The Sun is hot, really hot. How hot hot really is, depends on which part you’re talking about:
The sun has a core, a middle, a surface, and an atmosphere.
Starting from the inside out…
There’s the core, where the pressure and temperature are so great that atoms of hydrogen are fused into helium. Every second, 600 million tons of material go through this conversion, releasing vast amounts of gamma radiation. This is the hottest natural place in the Solar System, reaching temperatures of 15 million degrees Celsius. Photons generated at the core of the Sun are emitted and absorbed countless times over thousands of years on their journey to reach the surface.
Outside the core is the radiative zone. Here, temperatures dip down to where fusion reactions can no longer occur, ranging from 7 million down to 2 million degrees Celsius.
Next on our journey outwards from the centre of the Sun, is the convective zone, where bubbles of plasma carry the heat to the surface like a giant lava lamp. Temperatures at the bottom of the convective zone are 2 million degrees.
Finally, the surface, the part of the star that we can see. This is where the temperature is a relatively cool 5,500 degrees Celsius.
Here’s the strange part, as you move further away from the Sun into its atmosphere, the temperature rises again. Above the surface is the chromosphere, where temperatures rise back up to 20,000 degrees Celsius.
Then there is the corona, the Sun’s outer atmosphere. The corona as a wispy halo around the Sun, visible during eclipses, that stretches millions of kilometres out into space. In the corona, the gases from the Sun are superheated to more than a million degrees – some parts of can even rise to 10 million degrees Celsius.
How can the atmosphere of the Sun get hotter than regions inside it? Astronomers aren’t really sure, but there are two competing theories. It’s possible that waves of energy are released from the surface of the Sun, sending their energy high into the solar atmosphere. Or perhaps the Sun’s magnetic field releases energy into the corona as currents collapse and reconnect.
Stars can get much hotter or colder than our Sun. From the coldest, dimmest red dwarf stars to the hottest blue giants; it’s an amazing Universe out there.
Dinosaurs roamed the Earth for 135 million years. Filling every ecological niche, from the oceans, forests and plains; even the skies.
Then, 66 million years ago, something terrible happened. In a geological instant, 75% of the plants and animals on Earth went extinct. And all of the land dinosaurs were wiped off the Earth forever.
What happened? What killed them off?
What could have caused that much damage in such a short amount of time?
The key to this mystery was found in a strange layer of ash sandwiched between layers of rock deposited 66 million years ago. This line, known as the Cretaceous-Paleogene boundary, is found across the world in the geologic record and it marks the moment when everything DIED. What’s interesting about this layer is that it’s rich in iridium, a rare element on Earth, but abundant in asteroids.
And so, geologists found the most likely culprit: an asteroid.
This evidence matched the discovery of an enormous asteroid impact basin in the Yucatán Peninsula in Mexico, centered near the town of Chicxulub. The rock debris in this area could be dated back to approximately 66 million years old, matching the worldwide layer of ash.
We now know that an asteroid at least ten kilometres across slammed off the coast of Mexico 66 million years ago, releasing 2 million times more energy than the most powerful nuclear bomb ever detonated.
The effect of this impact is mindblowing.
Chicxulub CraterMillions of tonnes of rock were ejected into space on ballistic trajectories. Reheated by atmospheric re-entry, this debris superheated the air across the entire planet, catching the world’s forests on fire.
Shockwaves radiated outward from the impact site, inducing earthquakes and volcanoes along their path. Mega tsunamis thousands of meters high spread out from the impact site, pounding coastlines around the world.
Dust rained down across the planet. It filled the air, darkening the skies for decades, and preventing photosynthesis. Plants on land and in the oceans were unable to produce energy.
The planet cooled from the choking dust and aerosols, followed by years of acid rain, and then even global warming as the carbon from the blasted life filled the atmosphere.
Artists concept of asteroid impact eventThe effects to life were devastating.
It’s no surprise the land dinosaurs didn’t make it through this impact event. In fact, it’s a bigger surprise that our ancient ancestors, hardy early mammals could endure.
And our final sobering thought is that impacts of this scale have happened many times in the past, and will happen again in the future.
Have you ever noticed that the Moon always looks the same? Sure, the phase changes, but the actual features on the Moon always look the same from month to month.
Does the Moon rotate? What’s going on?
From our perspective here on Earth, the Moon always shows us the same face because it’s tidally locked to our planet. At some point in the distant past, the Moon did rotate from our perspective, but the Earth’s gravity kept pulling unevenly at the Moon, slowing its rotation. Eventually the Moon locked into place, always displaying the same side to us.
But if you looked down on the Earth-Moon system from the north celestial pole, from the perspective of Polaris, the North Star, you’d see that the Moon actually does rotate on its axis. In fact, as the Moon travels around the Earth in a counter-clockwise orbit every 27.5 days, it also completes one full rotation on its axis – also moving in a counter-clockwise direction.
If you look at a time lapse animation of the Moon moving entirely through its phases over the course of a month, you’ll notice a strange wobble, as if the Moon is rocking back and forth on its axis a bit.
This is known as libration.
On average, the Moon is tidally locked to the Earth’s surface. But its actual orbit is elliptical, it moves closer and then more distant from the Earth.
When the Moon is at its closest point, it’s rotation is slower than its orbital speed, so we see an additional 8 degrees on its eastern side. And then when the Moon is at the most distant point, the rotation is faster than its orbital speed, so we can see 8 degrees on the Western side.
Libration allowed astronomers to map out more of the Moon’s surface than we could if the Moon followed a circular orbit.
Until the space age, half the Moon was hidden from us, always facing away. This hemisphere of the Moon was finally first observed by the Soviet Luna 3 probe in 1959, followed by the first human eyes with Apollo 8 in 1968.
The two hemispheres of the Moon are very different.
While the near side is covered with large basaltic plains called maria, the far side is almost completely covered in craters. The reasons for this difference is still a mystery to planetary scientists, but it’s possible that a second Moon crashed into it, billions of years ago, creating the strange surface we see today.
So yes, the Moon does rotate.
But its rotation exactly matches its orbit around the Earth, which is why it looks like it never does.
What's the Most Earth-Like Planet In The Solar System?
Life on Earth got you down? Thinking you’d like to pick up and move to another planet? I’ve got bad news for you. Without protection, there’s no place in the entire Solar System that wouldn’t kill you in few seconds.
You’re looking at scorching temperatures, poisonous atmospheres, crushing gravity, bone chilling cold, a complete lack of oxygen, killer radiation, and more.
The entire Solar System is hostile to life as we know it.
If we had to choose from a range of terrible options, what would be the most Earthlike place in the Solar System?
We would want a world that has a similar gravity, similar atmospheric pressure and composition, protection from radiation, and a comfortable temperature. Just like the Earth.
Let’s look at a few candidates:
The Moon looks good. It’s close and… well, it’s close. It’s an airless world, so you’d need a spacesuit. Low gravity is bad news for your bones, which will lose mass and become brittle. Temperatures range from freezing cold to scorching hot, and there’s no atmosphere or significant magnetic field to protect you from the radiation of space.
While we’re suggesting moons, how about Titan, Saturn’s largest Moon?
It’s only 15% of Earth’s gravity, and the temperatures dip down to minus -179 degrees C; cold enough that it rains liquid methane. Even though the atmosphere is unbreathable, the good news is that the pressure is only a little higher than Earth’s. Which means you wouldn’t need a pressurized spacesuit, just a really, really warm coat.
The gravity of Mars is only 38% the gravity of Earth; and we don’t know what effect a long stay in this gravity would have on the human body. The atmosphere is poisonous carbon dioxide, and the pressure is less than 1% of sea level on Earth. So, you’d better pack a spacesuit. The temperatures can rise as high as a comfortable 35 degrees C, but then plunge down to -143 degrees C at the poles. One big problem with Mars is a total lack of magnetosphere. Radiation from space would be a constant hazard for anyone on the surface of the planet.
Atmosphere of Venus. Credit: ESAPerhaps another planet? How about Venus?
On the surface, it’s right out of the running. The temperature is an oven-like 462 degrees C, with a surface pressure 92 times more than Earth. The atmosphere is almost entirely carbon dioxide, with clouds of sulphuric acid. On the plus side, it has gravity roughly similar to Earth, and a thick atmosphere that would protect you from radiation.
Unfortunately, you’d die faster on the surface of Venus than almost anywhere else in the Solar System.
But… there is a place on Venus that’s downright lovely.
Up in the clouds.
Amazingly, if you rise up through the clouds of Venus to an altitude of 50-60 kilometers, the atmospheric pressure and temperature are the same as on Earth. The atmosphere would still be toxic carbon dioxide, but breathable air would be a “lifting gas” on Venus. You could float around the skies of Venus in a balloon made of breathable air. Stand out on the deck of your Venusian sky city in shorts and a T-shirt, soaking up the sunlight in regular Earth gravity.
Sounds idyllic, right?
So, opinions will vary. Some think Mars is the most Earthlike place in the Solar System, but in my opinion, the clouds of Venus are the place to go.
There is almost nothing that could completely destroy the earth.
Follow your instincts and ignore anyone raising alarms about its imminent demise.
Oh sure, there’s a pile of events that could make life more difficult, and a laundry list of things that could wipe out all of humanity. Including: asteroid strikes, rising temperatures, or global plagues
In order to actually destroy the Earth, you would need significantly more energy, and there just happens to be enough, a short 150 million kilometers away: the Sun.
The Sun has been in the main sequence of its life for the last 4.5 billion years, converting hydrogen into helium. For stars this massive, that phase lasts for about 10 billion years, meaning we’re only halfway through.
When the Sun does finally run out of hydrogen to burn, it’ll begin fusing helium into carbon, expanding outward in the process. It will become a cooler, larger, red giant star, consuming the orbits of Mercury and Venus.
Scientists are still unsure if the red giant phase of the Sun will consume the Earth. If it does, the Earth’s story ends there. It’ll get caught up inside the Sun, and spiral inward to its demise.
Death by red giant in 5.5 billion years.
If the Sun doesn’t consume the Earth then we’ll have a long, cold future ahead of us. The Sun will shrink down to a white dwarf and begin cooling down to the background temperature of the Universe. The Earth and the rest of the surviving planets will continue orbiting the dying Sun for potentially trillions of years.
Planet orbiting a dead star. Credit: NASAIf we’re exceedingly lucky, the Sun will get too close to another star, and the gravitational interactions will capture Earth in orbit, giving our planet a second chance for life. If not, the Earth will continue following the dying Sun around and around the Milky Way for an incomprehensible amount of time.
At this point, the main risk to the planet is a collision. Or maybe it’ll spiral inward over vast periods of time to be destroyed by the Sun, or collide with another planet. Or perhaps the entire Solar System will slowly make its way into the supermassive black hole at the center of the Milky Way.
One last possibility. Physicists think that protons – the building blocks of atoms – might eventually decay, becoming smaller particles and pure energy. After an undecillion years – a 1 followed by 36 zeros – half of the Earth will have just melted away into energy.
But if protons don’t decay, the Earth could theoretically last forever.
Look up into the night sky and count the moons. You can see only one moon, “the” Moon. But does the Earth have any other moons? Around the Solar System, multiple moons are the rule. Jupiter has 67 natural satellites, even Mars has two asteroid-like moons.
Could Earth have more than one?
Officially, the answer is no. The Earth has a single moon.
Today.
It’s possible Earth had more than one moon in the past, millions or even billions of years ago. Strange terrain on the far side of the Moon could be explained by a second moon crashing into it, depositing a layer of material tens of kilometers deep.
Moons could come and go over the billions of years of the Earth’s history.
For example, Mars has two Moons, but not for long. Phobos, the larger moon, is spiraling inward and expected to crash into the planet within the next 10 million years. And so, in the future, Mars will only have a single Moon, Deimos.
It’s also possible that the Earth might capture a Moon in the future. Neptune’s largest moon, Triton, orbits in the opposite direction from the rest of the moons around the planet. This suggests that Triton was actually a captured Kuiper Belt Object which strayed too close to the planet.
2006 RH120In fact, we did capture a 5-metre asteroid called 2006 RH120. It orbited the Earth four times during 2006/2007 before getting ejected again.
So we can assume events like this have happened in the past.
Additionally, we might have more moons, but they haven’t been discovered yet because they’re just too small. Researchers have calculated that there could be meter-sized asteroids in orbit around the Earth, remaining in orbit for hundreds of years before gravitational interactions push them out again.
And there are other objects that interact with Earth’s orbit in strange ways. Scientists don’t consider them moons, but they do stick around in our neighbourhood:
Asteroid 3753 Cruithne is in an orbital resonance with the Earth. It has a highly eccentric orbit, but takes exactly one year to orbit the Sun. From our perspective, it follows a slow, horse-shoe shaped path across the sky. Since the discovery of Cruithne in 1986, several other resonant near-Earth objects have been discovered.
2007 TK7There’s 2010 TK7, the Earth’s only known Trojan asteroid. It leads the Earth in the exact same orbit around the Sun, in a gravitationally stable point in space.
So, the answer… Earth only has a single Moon. Today. We might have had more moons in the past, and we might capture more in the future, but for right now… enjoy the one we’ve got.
If you could travel from world to world, from star to star, out into the gulfs of intergalactic space, you’d move away from the warmth of the stars into the vast and cold depths of the void.
The short answer is, the average distance to the Moon is 384,403 km (238,857 miles). But before you go thinking that this is the final answer, you need to consider a few things. For starters, note the use of the word “average”. This refers to the fact that the Moon orbits around the Earth in an elliptical pattern, which means that at certain times, it will be father away; while at others, it will be closer.
Hence, the number 384,403 km, is an average distance that astronomers call the semi-major axis. At its closest point (known as perigee) the Moon is only 363,104 km (225,622 miles) away. And at its most distant point (called apogee) the Moon gets to a distance of 406,696 km (252,088 miles).
This means that distance from the Earth to the Moon can vary by 43,592 km. That’s a pretty big difference, and it can make the Moon appear dramatically different in size depending on where it is in its orbit. For instance, the size of the Moon can vary by more than 15% from when it’s at its closest to when it’s at the most distant point.
It can also have a dramatic effect on how bright the moon appears when it is in its Full phase. As one might expect, the brightest full Moons occur when the Moon is at the closest, which are typically 30% brighter than when it’s fathest away. When it’s a Full Moon, and it’s a close Moon, it’s known as a Supermoon; which is also known by it technical name – perigee-syzygy.
To get an idea of what this all looks like, check out the animation above that was released by the Goddard Space Flight Center Scientific Visualization Studio in 2011. The animation shows the geocentric phase, libration, position angle of the axis, and apparent diameter of the Moon throughout the year, at hourly intervals.
At this point, a good question to ask would be: how do we know how far away the Moon is? Well, that depends on when we’re talking. In the days of ancient Greece, astronomers relied on simple geometry, the diameter of the Earth – which they had already calculated to be the equivalent of 12,875 km (or 8000 miles) – and the measurements of shadows to make the first (relatively) accurate estimates.
Having observed and recorded how shadows work over a long period of history, the ancient Greeks had determined that when an object is placed in front of the Sun, the length of a shadow this generates will always be 108 times the diameter of the object itself. So a ball measuring 2.5 cm (1 inch) across and placed on a stick between the Sun and the ground will create a triangular shadow that extends for 270 cm (108 inches).
This reasoning was then applied to the phenomena of Lunar and Solar Eclipses.
In the former, they found that the Moon was imperfectly blocked by the shadow of the Earth, and that the shadow was roughly 2.5 times the width of the Moon. In the latter, they noted that the Moon was of sufficient size and distance to block out the Sun. What’s more, the shadow it would create terminated at Earth, and would end in the same angle that the shadow of the Earth does – making them different-sized versions of the same triangle.
Using the calculations on the diameter of the Earth, the Greeks reasoned that the larger triangle would measure one Earth diameter at its base (12,875 km/8000 miles) and be 1,390,000 km (864,000 miles) long. The other triangle would be the equivalent of 2.5 Moon diameters wide and, since the triangles are proportionate, 2.5 Moon orbits tall.
Adding the two triangles together would yield the equivalent of 3.5 Moon orbits, which would create the largest triangle and gave the (again, relatively) accurate measurement of the distance between the Earth and the Moon. In other words, the distance is 1.39 million km (864,000 miles) divided by 3.5, which works out to around 397,500 km (247,000 miles). Not exactly bang on, but not bad for ancient peoples!
Lunar Laser Ranging Experiment from the Apollo 11 mission. Credit: NASA
Today, millimeter-precision measurements of the lunar distance are made by measuring the time it takes for light to travel between LIDAR stations here on the Earth and retroreflectors placed on the Moon. This process is known as the Lunar Laser Ranging experiment, a process that was made possible thanks to the efforts of the Apollo missions.
When astronauts visited the Moon more than forty years ago, they left a series of retroreflecting mirrors on the lunar surface. When scientists here on Earth shoot a laser at the Moon, the light from the laser is reflected right back at them from one of these devices. For every 100 quadrillion photons shot at the Moon, only a handful come back, but that’s enough to get an accurate appraisal.
Since light is moving at almost 300,000 kilometers (186,411 miles) per second, it takes a little more than a second to make the journey. And then it takes another second or so to return. By calculating the exact amount of time it takes for light to make the journey, astronomers are able to know exactly how far the Moon is at any time, down to millimeter accuracy.
From this technique, astronomers have also discovered that the Moon is slowly drifting away from us, at a glacial rate of 3.8 cm (1.5 inches) a year. Millions of years in the future, the Moon will appear smaller in the sky than it does today. And within a billion years or so, the Moon will be visually smaller than the Sun and we will no longer experience total solar eclipses.
There are as many as four-hundred billion stars in our galaxy: the Milky Way. And there are more than one-hundred-and-seventy billion galaxies in the observable Universe. Most of those stars have planets, and many of those planets have got to contain useful minerals and fall within their star’s habitable zone where liquid water is present.
The conditions for life are probably everywhere.
But where are all the aliens?
And think about this.
The Universe has been around for 13.8 billion years. Human beings originated 200,000 years ago, so we’ve only been around for 0.01% of the age of the Universe. An intelligent species could arise on any one of those countless worlds, and broadcast their existence to the entire galaxy.
Once a species developed interstellar travel, they could completely colonize our galaxy within a few tens of millions of years; just a heartbeat in the age of the Universe.
So where are they?
As far as we know, Earth is the only place in the Universe where life has arisen, let alone developed an intelligent civilization.
This baffling contradiction is known as the Fermi Paradox, first described in 1950 by the physicist Enrico Fermi.
Scientists have been trying to resolve this mystery for decades, listening for radio signals from other worlds. We’ve only sampled a fraction of the radio spectrum, and so far, we haven’t detected anything that could be a signal from an intelligent species.
How can we explain this?
Maybe we really are the only planet in the entire Universe to develop life. Maybe we’re the first civilization to reach this level of advancement in the entire galaxy. But with so many worlds out there, that really seems unlikely.
Artist impression of an asteroid impact on early Earth (credit: NASA)Maybe civilizations destroy themselves when they reach a certain point. Nuclear weapons, global warming, killer epidemics, and overpopulation could all end humanity. Asteroids could strike the planet and wipe us out. But would this happen to every single civilization? one-hundred-percent of them? Even if ninety-nine-percent of civilizations destroy themselves, we’d still have a couple that made it through and fully colonized the galaxy.
Maybe they’re just too far away, and our signals can’t reach each other. But then, self-replicating probes could traverse those distances and leave a local artifact in every single star system.
Maybe we can’t understand their signals or recognize their artifacts. Maybe, but if aliens constructed a series of artifacts on Earth, I think we’d notice them. The aliens would have experience creating obvious structures.
Maybe they’re just too alien and we just can’t understand them. Maybe we’re too insignificant, and they don’t think we’re even worth talking to. We don’t need to talk to them to know they exist. If they flew through our Solar System, ignoring us, we’d still know they’re around.
Maybe they’re not talking to us on purpose, and we’re really in some kind of galactic zoo. Or aliens have a Prime Directive, and they’re not allowed to talk to us. Again, all the aliens? Not a single one has gotten through and snuck us some evidence? Milky Way. Image credit: NASA
There are many other potential solutions to the Fermi Paradox, but I personally find them all insufficient. The Universe is big, and old, and if extraterrestrial life is anything like us, it wants to multiply and spread out.
Perhaps the most unsettling thought is that something happens to 100% of intelligent civilizations that prevents them from exploring and settling the galaxy. Maybe something good, like the discovery of a transportation system to another Universe. Or maybe something bad, like a destructive technology that has destroyed every single civilization before us.
How do you feel about the Fermi Paradox? How do you resolve the contradictions? Whatever the solution, it’s really fun to think about.
Earth is the third planet from the Sun, and the climate here is just right for life. Here in our Solar System, there are planets both hotter and colder than Earth.
So… which one is the hottest?
You might think it’s Mercury, the planet closest to the Sun. Mercury orbits at a distance of only 58 million kilometers, travelling in a blast-furnace of scorching radiation. Its temperature can skyrocket to 700 Kelvin, or 426 degrees Celsius on the sunward side. In the shadows, temperatures plunge down to 80 Kelvin, which is -173 degrees Celsius
Mercury sure is hot, but Venus is hotter.
Venus imaged by Magellan Image Credit: NASA/JPLVenus is much further from the Sun, orbiting at a distance of more than 108 million kilometers. Average temperature there is a hellish 735 Kelvin, or 462 degrees Celsius – hot enough to melt lead.
Venus remains that same temperature no matter where you go on the planet. At the North Pole? 735 Kelvin. At night? 735 Kelvin. Daytime at the equator? You get the point.
So, why is Venus so much hotter than Mercury, even though it’s further away from the Sun? It’s all about the atmosphere.
Mercury is an airless world, not unlike the Moon. Venus, has a very thick atmosphere of CO2, which adds incredible pressure, and traps in the heat.
Atmosphere of Venus. Credit: ESAConsider our own planet. When you stand at sea level on Earth, you’re experiencing one atmosphere of pressure. But if you could stand on the surface of Venus – and trust me, you don’t want to – you’d experience ninety-two times as much atmospheric pressure. This is the same kind of pressure as being a kilometer underneath the surface of the ocean.
Venus also shows us what happens when carbon dioxide levels just keep on rising. Radiation from the Sun is absorbed by the planet, and the infrared heat emitted is trapped by the carbon dioxide, which creates a runaway greenhouse effect.
You might think a planet this hot with such extreme temperature and pressure, would be impossible to explore.
And if you did, you’d be wrong.
The Soviets sent a series of spacecraft called Venera, which parachuted down through the thick atmosphere and returned images from the surface of Venus. Although the first few missions were failures, this taught the Soviets just how hellish the Venusian environment really is. Surface of Venus by Venera.
Venera 13 made it down to the surface in nineteen-eighty-one and survived for one-hundred-and-twenty-seven minutes, sending back the first color pictures of Venus’ surface.
The hottest planet in our solar system is Venus,
When it comes to temperature, distance from the Sun matters, but it takes a backseat to wrapping a planet in a atmospheric blanket of carbon dioxide.
Then there is the corona, the Sun’s outer atmosphere. The corona as a wispy halo around the Sun, visible during eclipses, that stretches millions of kilometres out into space. In the corona, the gases from the Sun are superheated to more than a million degrees – some parts of can even rise to 10 million degrees Celsius.
