Rivers of ink have been spilled on this question and countless movies have been produced exploring the fascination that the possible existence of alien life carries with it. However, the first concrete step towards answering this question came in 1995 with the announcement of the detection of the exoplanet 51 Peg b. Exoplanets are planets orbiting stars other than the Sun and 51 Peg b was the first exoplanet detected to orbit a solar-like star. This discovery led Michel Major and Didier Queloz to win the Physics Nobel Prize in 2019. Since then, within a few years, the number of planets known to us has grown more than 500 times and currently amounts to approximately 4,500, though new ones are discovered and announced daily. Furthermore, the results of the observations conducted so far indicate that planets with a mass and radius smaller than that of Neptune constitute the largest population of planets in our galaxy. This is remarkable, because these are the planets that have the highest chance of having developed, or developing in the future, a habitable environment, and thus possibly hosting life.
Since the detection of the first exoplanets, considerable effort of the research community has gone and is going into identifying the ways in which a habitable environment can develop under different conditions and into devising techniques for recognizing the presence of life on distant planets. However, the scientific community was soon confronted with the surprise that the first detected exoplanets are rather large and orbit very close to their host star, both factors hindering the development of life. Indeed, almost all the planets known to date do not have characteristics adequate to host life as we know it on Earth. It is therefore interesting to spot, among the planets known to date, those that possess the least favorable conditions for the development of life. I have come up with five such extreme planets that almost certainly do not host and most likely never will host life. The selection was done by considering several factors, such as the extreme planetary temperature or irradiation. Although the selection criteria may be somewhat arbitrary, these planets reflect well the range of odd and crazy conditions that some planets sustain. The five planets are WASP-12b, KELT-9b, AU Mic b, PSR J1719-1438 b, and PSO J 318.5-22.
WASP-12b (~1,400 light years away from us)
This is a so-called “hot Jupiter”, namely a planet of roughly the size and mass of Jupiter, but orbiting very close to the host star. WASP-12b orbits within about 1 day a star that is somewhat more massive and hotter than the Sun. In fact, the region of the Sun that emits into space the light we see with our eyes (in jargon called the photosphere) is approximately 5,500 °C, while the photosphere of WASP-12, the host star, is approximately 6,000 °C. The temperature of the host star and the short orbital separation lead to a temperature of the planet on the order of 2,000 °C. The distance between the planet and the photosphere of the star is just one stellar diameter. For planets orbiting so close to their host star, such as WASP-12b, the length of a day is equal to the length of a year meaning that the planet always shows the same face to the star, similarly to the Moon with the Earth.
Many hot Jupiters are known to be subject to temperatures as high as 2,000 °C and higher (see for example the next planet, KELT-9b), but WASP-12b has a peculiarity that is not shared by many. The planet has a rather large radius for its mass, implying that it has low gravity, which, together with the short distance to a heavy star, leads to distortion of the planet, which takes on the shape of an egg (“Roche lobe” in jargon), with the “pointy side” facing the star. This is a characteristic of many hot Jupiters, but in the case of WASP-12b this elongation is extreme: the radius of the planet in the direction of the star is almost twice that in the perpendicular direction. The low gravity and high temperature of the planet, together with the shape of the atmosphere, leads to a strong and continuous flow of gas from the lower to the upper part of the atmosphere. The gas is then ejected into space, where it is captured by the gravity of the nearby host star. Therefore, as the planet orbits around the star, there is continuous flow of a considerable amount of gas from the planet towards the star. Mirroring this on the Earth, it would be as if the air we breathe continuously flowed upwards and, once it reached the altitude at which stratospheric balloons fly, was ejected into space at high speed and then sucked up by the Sun. Given the large difference in mass between the star and the planet, the star will hardly notice that it has eaten a planet and no trace of it will be left at the end. WASP-12b is definitely not a place where life could emerge and certainly not a destination for a relaxing holiday.
KELT-9b (~650 light years away from us)
KELT-9b is by far the hottest planet known and the progenitor of what astrophysicists call, without much imagination, “ultra-hot Jupiters”. These are hot Jupiters with temperatures greater than 2,000 °C. The temperature on KELT-9b is almost 5,000 °C and about 2,000-2,500 degrees hotter than the second hottest planet known. In fact, a temperature of 5,000 °C is higher than that of the photosphere of many stars. But how can a planet reach such a high temperature? The reason is that the planet orbits very close (with a period of just about 1.5 days) to a star that is more than twice as heavy as the Sun and almost twice as hot. The temperature of the photosphere of KELT-9, the host star, is approximately 10,000 °C. Like all hot Jupiters, KELT-9b always faces the same side to the star and is characterized by strong winds bringing the hot gas from the day side towards the night side and vice versa. The difference is that in classical hot Jupiters the not too extreme day-side temperature enables the gas to cool down considerably on the night side, even leading to the formation of clouds, but on KELT-9b, the day-side temperature is so high that even the night side, which is never exposed to the stellar light, is unable to cool to temperatures below 2,500 °C. This is definitely not a place where life could emerge.
Like WASP-12b, this planet also has a rather egg-like shape, but not as extreme because KELT-9b is as large as WASP-12b, but twice as heavy, meaning that KELT-9b has stronger gravity, and it has a slightly longer orbital distance. KELT-9b is also losing its atmosphere at a very high rate, but most likely will not be completely consumed before the star engulfs the planet in the next few hundred million years. This engulfment does not happen because the planet slowly moves towards the star, as for WASP-12b, but because the star is not going to stay as it is for very long. Within the next few hundred million years, the star will start to considerably increase in size, engulfing the planet, which then will quickly spiral into the star where it will be torn to pieces and vaporized.
AU Mic b (~30 light years away from us)
AU Mic b is a so-called Super-Earth, that is a planet typically less than 10 times the mass of the Earth and slightly larger than the Earth, orbiting within about 8 days around the host star AU Mic, which is a star that is two times lighter than the Sun and has a photospheric temperature of about 3,500 °C, so significantly cooler than the Sun. What makes this system special is its young age of only about 20 million years, which is nothing compared to the several thousands of millions of years that these stars live. Stars like AU Mic, and like the Sun, are a bit like human beings: they are active and have lots of energy when they are young and then calm down as they age. An active star produces frequent and random explosions and eruptions. The outcome of these explosions is the ejection of gas (charged particles; so-called coronal mass ejections) into space at high speed and the emission of extremely intense X-ray radiation. The Sun also produces such eruptions, which sometimes reach us, potentially disrupting telecommunications, but they are significantly weaker and happen seldomly.
The result of these coronal mass ejections and X-ray radiation reaching the planet is twofold. First, if the planet is not protected by a magnetic field, like the Earth is, the particles ejected by the star during an eruption penetrate deep into the planetary atmosphere and kick away some of the gas composing the atmosphere, while the X-rays heat up the upper part of the atmosphere that will expand and escape into space. This leads to the planet slowly losing its atmosphere, which is not necessarily a bad thing for the advent of life (see below). Second, the molecules irradiated by the X-rays are destroyed (in jargon “dissociated”). This implies that, if molecular bonds, like those keeping DNA together, could form on the surface of this planet, they would be broken apart every time a strong eruption happened on the side of the star facing the planet, which is roughly every couple of hours.
The interesting thing is that practically every planet, including the Earth, has gone through what AU Mic b is going through now, simply because all old stars were once young, and thus active, in the past. However, AU Mic b is special, because it is one of the few young planets known, and one of the closest to us. Only a few young planets are known, because they are difficult to find due to their high activity, which interferes with the observational signatures used to detect planets.
The fact that young stars induce the loss of the atmosphere of planets orbiting close enough around them is important for the advent of life on a planet. It is believed that most planets are born surrounded by a large atmosphere composed primarily of hydrogen. Hydrogen is an extremely efficient greenhouse gas, which renders any kind of life on the rocky surface of a planet underneath it essentially impossible. The high activity of young stars enables rocky planets to get rid of this hydrogen atmosphere, leaving space for the possible advent of an atmosphere with a more complex composition and possibly favorable for the advent of life.
PSR J1719-1438 b (~3,900 light years away from us)
The planets described above definitely do not possess conditions adequate for hosting life and have pretty crazy characteristics, but overall they are relatively “normal” planets for what we know. PSR J1719-1438 b, instead, goes beyond the imaginable. This is a planet more massive than Jupiter that orbits in just over 2 hours around a neutron star. Neutron stars are what remains after supernova explosion of stars more massive than 8 times the Sun. These are the most powerful explosions in nature. Neutron stars are made of the second densest matter known to exist, after that of black holes, which is even denser. The host star, PSR J1719-1438, is slightly more massive than the Sun, but only 4 times bigger than the Earth. This means that on average a 1 cm cube composed of neutron star matter weighs roughly 30 kg. In comparison, a cube of the same size composed of the densest element on Earth, osmium, weighs just 22 grams.
Why is such a planet not adequate for hosting life? First, neutron stars emit a negligible amount of optical and infrared light, the type of light we see with our eyes and that heats up the Earth to the temperature it has, respectively. In such an environment, for example, plants would not be able to start photosynthesis. However, even if it were somehow possible to irradiate this planet with optical and infrared light, the radiation emitted by the neutron star would kill and vaporize any living form present on the planet. This is because neutron stars emit a huge amount of X-ray radiation, which efficiently dissociates molecules. This star is the perfect life killer and the planet is definitely too close to it to be anywhere near a possible environment for hosting life.
How can a planet be orbiting the remnant of such a powerful explosion? We can only speculate about this. Starting from the assumption that no planet can survive intact anywhere near a supernova, there are two possibilities. One is that the planet was wandering in space, possibly at the outskirts of another planetary system, and it has been captured by the neutron star ending up in the orbit in which we find it now. This means that the planet may have passed from a condition in which it was not receiving any stellar light, or very little, to another in which it receives a huge amount of very energetic radiation. A big change! The other possibility is that the planet formed after the supernova explosion, probably out of the material ejected by the dying star.
PSO J318.5-22 (~80 light years away from us)
So far, we have been through planets on which life could not be present, mostly as a consequence of the properties of their host star and/or characteristics of their orbit. The case of PSO J318.5-22 is exactly the opposite: this planet is not suitable for life because it has no star. PSO J318.5-22 is a so-called “rogue planet”, that is a planet wandering in space without orbiting any star. This is a young planet, which is the characteristic that enabled us to find it. Young planets are still shrinking in size (contracting), which leads them to increase their internal temperature and then to emit infrared light. PSO J318.5-22 is roughly 6 times more massive than Jupiter and 1.5 times larger. Because of the internal heat coming from the inside, the temperature of the portion of the planetary atmosphere that emits light is at about 500 °C, but, in contrast to what happens with planets illuminated by a star, the temperature decreases steeply with increasing altitude, up to the freezing temperatures of outer space. The eternal night to which this planet has been doomed is not what could be considered a possible habitable environment.
To conclude... a curiosity. A careful reader may have noticed that, in contrast to the other planets listed above, the name of this planet does not end with a letter, e.g., “b”. This is because of the naming convention adopted for exoplanets. Every star we know of has a name assigned to it, typically a short sequence of letters indicating the name of a catalog or the facility that discovered it followed by a number indicating either the position in the catalog or its approximate coordinates in the sky. In addition, bright stars have a proper name, that is however commonly used just for a few of them, and/or a name related to the constellation in which they lie. Planets take the name of their host star, followed by the letter “b” for the first detected planet in the system or closest planet to the star, and continuing with the other letters of the alphabet for the remaining planets in order of their discovery or distance to the host star. The letter following the name of the star is written in lowercase, because capital letters (e.g., “A”, “B”, “C”) are used to identify the different components of stars that are gravitationally bound forming binary or multiple systems.
At a glance
Luca Fossati studied Physics at the University of Pavia, in Italy. Following work experience at the European Southern Observatory in Chile, he moved to the University of Vienna, where he obtained his PhD in Astrophysics in 2009. He then moved for a 3-year postdoc position at the Open University (UK), followed by a further 3-year postdoc position at the University of Bonn (Germany) where he received the Humboldt fellowship. Since 2015, he has led the exoplanet research group at the Space Research Institute of the Austrian Academy of Sciences. He is a scientific team member of the CHEOPS (ESA), CUTE (NASA), and ESCAPE (NASA) missions. He is also involved in the PLATO (ESA) and Ariel (ESA) mission and in the development of the next generation NASA space telescopes.