Earth-like planets might not look like exact replicas of Earth. The only common defining feature is that they have a solid surface with rock underneath it. The interior is often structured in layers, with a hot, sometimes liquid core of iron and other heavy elements. Above that might be a mantle of silicates and a crust, which is also made of silicates and other light components. This is sometimes followed by a layer of air, i.e. atmosphere. Some planets have water on the surface—a hydrosphere.
Hot Earth-like Planets (Venus-like)
These rocky planets have a very high temperature—several hundred degrees, like Venus in our solar system—either because of proximity to their sun, or because they have dense enough atmosphere to create a greenhouse effect.
Super-Earths
So far, most of the rocky planets discovered belong to the group of Super-Earths, meaning their masses are greater than that of Earth. There are various definitions, ranging from between 1 and 14 times to between 5 and 10 times Earth’s mass. However, the measurements are often subject to errors. Determining the diameter is insufficient to distinguish between a planet with a small rocky core plus a large gas cover, and a genuine rocky planet.
If the original dust disk is large enough, huge rocky planets can form, so-called Mega-Earths. This would, in particular, be the case for planets in orbit around giant stars of the spectral classes B and O, that have up to 150 times the mass of the sun. Such giant Earths could weigh up to 4,000 times as much as our Earth.
However, this is certainly not the only classification system, and scientists consider it rather impractical. A different method does not look at the outside of a planet, but instead focuses solely on its composition. This distinguishes between:
Metallo-silicate planets, similar to Mercury and Earth
Silicate planets like Europa and Io; and Earth’s moon
Hydro-silicate planets, comparable to Ganymede, Callisto, Titan, and Pluto
Ice planets like Enceladus, with very low silicate content
Gas giants with methane clouds below 80 degrees Kelvin
Gas giants with ammonia clouds below 150 Kelvin
Gas giants with water vapor clouds, 150 to 350 Kelvin
Gas giants with an albedo around 12%, 350-900 Kelvin, ‘Hot Jupiters’
Gas giants with alkali absorption, 900-1500 Kelvin
Gas giants with silicon dioxide clouds above 1500 Kelvin
Planets in Systems with Multiple Stars
Planets can also develop in systems with multiple stars. In these systems, however, it is more difficult for them to reach permanently-stable orbits. Conditions are most favorable if both stars are very far away from each other—or very close. In the former case we basically have separate planetary systems. For instance, Proxima b orbits its mother star Proxima Centauri, while that star moves around a common center of gravity with the binary pair, Alpha Centauri A and Alpha Centauri B. In the case of stars being close together, planets usually move around both and are called circumbinary planets. Such combinations seem to be relatively common. The Kepler telescope investigated 1,000 binary star systems and has found seven with planets.
Habitability and Life
It is very difficult to determine from afar whether a planet could, in principle, harbor life. The problems lie in the fact that we only know one system of life so far: life on Earth. It is generally based on surface water in its liquid state. There might be many other types of life that would be based on completely different preconditions.
But the researchers had to agree on something. Therefore, they define the habitable zone around a star as that area in which water could exist on the surface of a planet orbiting it. This does not mean that water actually exists. And what if some lifeforms do not acquire their energy from sunlight but instead from heat? Then they could also use water below the surface, which is closer to the core of the planet.
How far the habitable zone reaches depends on how much energy a star emits. And this can change due to the aging process. If the sun keeps increasing its output, which is a typical phenomenon of aging, Earth will at some point no longer be in the habitable zone, as it will get too hot. And if the sun becomes a red giant at the end of its lifecycle, then conditions near Jupiter and Saturn will turn more advantageous. Then Saturn’s moon Titan might become the most fertile place in the solar system.
But apart from the size and the distance of the habitable zone, the nature of the main star has a great influence on the possibility of life. Very bright giant stars often do not last long enough for life to develop on their planets. Red dwarfs, on the other hand, emit strong X-ray and UV radiation. This could be an obstacle for the development of life, because in these cases the habitable zone would be very close to the star. It is very important for the development of life that the star should provide a constant energy output over a long period of time. Sudden eruptions or fluctuations can have disastrous consequences. The 11-year solar cycle has a significant effect on the climate of Earth, even though the energy output only changes by 0.1 percent. Therefore stars with stronger cycles are presumed problematic for the development of life.
Several other characteristics of the planet also must be considered. A dense atmosphere can retain solar energy better than a thin one, due to the greenhouse effect. Mars, for instance, would be noticeably colder than Earth, even if it had the same orbit around the sun. A strong magnetic field prevents the solar wind from ripping away the atmosphere over time. It also protects against radiation eruptions of the star, which is particularly helpful in the case of red dwarfs like Proxima Centauri. The issue of whether a planet always faces its star with the same side or whether it has a strongly elliptical orbit can also influence the chances for life on its surface.
Life as we know it is very particular, and the planet must not be too small. Smaller planets do not have enough gravity to retain a dense atmosphere. Their interior also cools off soon after they develop, leaving neither plate tectonics nor a magnetic field, both of which presuppose a liquid core. Therefore it is probably no accident that Earth is the densest of all rocky bodies in the solar system. Studies estimate that the lower limit for habitability is 0.9 Earth masses—looks like we humans were lucky—as the Earth is only slightly above that. However, with a growing planetary size, the risk of the atmosphere becoming too dense also increases. Then the greenhouse effect would make the surface too hot, so that a Super-Earth would have to orbit its star at a greater distance than Earth does around the sun.
And the planet had better move around its star in an almost circular orbit, because otherwise it would sometimes get too hot, and then too cold. The Earth is exemplary in this aspect, as the eccentricity of its orbit—a measure of being close to a circle—lies below 0.02. The exoplanets discovered so far, and whose eccentricities are known, have values above 0.25. Their oceans would alternately freeze and boil with the changing seasons.
Two final aspects: If a planet does not meet the criteria, either because it is too large or made of gas, it is possible that its moons, which usually are made of ice or rock, could still carry life.
And finally, life itself plays a role in its spread. The fact that there is so much oxygen in Earth’s atmosphere—which animals can breathe—is based on the good preparation by plant-based life that produces oxygen as a side product. So if we find a planet that seems to be suitable but does not have enough oxygen, we would just have to sow plants—and then wait a few million years. Patience is always helpful in space.
Planets without Stars
There is another class of planets which is so exotic that scientists have not even agreed on a name for it. These are objects that travel all alone through interstellar space, far away from the light and warmth of a star. Up to now, we only know of a handful of these lonely wanderers, but there are probably many more of them. Astronomers’ estimates diverge considerably: For each of the approximately 200 billion stars in the Milky Way, there might be a handful or up to 100,000 loner planets.
The
term most often used for them is Planetary Mass Object, sometimes shortened to Planemo, or simply PMO. Before we can determine their origins—and therefore their number—more precisely, we first have to find and examine one, and this is an enormous challenge. All the search methods for exoplanets, to be discussed in the next section, fail here. An Earth-like PMO far out in space is a frozen stone ball almost impossible to detect unless it happens to move in front of a star that, from our perspective, lies behind it. This bends the star’s light, which astronomers can record as a gravitation lens effect.
There are a few of these planets, though, that radiate in certain wavelengths, one reason being that they stored enough of the heat generated during their development process. An exciting example, cataloged as PSO J318.5-22, was discovered by a team in 2013.
Where do these lonely wanderers come from? Some, especially the smaller, Earth-like ones, are probably cosmic runaways that originally formed like normal planets in a protoplanetary disk. But then some accident hurled them out of their system—for example, the influence of a heavy neighbor or another star that approached the system. Many others have been solitary all their lives. They developed from interstellar nebulas in the same manner as stars or brown dwarfs. Astronomers believe there is a lower mass limit for objects to form this way. They estimate it to be between two and three Jupiter masses.
Methods for Discovering Exoplanets
Compared to stars, exoplanets are small, light, and extremely dim. It is therefore not surprising that the first ones detected were not near normal stars, but around a rotating neutron star, the Pulsar PSR 1257+12—also called Lich—2,300 light years from Earth. Pulsars send radio signals with extreme regularity due to their rotation, but in the case of Lich, astronomers noticed tiny delays. These could only be caused by several companions. At first they suspected the existence of two planets, but now we know there are three planets, Draugr, Poltergeist, and Phobetor, as well as the ‘exo-comet’ PSR 1257+12 e. Pulsars are the remnants of a supernova explosion. These three companions must either have survived the supernova, or they developed later and were captured by the pulsar. Up to now, only one other planet has been found this way.
What other methods are there which astronomers successfully used to discover planets?
Transit Method
The transit method presupposes that the course of the planet moves directly across the axis between the Earth and a star. This reduces the brightness of the star in specific intervals, which can be measured by telescopes. Space telescopes like Kepler are especially suited for this.
Using the transit method allowed scientists to detect about 80 percent of the currently 4,062 exoplanets in 3,038 systems as of May 2019.
The procedure is successful, but it suffers from a major disadvantage: The mass—and therefore the type—of the planet cannot be determined, only its size and orbit. Furthermore, only about one percent of all existing planets can be detected this way, as the others may be moving in different courses around their stars.
Radial Velocity Method
When considering the rotation of the Earth around the sun, one often imagines the sun as if it were stationary, twirling the Earth around it on a string, so to speak. This image is incorrect. In reality, both the Earth and sun—planet and star—move around a common center of gravity. So the star also turns in circles, though small ones, when it is influenced by the planet. We cannot see this circular motion from the Earth, but we can see this star move back and forth, away from us and toward us. The speed with which this happens is called the ‘radial velocity,’ and via the Doppler Effect, this slightly shifts the star’s spectral lines. We can measure this shift with special instruments and then calculate how heavy the planet—or planets—pulling on this star must be.
If just this technique is used, though, it yields only a lower limit for the planetary mass. In order to calculate the exact mass, and thus the density, the planet would also have to be detected by the transit method. About one in five of all planets has been found using this method.
Gravitation Lens Method
If the light of a background star passes by another star on its way to Earth, it can be bent and magnified, just like going through a lens. However, if the star in the foreground has planets, this effect will change periodically. With the help of this method, 19 planets have already been identified, often at large distances of several thousand light years. Unfortunately, such gravitation lenses are hard to find. In addition, these observations cannot be repeated, as the stars move on in the meantime. One advantage of this method, though, is that it also works for planets with a wide orbit or low mass. Scientists hope to get an overview this way, to determine how common Earth-like planets really are.
Direct Observation
Ten years ago, nobody would have considered observing an exoplanet through a telescope. Now significantly improved technology has increased the number of planets discovered this way to more than 20. Once the E-ELT at the ESO or NASA’s James Webb Telescope become operational in a few years, we should gain exciting new data about many planets in our neighborhood. A direct view of your target offers many more details than an indirect proof.
Today this method works well for young planets. They retain enough heat from the period when they came into being that they still radiate energy. The coldest exoplanet detected this way is 59 Virginis b, which is no more than 500 million years old and has an average temperature of 240 degrees. The smallest planet that has been directly observed is Fomalhaut b, with approximately two Jupiter masses.
Exoplanet Records
Which planets exhibit the most extreme features?
Farthest away: SWEEPS J175853.92-291120.6 b—27,700 lightyears
Closest: Proxima b—4.22 lightyears
Heaviest: DENIS-P J082303.1-491201 b—28.5 Jupiter masses
Lightest: Draugr—0.02 Earth masses
Biggest: HD 100546b—6.9 Jupiter radii
Smallest: Kepler-37 b—0.3 Earth radii
Densest: PSR J1719-1438 b—at least 23 g/cm3
Hottest: Kepler-70 b—several thousand degrees
Coldest: OGLE-2005-BLG-390L b—50 Kelvin or -223 degrees Celsius
Youngest: V830 Tau b—2 million years
Oldest: PSR B1620-26 b—13 billion years
Longest year: 2MASS J2126-8140—about 1 million Earth years
Shortest year: PSR J1719-1438 b—2.2 hours
Farthest away from its sun: HD 106906 b—about 650 astronomical units
Closest to its sun: PSR J1719-1438 b—0.004 astronomical units
Closest to other planets: Kepler-70 b approaches Kepler-70 c to within 0.0016 AU
Heaviest mother star: HD 13189 b—mother star with 4.5 sun masses
Lightest mother star: TRAPPIST-1b, c, and d—mother star with 0.08 sun masses
Most extensive planetary system: HD 10180—9 planets, 7 of them confirmed
Most mother stars: Kepler-64—orbits in a system with 4 stars
Eleven Interesting Exoplanets
Exoplanets appear in the most diverse forms—almost as if they sprang from the imagination of a science fiction writer.
Proxima b
Proxima b is the exoplanet closest to our sun and therefore the obvious destination for the expedition depicted in this novel. If you ever go there, you will find most things just as described. I imagined the life forms on my own, based on scientific knowledge, of course. The planet is 30 to 50 percent heavier than Earth. Due to the planet’s tight orbit, its star surely must have forced it to always direct the same side toward the sun, as the moon does to Earth. One orbit around its mother star takes 11.2 days. However, one would not notice this on the planet, as there are no seasons, and it is always day if you’re on the ‘front’ side. Proxima b is located within the habitable zone, so water could exist on its surface in liquid form. Compared to Earth, 30 times more UV radiation and 250 times more X-rays reach the surface. Whoever wants to live there has to adapt to high radiation levels. A magnetic fi
eld, which has not yet been proven to exist, could mitigate their effects considerably. This also applies to the radiation eruptions of the mother star.
WASP-17b
Almost all known planets rotate the right way—meaning that their orbits follow the rotation of their central star. This is only logical, because planets form from the swirling disk of matter around a rotating protostar. It is different in the case of WASP-17b. This world has an orbital inclination of 149 degrees, which means it completes a retrograde orbit around its star every 3.74 Earth days. In addition it is very bloated and therefore has an extremely low density.
The reason for this strange orbit is unknown: Some researchers suspect that a near-collision, or the gradual gravitational effect of another planet might be responsible for it. WASP-17b was the first of the retrograde-orbiting planets to have been discovered. The mass of the planet is 0.5 of Jupiter’s and its radius 1.5 to 2 times the radius of Jupiter.
Kepler-70b
Kepler-70b is really fast—the planet moves around its central star, a red giant, in only 5.76 Earth hours or 0.24 Earth days. This is the shortest orbital period of all planets known today, and the velocity lies slightly below five percent of the speed of light. It is believed that this planet used to be a Hot Jupiter, but that now only a remnant of the former gas giant is left, with less than half the mass of Earth. Due to the tight orbit of Kepler-70b, 65 times closer to its sun than Mercury is to ours, it has such extreme temperatures that it is one of the hottest exoplanets.
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