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.
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 Exemplary 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 field, 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.
Its central star probably expanded into a red giant approximately 18 million years ago and in doing so swept away the planet’s atmosphere. This is what might happen to Earth a few billion years from now. The planet might have been engulfed by the atmosphere of its star at one time, but its rocky core survived, nevertheless.
WASP-12b
When WASP-12b was discovered in 2008, it ran counter to all expectations. Since then, it has been considered one of the hottest planets. It is more than 50 percent larger than Jupiter. But this Hot Jupiter is particularly interesting because it is being eaten alive. It orbits its sun so closely—one revolution takes 1.1 Earth days—that it probably loses 6 trillion kilos of mass every second, while its atmosphere is being blown away. It is assumed that the planet will die within 10 million years.
In addition, the planet might exhibit a high concentration of carbon in the form of carbon monoxide and methane. This means it could have a solid core containing a lot of diamond. Perhaps millions of years from now only a gigantic diamond will be left of WASP-12b. In addition, this planet was long considered the fastest-moving known planet. It moves at an impressive speed of 830,000 km/h.
Gliese 436 b
Gliese 436 b acts like a comet, because it drags a long tail behind itself. In its orbit, it seems to lose between 100 and 1,000 tons of hydrogen per second. It is assumed that during its existence Gliese 436 b has lost up to ten percent of its atmosphere. But its huge tail, which is approximately 50 times larger than the central star, obscures the sun during its orbit of less than three days.
There had been suggestions that planets with comet-like tails might exist, but Gliese 436 b was the first one actually discovered. Due to its size and proximity to its central star, it is called a Hot Neptune. Its gaseous tail might continue to exist for a longer period, because the sun that the planet orbits is a relatively cool red dwarf.
Janssen
On this world, one of the few with a real name, diamonds aren’t just a ‘girl’s best friend.’ Janssen, alias 55 Cancri e, is a Super-Earth and one of the five planets orbiting its star, the A component of the binary system Copernicus. We used to believe that a lot of water exists on 55 Cancri e. However, today researchers assume that the planet consists mainly of carbon in the form of graphite, diamond, and other minerals. An entire third of the planetary mass, about three times the mass of Earth, could be a single huge diamond.
Due to these findings, it is assumed that far-away rocky planets don’t have to be similar to Earth. They could be completely different, and Janssen was the first rocky planet to be detected that had a totally different composition than ours. On the day side, the planet reaches temperatures up to 2,400 degrees, while the night side is refreshingly cool with a maximum of 1,100 degrees.
HAT-P-1b
When astronomers discovered HAT-P-1b in 2006, they were amazed to find that it is almost twice the size of Jupiter, while weighing only half as much. Accordingly, its density is only a quarter of the density of water and it is lighter than cork. In a giant bathtub, it would float three times higher than Saturn. So far, nobody knows why this is the case. Perhaps additional heat reaches the interior of the planet, but there is also no explanation for that. One possibility is that the planet might be ‘lying on its side’ and rotating vertically to its orbit, like Uranus in our system. However, as this position is very rare, and other ‘bloated’ planets have already been discovered, this theory definitely does not apply to all of them. The planet’s star, by the way, is part of a binary system.
Gliese 1214 b
Gliese 1214 b is a Super-Earth. Its mass reaches almost 7 times that of Earth, and its radius is estimated to be more than two and a half Earth radii. It orbits its star at a distance of only 2 million kilometers. The most interesting aspect is that observations indicate its atmosphere consists mostly of water vapor.
The density of the planet might be around 2 grams per cubic centimeter. For comparison, that of Earth is 5.5 grams, and water weighs 1 gram per cubic centimeter. Scientists concluded that there must be lots of water there from the time the planet was farther away from its sun, in the habitable zone. The current close orbit and the high temperatures evaporate the water into a hot haze enveloping the planet. The planet is still considered one of the exoplanets most likely to have an ocean, but with a surface temperature of 120 to 280 degrees it would be so hot you’d better not jump into the water.
HD189733 b
The next time you stand in the rain, you might want to think of the inhabitants of HD 189733 b, even though it is rather unlikely they exist. On this planet there is not only a scorching temperature of 850 degrees, but perhaps also a rain of glass falling sideways, driven through the
atmosphere by winds reaching up to 8,700 km/h. The cobalt blue color of the planet is not caused by oceans but by silicate particles in the clouds of its atmosphere. When these silicates condense in the extreme heat, they are turned into small drops of glass, which not only create the blue light, but are also carried around the planet by hurricanes. The planet is 30 times closer to its sun than Earth is to ours, and it has a captured rotation, meaning it always faces its star with the same side. The enormous temperature difference further reinforces the storms.
HD80606 b
All the planets in our solar system have relatively circular orbits, so that their closest and most distant points from the sun are not so different. The orbit of HD 80606 b, on the other hand, is strongly elliptical. During an orbit lasting 111 Earth days, the distance of HD 80606 b from its sun varies between 0.03 AU and 0.88 AU, where one astronomical unit equals the distance from the Earth to the sun. When it approaches the point closest to the sun, the temperature rises from 500 to 1,200 degrees within just six hours. Accordingly, the seasons on HD 80606 b are not determined by the angle of inclination but by its orbit. If you stayed high up in the atmosphere of the planet for one orbit, you would observe its star getting 30 times as large as the apparent size of our sun in our sky, while increasing its brightness by a factor of 1,000. The extreme temperature changes must create storms just as extreme, with winds blowing at 18,000 km/h.
Proxima Trilogy: Part 1-3: Hard Science Fiction Page 26