1,963. That is the number of confirmed exoplanets as of March 2016, according to NASA. With a backlog of 4,696 potential exoplanet candidates, the number is likely to keep rocketing upwards, just as it has since the very first confirmation in 1991. In 2014 alone, NASA confirmed the presence of 715 new exoplanets. New technologies, the advent of crowdsourcing to amateur astronomers, and a renewed excitement for discovering alien worlds are all responsible for this surge in discoveries, and that excitement is wholly warranted. Beyond potentially habitable, Earth-like planets, astronomers have discovered some planets far stranger and more exciting than we could ever have imagined.
The Diamond Planet
What if there were a planet whose net worth was 384 quadrillion times the global GDP of our humble planet Earth? That’s right, orbiting a star in the Cancer constellation a mere forty light years away is a planet whose mass may be one-third diamond. Clocking in at twice the size and eight times the mass of Earth, 55 Canceri e is composed primarily of graphite — a form of carbon which, when combined with 55 Canceri e’s 2,150 degree Celsius (3,900 Fahrenheit) surface temperature, creates the perfect environment for diamond formation. Forbes Magazine estimates the exoplanet’s worth is $26.9 nonillion — $26.9 followed by 30 zeros.
The planet is also interesting because its unique chemical makeup suggests that a greater diversity of planets exists than scientists previously anticipated. The star system that houses 55 Canceri e is predominantly composed of carbon as opposed to the oxygen and silicates that make up our own solar system. This finding hints that more carbon- and diamond-rich planets may be found around Canceri itself or around similar stars.
The Dark World
No, not the summer blockbuster starring everyone’s favorite hammer-wielding Norse god of thunder. This strange exoplanet may be so inhospitable that Thor himself would not even dare to visit. 750 light years away in the Draco constellation lives TReS-2B, a gas giant darker than the darkest coal or any black paints we are capable of making on earth. TReS-2B lacks the sort of reflective clouds that give Jupiter its distinctive colors. Due to an unknown compound in the atmosphere surrounding the planet, the gasses reflect less than one percent of light hitting it from the system’s sun.
Researchers suggest that the extreme light-absorbing properties of the planet may be due to gaseous titanium oxide, sodium, and potassium, or a yet-unknown chemical. To add to the ominous appearance of this planet, the 980 degree Celsius (1,800 Fahrenheit) atmosphere actually results in a surrounding faint-red glow due to blackbody radiation.
The Primordial Planet
Aptly nicknamed Methuselah after the nearly 1000-year-old man in the Hebrew Bible, PSR B1620-26 b is a planet nearly as old as our universe. At 13 billion years old — likely formed less than a billion years after the Big Bang — this planet is the oldest known exoplanet, outstripping Earth by approximately nine billion years. The prehistoric giant is located 5,600 light years from Earth in a globular star cluster. These dense clusters of old stars are smaller than galaxies and are often formed in the early universe. Until the discovery of Methuselah, it was thought that exoplanets could not exist in globular clusters, since they often lack heavy elements necessary for planet formation in their star-crowded centers.
Methuselah probably once orbited a yellow, main-sequence star like our sun. Now, having survived the star’s expanding red giant phase, Methuselah orbits the chaotic remnants of the star: a white dwarf and a fast-spinning pulsar. Not only has this primordial planet survived the test of time, but also the extremely violent conditions of such an orbit, which usually tears lesser planets apart. Methuselah’s existence hints that exoplanets may be abundant in old star clusters and implies that planets may be able to form in even the most heavy-element-depleted of star systems.
The Pink Planet
As surprising as the festive color of GJ 504b may be, it is actually not the most interesting thing about the young planet. Scientists speculate that the fuchsia atmosphere results from the residual heat of its recent formation. However, what is most mysterious about the magenta world is its distance from the host star. GJ 504b is an estimated 43.5 AU (astronomical units — 1 AU, or 93 million miles, is the distance from Earth to the sun) from its star. In contrast, Neptune, the most distant planet in our solar system, is a mere 30.1 AU from the sun.
The Pink Planet is four times the mass of Jupiter, which is not especially uncommon among exoplanets; however, at nine times further away from its host star than Jupiter is from the sun, the Pink Planet is challenging current models of planet formation. The present model assumes that accretion of debris — such as asteroid chunks and space dust — orbiting a star is necessary to form a planet’s core. Subsequently, scientists previously thought that giant-planet formation at great distances from stars was impossible due to the much lower density of this kind of orbiting debris.
Amongst a veritable rouges’ gallery of planets made of diamond or magenta gases, these nondescript planets seem out of place. But it is exactly the mundane qualities of these worlds that make them so interesting; they are not too cold and they are not too hot — in fact, they may be just right.
Kepler-186f and Kepler-452b lie in the Goldilocks Zone, or habitable zone, of their respective stars. Much like the porridge of the fairytale character for which it is named, the Goldilocks Zone is the area surrounding a star that is neither too hot nor too cold, but rather the optimal temperature for liquid water to exist. Water is necessary to support life as we know it and thus makes planets in the Goldilocks Zone good contenders in the search for extraterrestrial life. Naturally, the range of distances from a star that qualify as a Goldilocks Zone vary depending on the size and heat of the star.
Kepler-186f, confirmed in 2014, holds a short 130-day orbit around a relatively weak star 500 light years from Earth, but is only 10-11% larger than Earth. Therefore, Kepler-186f has survivable gravity and the possibility of water and a rocky terrain, all of which are ingredients for potential human habitation.
Kepler-452b, confirmed in July of 2015, is an even more exciting prospect. At 1,400 light years away, the rocky planet Kepler-452b holds a 385-day orbit around a yellow star similar to our sun. It has resided in the habitable zone for six billion years — two billion years longer than Earth — and researchers suggest that this duration gives life “substantial opportunity” to have formed. Kepler-452b has a diameter 60% larger than that of Earth and, while it is considered a “super-Earth” in terms of size, humans would still be capable of surviving the stronger gravity.
While it may take many decades or even centuries more to answer the question of life on other planets, we have conclusively shown that our Earth and our solar system are not alone in under two decades.
As it is with most aspects of astronomy and astrophysics, the goal of the Great Exoplanet Hunt of the last decade has been to understand more about the universe in which we live. However, at the root of our fascination with exoplanets, strange or otherwise, is the fundamental question: Are we alone in the universe? While it may take many decades or even centuries more to answer the question of life on other planets, we have conclusively shown that our Earth and our solar system are not alone in under two decades. It will likely be a long time, if ever, before we can visit any of these exoplanets to confirm their “out-of-this-world” properties. However, in the meantime, NASA’s Jet Propulsion Laboratory has developed a series of travel posters and an interactive trip planner for some of our universe’s strangest exoplanets.