Harvard University, where she works on cosmology and particle physics.

## If there was air between the Sun and Earth, how warm would we get? - Astronomy

When we "feel" heat, is it because electromagnetic radiation is exciting water molecules in our bodies?

First, a quick warning: I am not a biologist, but I've supplemented my vague memories from my high school Anatomy & Physiology class by reading some websites, so hopefully the biological part of my answer will be fairly accurate.

Now, let me distinguish between two uses of the word "heat." Heat is a sensation that occurs when temperature-sensitive nerves in our skin detect a difference between the temperature at the skin surface, and temperature deeper in your body. However, the term "heat" also has a specific meaning in physics, meaning thermal energy.

The sensation of heat comes from nerve-endings that detect the temperature of the skin. The temperature of the skin increases when heat energy flows into the skin. For moderate ranges of temperature, the nerve endings tend to adapt this is why when you first get into a hot shower, it can seem VERY hot, but as time goes by you get used to it. For this reason, the nerve-endings are most sensitive to changes in temperature.

In general, there are three ways for heat to flow from one place to another: convection, conduction, and radiation.

Convection occurs in fluids, when parts of the fluid that are warm tend to rise--but it is not relevant here.

Conduction occurs when heat flows between two objects that are in direct contact. For example, when you wrap your hands around a warm coffee mug, the heat flows directly from the warm mug to your hands. This raises your skin temperature, and you feel the sensation of heat. This all occurs without any exchange of photons--just molecules banging into one another.

Radiation can carry heat in the form of photons. There doesn't have to be direct contact between a hot object and the person for radiation to carry heat, because photons can travel through air, or even a vacuum.

We often think of infrared as "heat radiation" because many of the objects that we have daily contact with (anything with a temperature less than about 500 degrees centigrade) radiate most of their energy in the infrared. However, all wavelengths of light carry heat. The Sun is so hot that it radiates most of its light in visible wavelengths, and these photons heat the Earth (including the people on it).

Also, any object can absorb the photons, not just water molecules. For example, as anyone who likes to go barefoot knows, a perfectly dry sidewalk can get very hot on a sunny day. You may be thinking of a microwave oven, which radiates photons which are absorbed very effectively by water molecules (and also other molecules common in foods, like fats). Your body can absorb microwaves, but they are not produced in great quantities by the Sun or other objects.

Now, when photons strike your skin, some of them are reflected. That's how we can see people! We see the photons visible that are reflected from their skin. But not all the photons are reflected--if they were, then people would look pure white. The photons that aren't reflected are absorbed. The absorbed photons transfer their energy to the skin, increasing its temperature, and again, we feel the sensation of heat.

#### Britt Scharringhausen

Britt studies the rings of Saturn. She got her PhD from Cornell in 2006 and is now a Professor at Beloit College in Wisconson.

## If there was air between the Sun and Earth, how warm would we get? - Astronomy

When we "feel" heat, is it because electromagnetic radiation is exciting water molecules in our bodies?

First, a quick warning: I am not a biologist, but I've supplemented my vague memories from my high school Anatomy & Physiology class by reading some websites, so hopefully the biological part of my answer will be fairly accurate.

Now, let me distinguish between two uses of the word "heat." Heat is a sensation that occurs when temperature-sensitive nerves in our skin detect a difference between the temperature at the skin surface, and temperature deeper in your body. However, the term "heat" also has a specific meaning in physics, meaning thermal energy.

The sensation of heat comes from nerve-endings that detect the temperature of the skin. The temperature of the skin increases when heat energy flows into the skin. For moderate ranges of temperature, the nerve endings tend to adapt this is why when you first get into a hot shower, it can seem VERY hot, but as time goes by you get used to it. For this reason, the nerve-endings are most sensitive to changes in temperature.

In general, there are three ways for heat to flow from one place to another: convection, conduction, and radiation.

Convection occurs in fluids, when parts of the fluid that are warm tend to rise--but it is not relevant here.

Conduction occurs when heat flows between two objects that are in direct contact. For example, when you wrap your hands around a warm coffee mug, the heat flows directly from the warm mug to your hands. This raises your skin temperature, and you feel the sensation of heat. This all occurs without any exchange of photons--just molecules banging into one another.

Radiation can carry heat in the form of photons. There doesn't have to be direct contact between a hot object and the person for radiation to carry heat, because photons can travel through air, or even a vacuum.

We often think of infrared as "heat radiation" because many of the objects that we have daily contact with (anything with a temperature less than about 500 degrees centigrade) radiate most of their energy in the infrared. However, all wavelengths of light carry heat. The Sun is so hot that it radiates most of its light in visible wavelengths, and these photons heat the Earth (including the people on it).

Also, any object can absorb the photons, not just water molecules. For example, as anyone who likes to go barefoot knows, a perfectly dry sidewalk can get very hot on a sunny day. You may be thinking of a microwave oven, which radiates photons which are absorbed very effectively by water molecules (and also other molecules common in foods, like fats). Your body can absorb microwaves, but they are not produced in great quantities by the Sun or other objects.

Now, when photons strike your skin, some of them are reflected. That's how we can see people! We see the photons visible that are reflected from their skin. But not all the photons are reflected--if they were, then people would look pure white. The photons that aren't reflected are absorbed. The absorbed photons transfer their energy to the skin, increasing its temperature, and again, we feel the sensation of heat.

#### Britt Scharringhausen

Britt studies the rings of Saturn. She got her PhD from Cornell in 2006 and is now a Professor at Beloit College in Wisconson.

## Earth climate models and the search for life on other planets

Illustration of an exoplanet. Credit: NASA’s Goddard Space Flight Center/Chris Smith

In a generic brick building on the northwestern edge of NASA's Goddard Space Flight Center campus in Greenbelt, Maryland, thousands of computers packed in racks the size of vending machines hum in a deafening chorus of data crunching. Day and night, they spit out 7 quadrillion calculations per second. These machines collectively are known as NASA's Discover supercomputer and they are tasked with running sophisticated climate models to predict Earth's future climate.

But now, they're also sussing out something much farther away: whether any of the more than 4,000 curiously weird planets beyond our solar system discovered in the past two decades could support life.

Scientists are finding that the answer not only is yes, but that it's yes under a range of surprising conditions compared to Earth. This revelation has prompted many of them to grapple with a question vital to NASA's search for life beyond Earth. Is it possible that our notions of what makes a planet suitable for life are too limiting?

The next generation of powerful telescopes and space observatories will surely give us more clues. These instruments will allow scientists for the first time to analyze the atmospheres of the most tantalizing planets out there: rocky ones, like Earth, that could have an essential ingredient for life—liquid water—flowing on their surfaces.

For the time being, it's difficult to probe far-off atmospheres. Sending a spacecraft to the closest planet outside our solar system, or exoplanet, would take 75,000 years with today's technology. Even with powerful telescopes nearby exoplanets are virtually impossible to study in detail. The trouble is that they're too small and too drowned out by the light of their stars for scientists to make out the faint light signatures they reflect—signatures that could reveal the chemistry of life at the surface.

In other words, detecting the ingredients of the atmospheres around these phantom planets, as many scientists like to point out, is like standing in Washington, D.C., and trying to glimpse a firefly next to a searchlight in Los Angeles. This reality makes climate models critical to exploration, said chief exoplanetary scientist Karl Stapelfeldt, who's based at NASA's Jet Propulsion Laboratory in Pasadena, California.

"The models make specific, testable predictions of what we should see," he said. "These are very important for designing our future telescopes and observing strategies."

Is the solar system a good role model?

In scanning the cosmos with large ground-based and space telescopes, astronomers have discovered an eclectic assortment of worlds that seem drawn from the imagination.

"For a long time, scientists were really focused on finding sun- and Earth-like systems. That's all we knew," said Elisa Quintana, a NASA Goddard astrophysicist who led the 2014 discovery of Earth-sized planet Kepler-186f. "But we found out that there's this whole crazy diversity in planets. We found planets as small as the moon. We found giant planets. And we found some that orbit tiny stars, giant stars and multiple stars."

Indeed, most of the planets detected by NASA's Kepler space telescope and the new Transiting Exoplanet Survey Satellite, as well as ground-based observations, don't exist in our solar system. They fall between the size of a terrestrial Earth and a gaseous Uranus, which is four times bigger than this planet.

Planets closest in size to Earth, and most likely in theory to have habitable conditions, so far have been found only around "red dwarf" stars, which make up a vast majority of stars in the galaxy. But that's likely because red dwarfs are smaller and dimmer than the sun, so the signal from planets orbiting them is easier for telescopes to detect.

Because red dwarfs are small, planets have to lap uncomfortably close—closer than Mercury is to the sun—to stay gravitationally attached to them. And because red dwarfs are cool, compared to all other stars, planets have to be closer to them to draw enough heat to allow liquid water to pool on their surfaces.

Among the most alluring recent discoveries in red dwarf systems are planets like Proxima Centauri b, or simply Proxima b. It's the closest exoplanet. There are also seven rocky planets in the nearby system TRAPPIST-1. Whether or not these planets could sustain life is still a matter of debate. Scientists point out that red dwarfs can spew up to 500 times more harmful ultraviolet and X-ray radiation at their planets than the sun ejects into the solar system. On the face of it, this environment would strip atmospheres, evaporate oceans and fry DNA on any planet close to a red dwarf.

Yet, maybe not. Earth climate models are showing that rocky exoplanets around red dwarfs could be habitable despite the radiation.

The magic is in the clouds

Anthony Del Genio is a recently retired planetary climate scientist from NASA's Goddard Institute for Space Studies in New York City. During his career he simulated the climates of Earth and of other planets, including Proxima b.

Del Genio's team recently simulated possible climates on Proxima b to test how many would leave it warm and wet enough to host life. This type of modeling work helps NASA scientists identify a handful of promising planets worthy of more rigorous study with NASA's forthcoming James Webb Space Telescope.

"While our work can't tell observers if any planet is habitable or not, we can tell them whether a planet is smack in the midrange of good candidates to search further," Del Genio said.

Proxima b orbits Proxima Centauri in a three-star system located just 4.2 light years from the sun. Besides that, scientists don't know much about it. They believe it's rocky, based on its estimated mass, which is slightly larger than Earth's. Scientists can infer mass by watching how much Proxima b tugs on its star as it orbits it.

2014, NASA’s Swift mission detected a record-setting series of X-ray flares unleashed by DG CVn, a nearby binary consisting of two red dwarf stars, illustrated here. At its peak, the initial flare was brighter in X-rays than the combined light from both stars at all wavelengths under normal conditions. Credit: NASA’s Goddard Space Flight Center

The problem with Proxima b is that it's 20 times closer to its star than Earth is to the sun. Therefore, it takes the planet only 11.2 days to make one orbit (Earth takes 365 days to orbit the sun once). Physics tells scientists that this cozy arrangement could leave Proxima b gravitationally locked to its star, like the moon is gravitationally locked to Earth. If true, one side of Proxima b faces the star's intense radiation while the other one freezes in the darkness of space in a planetary recipe that doesn't bode well for life on either side.

But Del Genio's simulations show that Proxima b, or any planet with similar characteristics, could be habitable despite the forces conspiring against it. "And the clouds and oceans play a fundamental role in that," Del Genio said.

Del Genio's team upgraded an Earth climate model first developed in the 1970s to create a planetary simulator called ROCKE-3-D. Whether Proxima b has an atmosphere is an open and critical question that will hopefully be settled by future telescopes. But Del Genio's team assumed that it does.

With each simulation Del Genio's team varied the types and amounts of greenhouse gases in Proxima b's air. They also changed the depth, size, and salinity of its oceans and adjusted the ratio of land to water to see how these tweaks would influence the planet's climate.

Models such as ROCKE-3-D begin with only grains of basic information about an exoplanet: its size, mass, and distance from its star. Scientists can infer these things by watching the light from a star dip as a planet crosses in front of it, or by measuring the gravitational tugging on a star as a planet circles it.

These scant physical details inform equations that comprise up to a million lines of computer code needed to build the most sophisticated climate models. The code instructs a computer like NASA's Discover supercomputer to use established rules of nature to simulate global climate systems. Among many other factors, climate models consider how clouds and oceans circulate and interact and how radiation from a sun interacts with a planet's atmosphere and surface.

When Del Genio's team ran ROCKE-3-D on Discover they saw that Proxima b's hypothetical clouds acted like a massive sun umbrella by deflecting radiation. This could lower the temperature on Proxima b's sun-facing side from too hot to warm.

Other scientists have found that Proxima b could form clouds so massive they would blot out the entire sky if one were looking up from the surface.

"If a planet is gravitationally locked and rotating slowly on its axis a circle of clouds forms in front of the star, always pointing towards it. This is due to a force known as the Coriolis effect, which causes convection at the location where the star is heating the atmosphere," said Ravi Kopparapu, a NASA Goddard planetary scientist who also models the potential climates of exoplanets. "Our modeling shows that Proxima b could look like this."

In addition to making Proxima b's day side more temperate than expected, a combination of atmosphere and ocean circulation would move warm air and water around the planet, thereby transporting heat to the cold side. "So you not only keep the atmosphere on the night side from freezing out, you create parts on the night side that actually maintain liquid water on the surface, even though those parts see no light," Del Genio said.

This is an excerpt of Fortran code from the ROCKE-3D model that calculates the details of the orbit of any planet around its star. This has been modified from the original Earth model so that it can handle any kind of planet in any kind of orbit, including planets that are “tidally locked,” with one side always facing the star. This code is needed to predict how high in the sky of a planet the star is at any time, and thus how strongly heated the planet is, how long day and night are, whether there are seasons, and if so, how long they are. Credit: NASA’s Goddard Institute for Space Studies/Anthony Del Genio

Taking a new look at an old role model

Atmospheres are envelopes of molecules around planets. Besides helping maintain and circulate heat, atmospheres distribute gases that nourish life or are produced by it.

These gases are the so-called "biosignatures" scientists will look for in the atmospheres of exoplanets. But what exactly they should be looking for is still undecided.

Earth's is the only evidence scientists have of the chemistry of a life-sustaining atmosphere. Yet, they have to be cautious when using Earth's chemistry as a model for the rest of the galaxy. Simulations from Goddard planetary scientist Giada Arney, for instance, show that even something as simple as oxygen—the quintessential sign of plant life and photosynthesis on modern Earth—could present a trap.

Arney's work highlights something interesting. Had alien civilizations pointed their telescopes toward Earth billions of years ago hoping to find a blue planet swimming in oxygen, they would have been disappointed maybe they would have turned their telescopes toward another world. But instead of oxygen, methane could have been the best biosignature to look for 3.8 to 2.5 billion years ago. This molecule was produced in abundance back then, likely by the microorganisms quietly flourishing in the oceans.

"What is interesting about this phase of Earth's history is that it was so alien compared to modern Earth," Arney said. "There was no oxygen yet, so it wasn't even a pale blue dot. It was a pale orange dot," she said, referencing the orange haze produced by the methane smog that may have shrouded early Earth.

Findings like this one, Arney said, "have broadened our thinking about what's possible among exoplanets," helping expand the list of biosignatures planetary scientists will look for in distant atmospheres.

Building a blueprint for atmosphere hunters

While the lessons from planetary climate models are theoretical—meaning scientists haven't had an opportunity to test them in the real world—they offer a blueprint for future observations.

One major goal of simulating climates is to identify the most promising planets to turn to with the Webb telescope and other missions so that scientists can use limited and expensive telescope time most efficiently. Additionally, these simulations are helping scientists create a catalog of potential chemical signatures that they will one day detect. Having such a database to draw from will help them quickly determine the type of planet they're looking at and decide whether to keep probing or turn their telescopes elsewhere.

Discovering life on distant planets is a gamble, Del Genio noted: "So if we want to observe most wisely, we have to take recommendations from climate models, because that's just increasing the odds."

## If there was air between the Sun and Earth, how warm would we get? - Astronomy

The Earth is always being pulled towards the Sun by gravity. If the Earth were stationary compared to the Sun, it would fall into the sun under the force of gravity. However the Earth is actually moving sideways compared to the center of the Sun at 3 km/second (

2 miles/second). The Earth is not moving fast enough to "escape" the Sun's gravity and leave the solar system, but it is going too fast to be pulled into the Sun. Therefore, it keeps going around and around - orbiting the Sun. It is rather like a tether ball. Think of the top of the post as the Sun and the ball as the Earth. The string between them is like the force of gravity keeping them the same distance apart. When you hit the tether ball it spins around the post. If there were no air or rope friction, the ball would spin forever without getting any closer to the post. That is essentially what the Earth is doing when it orbits the Sun - in the vacuum of outer space, it does not loose speed to air friction, so it just keeps going around the Sun.

Well, that's a good question, and Newton worried about the same thing! Actually, due to conservation of angular momentum, all the planets are in fairly stable orbits, with minor changes over millions of years, but no chance that they will fly off or anything! The earth's orbit is in the shape of an ellipse, which means that we get a little bit closer and farther from the sun over the course of a year. We also wobble in the tilt of our axis, so that the North Pole does not always point to the star Polaris, which is currently our north star.

But, the orbits are pretty stable, because there is a fairly constant gravitational force between the sun and the earth keeping the earth in its orbit. The strength of this force changes slightly over the course of an orbit, being a bit stronger when the earth is a bit closer - at those times(currently, when the northern hemisphere is having winter) the earth actually orbits a bit faster. (Not to be confused with spin!)

The Earth is "falling" around the Sun. The Earth has some initial momentum - it is moving in a direction, which is perpendicular to the direction of the Sun from the Earth. The Sun's gravity is enough to keep the Earth from flying off in a straight line, away from the Sun, but not enough to bring the Earth closer in - the Earth is continually changing its direction of movement, but in such a way that it follows a nearly circular path around the Sun.

If the Sun's gravity were stronger, it would pull the Earth in closer, but then the angle between the Earth's motion would also be changing more rapidly, so it would continue revolving around the Sun.

This concept is called the conservation of angular momentum, which is one of the basic principles of physics.

Kepler's law and Newtons laws explains this very well. If we could just STOP the Earth for a moment relative to the sun and then allow it to freely move it WOULD fall into the sun. But the earth was born from a ring of material that was MOVING around the sun on a stable orbit. So after the debris coagulated to form the earth, this initial orbital energy was retained. Hence the earth is moving on a stable orbit of fixed radius. Look up Kepler's laws, the period of revolution of a body squared is proportional to the distance between the sun and the body cubed.