Elon Musk has indeed sent the first astronauts to the ISS and is soon threatening to conquer Mars with the Starship, which is currently under construction. It is quite possible that the first flight to the Red Planet will happen within the next few years.
However, colonizing other celestial bodies, as Elon dreams, will not be easy since conditions on them are, to put it mildly, not very comfortable. Astrophysicist Neil deGrasse Tyson spoke in detail about how long a person could survive on various planets or even stars in an interview with Business Insider.
Sun
Image: NASA/ESA, SOHO
Obviously, the Sun would incinerate you instantly, as its surface temperature is 5,499°C (9930°F). Technically, the Sun doesn’t have a solid surface—this term refers to the area between its core and its corona. You would simply evaporate without a trace.
However, “instantly” is a relative term. Physicist Randall Munroe, a former NASA employee, believes that if you were teleported to the Sun for just one nanosecond (one-billionth of a second) and then returned, you would survive. Your skin would receive about five orders of magnitude less heat than a one-second touch of a butane burner, meaning you wouldn’t even notice.
But if you were teleported closer to the core, where temperatures reach 14,999,727°C (26,999,540°F), you would evaporate in one femtosecond (one millionth of a nanosecond, or one quadrillionth of a second).
Average lifespan: 10⁻¹⁵ seconds.
Mercury
Image: NASA
A day on Mercury lasts 59 Earth days, and its year lasts 88 Earth days. The planet has virtually no atmosphere, so the sky is always black, and the Sun appears two and a half times larger than it does from Earth. It also moves strangely across the sky. The daytime side of Mercury heats up to +427°C (806°F), while the night side cools down to −180°C (-292°F).
However, if you were to stand somewhere on the border between day and night (the so-called terminator), you could survive—as long as you could do without oxygen.
The surface of Mercury is almost a vacuum, so if there is still air in your lungs, they would likely burst, your body would start to swell, and your blood would begin to boil. In about 10-15 seconds, you would lose consciousness due to lack of oxygen, and after 1-2 minutes, you would die from simple hypoxia.
Average lifespan: 2 minutes.
Venus
Image: NASA
Venus has almost the same gravity as Earth, but its atmosphere is much denser, composed mostly of carbon dioxide. The air is so thick that it would be difficult to move—like being 914 meters (3,000 feet) underwater in the Pacific Ocean. A day on Venus lasts 116 Earth days, but the atmosphere is so thick that it’s very dark on the surface.
The greenhouse effect heats the planet up to 465°C (869°F), causing constant sulfuric acid rain, which turns into sulfuric acid fog near the surface.
Thus, if you were to land on Venus, you would immediately be crushed by the atmosphere and burned by the heat and sulfuric acid.
Average lifespan: less than 1 second.
Earth
Image: NASA
Mostly harmless.
Lifespan: from several seconds or minutes (if you find yourself near aggressive predators, hostile humans, over the ocean, inside a volcano crater, or at high altitudes with thin air) to 122 years (the official longevity record set by Frenchwoman Jeanne Calment).
Mars
Image: NASA
Mars is quite cold, ranging from −60 to +20°C, but it also has a very thin atmosphere composed mainly of carbon dioxide, with some nitrogen and argon. The low temperatures will not feel as extreme as they would on Earth. However, there’s nothing to breathe.
You would survive on Mars as long as you could hold your breath. If you brought an oxygen tank, you’d then be killed by the low atmospheric pressure (within a few minutes), the cold (in a few hours), Martian dust damaging your lungs (within a few weeks), or radiation (within a few months).
Average lifespan: 2 minutes.
Jupiter
Image: NASA
Jupiter is a gas giant, meaning it has no solid surface to land on. If you were to fall into it from a great height, you would likely be killed by intense radiation long before reaching the atmosphere.
If you somehow survived this and reached the upper layers of the atmosphere, you would plummet through them at 180,000 km/h (due to Jupiter’s stronger gravity, you would fall faster). At around 250 kilometers in altitude, you would encounter ammonia clouds and experience temperatures of −150°C and strong winds—hurricanes in Jupiter’s hydrogen atmosphere reach speeds of 482 km/h. At this point, the pressure would already be enough to kill you.
If you managed to survive this, after 12 hours of continuous falling, you would reach the lower layers of the atmosphere, where total darkness reigns, the pressure is 2,000,000 times greater than on Earth, and temperatures are higher than on the surface of the Sun. Nothing could save you there.
Average lifespan: less than 1 second.
Saturn
Image: NASA
Everything said about Jupiter also applies to the other gas giants. Saturn is no exception—if you fell into its atmosphere, you would be crushed by enormous pressure and destroyed by high temperatures.
Average lifespan: less than 1 second.
Uranus
Image: NASA
Another gas giant. Pressure, temperature, and radiation are guaranteed.
Average lifespan: less than 1 second.
Neptune
Image: NASA
Although Neptune is called an ice giant, deep within its hydrogen-methane atmosphere, temperatures reach 476.85°C (890°F). The pressure is also extremely high. So, on this planet, the same fate awaits you as on Jupiter.
Because Europa, Jupiter’s moon, is thought to conceal an ocean of liquid water under its icy cover, it may be home to intelligent life from outside our solar system. Europa is perhaps more intriguing than any other moon in the galaxy. The frigid moon may be more habitable than previously believed. Europa, the ice moon, seems frigid and frightening at first view. However, it is misleading. Because there is a massive ocean of liquid water under its crust, providing possible conditions for life. Scientists have been slowly digging into the question of whether or not life exists on Europa in recent years. The outcome of which might be unexpected.
Ocean below Europa’s ice
This might be what it looks like when water vapor and water shoot out from the ice and form springs on Europa. (K. Retherford/SWRI/NASA/ESA)
Jovian satellite Europa is strangely beautiful, with a network of furrows that stretch for miles over its icy surface. These furrows resemble the cracks in a painting. However, this stunning scenery comes at a high cost, since no form of life could survive the -240°F (-150°C) temperatures, solid ice crust, and complete absence of air.
A secret habitat
According to the results, Europa likely has lakes underneath its frozen surface. (NASA/JPL/University of Arizona)
However, at depths of 6 to 9 miles (10 to 15 km), a whole other universe opens up. Here lies a vast ocean of salt water that wraps around Europa and is likely to be at least 60 miles (100 km) deep. This ocean may hold twice as much water as all the seas on Earth combined.
Again, details about this watery supermassive planet were gleaned through Galileo probe magnetic field data. Because it also picked up telltale abnormalities in Jupiter’s magnetic field surrounding Europa, the kind of disturbances usually caused by motions of conductive fluid. A magma ocean is unlikely on Europa since the moon is entirely covered in ice and, unlike Io, has no solid crust to keep the heat in. A subglacial sea of liquid salt water, on the other hand, would be conductive but also allow ice crusts to exist.
Since Europa is Jupiter’s second-most inner moon, powerful tidal forces are at play there. (NASA)
The key question, though, is: what exactly is it that keeps the water liquid under the ice in Europa? Tidal forces from Jupiter are a likely cause. Europa’s orbit is very elliptical and somewhat eccentric around its host planet. Because of this, the strength of Jupiter’s gravitational attraction varies throughout the orbit. Thus, the interior of Europa varies according to how squeezed it is by Jupiter at a given moment.
There is a little elliptical deformation of the whole moon when Jupiter is near enough to cause the ice, water, and rock on the side facing it to rise. However, when Europa migrates away from Jupiter, its form once again becomes roughly spherical. These continuous motions produce heat through subsurface friction, which may be sufficient to maintain a liquid water layer under the ice of Europa.
Europa’s cracks and their meaning
When did the cracks start taking on such a peculiar form?
This colorized Galileo satellite picture shows that the ice crust is fractured along many directions. (NASA/JPL/University of Arizona)
Europa’s ice crust is so stiff that it cannot adapt to the movement and fissures caused by the subglacial ocean’s continual ups and downs. Images taken by the Galileo spacecraft reveal a fascinating pattern of elongated trenches and furrows formed by these fissures. Water from the ocean rises to the surface at these fissures regularly and solidifies, thus, causing the darker hue of the lines.
Mysterious directional shifts in the fractures
But there’s something off about these lines. It’s unclear why these fractures have changed direction over time. Since Jupiter’s moon rotates in a tidal lock with the planet, the tidal pressures are always exerted in the same direction. Then why are the cracks not constantly forming in the same direction?
There are theoretically three potential causes: To begin, Europa’s icy crust may revolve a little faster than the rest of the moon. As a result, Jupiter’s tidal forces would gradually shift the crust’s orientation relative to the planet over time. Another idea is that, like Earth’s axis of rotation, Europa’s is somewhat inclined relative to its orbital motion. As a result, Europa would wobble more over time, and its crustal areas would alternately begin to draw closer to Jupiter.
A third prospect is that the fractures are scattered at random and that their direction is not related to lunar activities but could be related to local weak points in the ice sheet.
Europa’s wobbling axis
But this crust hypothesis came up on the losing end, with the models based on this scenario consistently failing to reproduce Europa’s usual cracking pattern. However, if the simulations were modified in a way that Europa’s axis swung back and forth by around 1 degree over time, the results would get quite similar to the fracture pattern seen in the ice crust, suggesting that the wobbling axis is more likely the reason for the directional changes in Europa’s fractures and ridges. Even a little axial tilt (or obliquity) accounts for a lot of this current phenomenon on Europa.
Liquid water under Europa’s ice sheet
What maintains the ocean’s fluidity on Europa? Sloshing water, or even subsea volcanoes. It’s from NASA and the Jet Propulsion Laboratory.
The usual breaking pattern of the ice crust may have been caused by Europa’s mild tumbling, which may also have led to the creation of a subglacial ocean. That’s because the heating impact of tidal forces can be amplified even by tiny variations in Europa’s orientation with regard to Jupiter.
Some scientists think that the subglacial ocean in Europa is continually sloshing back and forth under the unyielding crust due to the ice moon’s wobbling. Instead of being relatively motionless, the water would be marked by powerful currents. Also, the energy released by the motion of the water might be enough to keep Europa’s ocean liquid. Thus, the liquid water on Europa could be heated by the ocean itself, not by the surface or the subsurface. But, this is still only a theory at this point.
The possibility of life on Europa
Organisms under the ice
If water exists as a liquid on Europa, then there might be life on this icy moon of Jupiter. But there is more to it when it comes to subglacial lakes or seas. For thriving and reproducing, most organisms need either light or air, or at least certain gases. That’s why, for a long time, it was believed that the subterranean lakes of Antarctica were fairly unfriendly to life. But that didn’t turn out to be true.
Proof of life on Lake Vostok
A perspective image of the ice surface of Lake Vostok. (Michael Studinger / Lamont-Doherty Earth Observatory)
Lake Vostok, the biggest subglacial lake in the Antarctic on Earth has always been frozen solid. Having been covered by ice almost 2 miles (3 km) thick for the last 15 million years, its waters have been cut off from the surface. No light can make it through this ice sheet, the air pressure is very high, and food is probably in short supply. It’s undeniable that species on Earth that call Lake Vostok home must thrive under very harsh circumstances.
No samples of the lake’s liquid had been obtained due to concerns about contamination, so it was unclear whether or not there were organisms living in the waters of Vostok. However, in recent years, scientists have drilled just above the water level, collecting the first samples of frozen lake water from the boundary layer. This layer forms when the lake water freezes at the points of contact with the glacier ice and then collects on the underside of that glacier ice.
Incredible variety
The samples at depths of around 11,700 to 11,900 ft (3,550 to 3,600 m) came from the ice formed by the lake water and when the samples were studied, the scientists deciphered the DNA and RNA sequences frozen in the ice to discover the kind of species living in the Lake Vostok.
What they uncovered instead of an area devoid of diversity was the DNA of thousands of species living in impossible conditions. 94% of the species were bacteria, while the remaining 6% were either fungus or archaea, a unique class of single-celled creatures. This finding has forever changed our view of what is and isn’t deemed liveable.
Evidence of higher life
Future autonomous diving robots may help us learn more about the ecosystem of Lake Vostok, and potentially Europa as well. (NASA / JPL-Caltech)
What’s more, the investigations also uncovered the fact that many of the bacterial species detected in the ice are generally found in close connection with multicellular creatures. They are parasites and commensals that inhabit fish and other marine organisms such as crabs and worms. Some of the DNA samples could have even originated from these higher species.
This leads to the conclusion that there may be some complex organisms than single cells living in Lake Vostok which also suggests that life may exist on Europa’s cold surface if terrestrial creatures have colonized such purportedly hostile and severe habitats on Earth.
Europa’s ice crust is vital for possibility of life
Is it blocking life or safeguarding it?
Jupiter’s moon Europa has an ice cover that is 3,300 ft (1 km) thick, and it has a crucial role in the search for life on this icy moon. The ice cover serves as a blanket, protecting the subglacial ocean from the deadly cold of space and the lethal effects of radiation. As an added bonus, the ice crust, together with the rocky core, is likely the primary source of chemical building blocks for life. This is due to the coating of chemical compounds left on the surface of Europa’s ice sheet by meteorite strikes, radiation, and particles from Jupiter and the solar wind.
Does Europa have oxygen?
Those fissures in the ice are a hint that the crust of Europa regularly tears open and becomes permeable. (NASA/JPL)
When this radiation splits the water molecules in the ice, this in theory should release oxygen into Europa’s atmosphere. These gases and liquids can as well reach the ocean below if the ice sheet is thin enough to allow them to escape through its countless fractures. Yet, this thick ice crust can still be detrimental to life. And Europa’s ice crust is not precisely a thin sheet, being at least 6 miles (10 km) in thickness.
Recent models showed that enough oxygen could make its way down to the subglacial water from the glacier surface, and it is simply a matter of how long it would take. This is because tidal pressures on Europa could be causing the ice crust to shift and break apart on a regular basis, sending new, frozen ice to the top while pushing other sections of the surface layer deeper into the underwater ocean.
The upheaval in Europa
Numerous new bulges and seams can be seen all throughout Europa’s surface, proving that similar upheaval processes are still happening right now. In theory, oxygen was once restricted to the atmosphere’s outermost layers in Europa. But the simulations reveal that free oxygen might have been mixed in over the full thickness of the ice crust during the span of around 1 to 2 billion years of irradiation and upheaval.
Meanwhile, similar to the underside of ice floes in our terrestrial (surface) seas, a persistent, quicker interchange of thawing and freezing happens at the boundary layer between water and ice on Europa’s icy crust. In as short as half a million years enough oxygen might have been dissolved in Europa’s ocean water during this exchange at the ice-water interface to provide minimum oxygen saturation for life.
Getting ready for life
This amount of oxygen would support life on Earth for even the smallest crustaceans. In under 12 million years, oxygen levels in Europa’s atmosphere may have risen to match those of our seas, making breathing comfortable for even the biggest aerobic organisms. If Europa had started out without oxygen for 1 or 2 billion years, that may have been the perfect amount of time for life to evolve there since this is similar to the history of Earth.
The chemically hostile oxygen wasn’t there when the original building blocks of life developed on Earth. Circumstances altered and the increasing oxygen content of the atmosphere produced the conditions for the genesis of higher life forms only after the earliest single-celled creatures had existed. The water under Europa to possibly have enough oxygen to allow for the formation of even bigger life forms is, therefore, not inconceivable.
Lakes under Europa’s ice
Chaos Terrain for clues
To what extent the ice on Europa can be penetrated is a key aspect of the survival of any life on Jupiter’s moon. Europa’s icy shell may be more permeable than previously assumed. Images taken by the Galileo probe reveal strange landscapes on Europa such as uneven ridges, cracks, and plains that appear jumbled. This topography is called “Chaos Terrain.” The planets Mars and Mercury, as well as the dwarf planet Pluto, all have Chaos Terrains.
Scientists find the ice sheet mechanisms remarkably similar to those structures. This is because similar processes occur on glaciers and ice shelves that sit above subglacial volcanoes.
Caves under Europa’s ice
It is hypothesized that Europa may have subsurface lakes, like ice-filled caverns that sit about midway between the surface and the ocean. There are many cracks in the rough topography of the Chaos Terrain formations above the lakes, which might enable abundant oxygen and organic substances to infiltrate the waters in these shallow caves in the ice.
However, this may also mean that these subglacial lakes of Europa may support life. As time goes on, these tunnels might eventually burst apart due to massive fractures in the ice, allowing for a passageway to the ocean below. Even though Europa has a strong crust, scientists can now see that it may be home to enormous shallow lakes that facilitate “mixing.” This mechanism of mixing has the potential to improve the habitability of Europa’s ocean.
Connection to the ocean under the ice
Springs of water
Image data from the Hubble Space Telescope revealed that water vapor is coming from Europa’s south pole, sometimes in gigantic springs that reach heights of over 125 miles (200 km), providing more proof of a connection between Europa’s surface and its subglacial ocean.
A blue glow
The map above depicts the area where water vapor was found over the southernmost part of Europa. (Credits: NASA/ESA/L. Roth/SWRI/University of Cologne)
The spectrometer on the Hubble telescope picked up the faint light of excited oxygen and hydrogen atoms near the pole of Europa. In most cases, this is brought on by the disintegration of water molecules in response to a magnetic field. That implies that water vapor is present on Europa but at very low temperatures.
The moon Enceladus of Saturn is reported to experience a similar phenomenon. Geysers that are currently active here also send clouds of steam, ice, and dust hurtling into the void. However, only water vapor was found on Europa; whether the springs also include ice and dust particles is still unknown. Also, the origin of these springs remains unknown.
Connection to a hidden ocean
But do these openings reach the ocean under Europa’s ice crust? Or they might be created in the ice as a result of frictional stress close to the surface. In that case, there would be no need to delve into the subglacial ocean to learn more about its composition.
The water vapor auroras always appear when Jupiter’s moon is at the farthest point in its eccentric orbit. This phenomenon is likely caused by Jupiter’s tremendous gravitational pull and its tidal forces. Scientists believe that the large fractures and fissures on Europa’s ice are stretched farther away from the planet, allowing water vapor to escape. But as Europa returns to its orbit around Jupiter, the planet’s gravity squeezes the moon, causing the fractures to close.
Water vapor escaping near Europa’s south pole reinforces its status as a top contender for life in the Solar System.
Europa’s subglacial ocean may provide favorable circumstances for the emergence of life, if it were connected to the surface.
Chemical reactions taking place in the ice of Europa
A dynamic surface
There are possibly more than simply an ocean and lakes full of liquid water under Europa’s ice crust: Deep under the ice, the chemical reactions could be occurring at a remarkable rate between the frozen objects. This is thought to be invaluable. For one thing, at -305°F to -225°F (-187°C to -143°C), chemical processes just cannot take place on their own and extra energy would be required for them to happen.
Jupiter as an energy supplier
The areas where chemical substances have altered the ice are highlighted in false color. Acids and salts are seen in red. (NASA/JPL)
In theory, Jupiter provides one such energy supply. There is a steady release of radiation and energetic particles into the atmosphere through its radiation belts. If they were to land on Europa, they would set off chemical reactions. Nonetheless, these particles often only go down a few millimeters into the ice. For this reason, it was widely believed that considerable chemical activity could not have persisted in the depths of Europa’s ice crust.
However, this extra energy is actually still achievable without radiation and particle flow from Jupiter. Scientists conducted experiments in a high-vacuum room cooled to 50 to 100 Kelvin (minus 223° to minus 173°C) by spraying water vapor and sulfur dioxide gas onto mirrors. Instantaneously, the vapors froze into solid ice. Previous satellite observations have confirmed the presence of sulfur in Europa’s ice, most likely from the ice volcanoes on Jupiter’s moon Io but also from Europa’s subglacial ocean. However, what happened to this sulfur thereafter was a mystery.
The -280°F reaction
Do chemical processes take place on Europe’s ice cap? (NASA/JPL)
The scientists then observed the changes in the reaction chamber using infrared spectroscopy. Despite the subzero temperatures, the sulfur dioxide nevertheless managed to react with the water molecules, resulting in the formation of positive and negative ions. This reaction occurred very instantly at a temperature of -225°F (-143°C). After around half a day to a day at -280°F (-173°C), the reaction reached saturation.
A day may not seem like a little amount of time, but now compare it to the age of Europa, 4.5 billion years. After all, the process in the laboratory surprisingly transformed around 30% of the sulfur dioxide. What’s more, the positive and negative ions formed in this reaction readily combined with other molecules, triggering even other reactions.
What if the crust were more dynamic than we realized?
After this, they added carbon dioxide to the mixture to see whether the process would still occur in carbon dioxide ice, simulating the circumstances on Europa. This also froze up instantly on the mirrors while not halting the continuing reaction. If the frozen carbon dioxide had prevented the reaction this whole theory would have failed.
This, however, implies that Europa’s ice, and maybe the ice of other frozen moons like Ganymede and Callisto, may be chemically active. This means the sulfur dioxide under the surface of Europa is possibly interacting and forming chemical compounds, paving the way for the possibility of life.
We simply put ourselves and all sorts of other things on the scales to determine their weight. The mass of a planet is not quite so easy to determine. Over time, people have developed highly diverse types of scales—there are spring scales, beam scales, inclination scales, and many more. However, the basic principle is the same for all of them: Scales measure the weight force acting on the body lying on them. Since this weight force is proportional to the mass and the constant of proportionality is known—it is the acceleration due to gravity on the earth—the mass can be calculated from the weight force. Conveniently, most scales do not show the weight force in the unit of Newton but rather the mass in pounds or kilograms.
However, the acceleration due to gravity varies minimally from place to place. This is because it depends not only on the gravitational pull of the Earth but also on the centrifugal force acting at the respective location due to the Earth’s rotation. In addition, the planet is not an exact sphere but is slightly flattened. And the Earth’s surface is not smooth but has all kinds of unevenness with mountains and valleys. All this means that the gravitational acceleration is not the same everywhere. But in everyday life, these differences do not play a significant role since they are only in the per-mille range.
In addition to scales, another method used to determine masses is measurement by volume, like with a measuring cup. If the density of a substance is known, the mass is simply the volume multiplied by the density. There are different measuring cups for different substances that are adapted to their respective densities. While both measuring methods work well for all kinds of living beings and objects on earth, they are not suitable to weigh the planets and other celestial bodies in space.
Newton and His Law of Gravity
But physics also offers a solution here. The basis of weighing a planet or star is the law of gravitation discovered by Isaac Newton: It is the force of attraction that two masses exert on each other. According to Newton, the force is proportional to the two masses and inversely proportional to the square of the distance between the two masses. For example, if one of the masses is doubled – regardless of which one – the force of attraction is also doubled. If, on the other hand, you double the distance, the attractive force drops not only to half but to a quarter of the original value.
With this law, Newton successfully described the motion of the planets in the solar system in 1687. And it was also used to explain the law of free fall described by Galileo Galilei. The acceleration due to gravity is proportional to the mass of the Earth and inversely proportional to the square of the Earth’s radius. Since the size of the earth was already well known at Newton’s time, this gave Newton a possibility to determine the initially unknown constant of proportionality in his law of gravitation. But for this, he needed – also unknown by then – the mass of the earth.
Finding the Mass of Earth
In General Relativity, the bending of space-time by mass is what causes the force of gravity. (Image: T. PYLE/CALTECH/MIT/LIGO LAB)
To obtain the value in question, Newton first estimated the average density of our planet – based on the density of a rock. Since the radius and thus the volume of the earth was known, the estimated value for the density could be used to calculate the mass of the earth. The average density should be about five times greater than that of water, thus, Newton calculated the mass of Earth about 5.5 × 1024 kilograms. Considering the rather rough estimate, this is not a bad result: According to today’s measurements, Earth’s mass is 5.972 × 1024 kilograms. Newton was thus able to approximate the magnitude of the constant of proportionality in the law of gravitation.
A more precise determination of these gravitational constants was not possible until 1797 when the British natural scientist Henry Cavendish developed a “gravitational balance”. The gravitational attraction between small bodies could now be measured directly for the first time. The masses of the bodies were known and so the experiment allowed to calculate the proportionality constant in the law of gravitation from the measured gravitational attraction. The more exact value for the gravitational constant and the much more easily measurable gravitational acceleration now make it possible to determine the mass of the Earth with far greater accuracy than by volume. Earth was the first planet that scientists could weigh using Newton’s law of gravity.
Measuring the Weight of the Solar System
More than 99 percent of the solar system’s mass is found in the sun.
The solar mass is the standard unit of mass in the astronomical community. The Sun’s mass is equal to 1.9890 x 1030 kg, or around 333,000 Earths. More than 99 percent of the solar system’s mass is found in the sun.
Almost three decades before Cavendish’s experiment, researchers had already succeeded in scaling our solar system. Before that, the ratios between different distances in the solar system were known, but not absolute distances. Knowledge of both the absolute distances of celestial bodies and the gravitational constant finally enabled astronomers to weigh other planets as well. For if one disregards other forces – which play only a very minor role in celestial mechanics – gravity alone determines the orbit of a celestial body.
A common method for determining the masses of a planet is by determining the orbits of its moons or by passing space probes. Mass creates gravity, and the planet’s gravitational pull in turn determines the orbit of any object moving around the planet, in terms of size and orbital period.
This relationship can be illustrated using the example of a moon orbiting a planet. The gravitational pull of the planet that is keeping the moon in orbit can be calculated with the help of Newton’s law of gravity. The gravitational force acts here as a so-called centripetal force since it forces the moon into a circular orbit. This makes it possible to determine the mass of the planet solely from the orbital radius and orbital period of the moon. Astronomers have been able to weigh all the planets in the solar system by using this method except Mercury and Venus – which have no moons. This technique allows to find the mass of many celestial bodies, even including space probes.
Weighing the Exoplanet (Extrasolar) Planets
Astronomers must proceed somewhat differently if they want to weigh planets near other stars. Consider a planet and star that do orbit around their common center of gravity. This causes the star to periodically move toward and away from us. The motion is minimal, but it leaves traces in its light spectrum – caused by the so-called Doppler effect. The characteristic signatures of this light spectrum tell a lot about the star. And since the greater the planet’s mass (a bigger gravitational force), the more a star moves, scientists can use this method to determine the planet’s mass.
For this, however, not only the distance of the system must be known but also the type of star and thus its mass. However, both quantities are usually easy to determine in astronomy. Nevertheless, scientists usually get only a minimal value for the mass. Because as long as they do not know the orientation of the planet’s orbit in space, the actual distance between the star and its companion cannot be precisely quantified. This orientation could be better known only if the planet regularly passes in front of its star from Earth.
