In my last post, I detailed some of the hottest locations for astrobiology in our Solar System. Today, however, we’re going to be going farther afield- outside the Solar System entirely, in fact.
The discovery of exoplanets – planets that orbit other stars- has been one of the great scientific success stories of the last century. In less than 20 years, we’ve gone from a handful of early detections to literally over a thousand (plus thousands more “candidates” that are awaiting verification). Obviously, astrobiologists have been more than a little excited by this pace of discovery.
Detecting an exoplanet is no mean feat- such bodies are usually a million times dimmer than their host star, and the light of the star tends to overwhelm such faint emissions. However, several techniques have been developed to get around these limitations.
The earliest used, Doppler spectroscopy,takes advantage of the fact as a planet orbits a star, it “tugs” on its center of mass, causing it to “wobble” ever so slightly. The motion due to this wobble can be detected by looking for the resulting Doppler shift in the star’s spectra. However, this method is generally most effective in determining extremely large planets that orbit close to their parents stars (so called “hot Jupiters”), which are unlikely to host life.
The most successful method used to date has been transit photometry, which looks for tiny dips in the star’s light output as the planet crosses in front of it. This method does have some limitations- the star, the planet, and Earth have to be precisely aligned for the transit dip to be visible- but it’s a relatively easy signal to look for otherwise. Transit photometry has been used by a number of different observing missions, the most famous example being the spectacular planet-hunting Kepler space telescope.
A few other planets have been detected using more esoteric methods, such as gravitational microlensing or timing pulsations in stars and pulsars. A scant handful have even been directly imaged, although this only feasible if the planet is extremely large, hot, and widely separated from its host star.
Using these methods, a whole zoo of exoplanets has been detected. Most of them are likely to be uninhabitable- but let’s take a look at the ones that might be a bit more promising for seekers of extraterrestrial life.
One of the most Earth-like planets (at least in terms of mass and theoretical surface temperatures) yet discovered, Kepler-296e is 1.75 times the size of Earth. It orbits a red dwarf star 1089 light years away, which is part of a binary system. It is located within the habitable zone of the star, where the temperature is warm enough for water to be liquid on the surface. Kepler-296’s habitable zone is much closer than the Earth is to the sun, owing to the cooler temperature of the host star; the planet orbits its star in only 34 days.
Located 1,120 light years from Earth, Kepler-442b also orbits a cooler red dwarf star. It’s 2.34 times the size of Earth, and would have a surface gravity about 30% greater (definitely the planet to go to if you want to get a good workout).
Detected 1,200 light years from Earth in the Lyra constellation, Kepler-62e is a member of an older star system, being likely billions of years older than Earth. It is thought to have a rocky composition (like Earth’s), and computer modeling suggests the planet could be largely covered by oceans. It’s considered a strong enough candidate for habitability that it’s been targeted for observation by the SETI program.
Gliese 832 c
One of the closest potentially habitable planets detected, Gliese 832 c is a scant 16.1 lights away. It is thought to have an extremely elliptical orbit, as planets go- that is to say, the distance from its star varies considerably. Consequently, the surface temperature may swing from -40 degrees Celsius to 7 degrees Celsius, depending on where the planet is in its orbit; on average, however, the temperature is warm enough to allow liquid water. However, it is possible the planet may have developed a dense atmosphere, leaving it in an uninhabitably hot state similar to Venus. Further observation will be required to determine how friendly to life the planet really is.
Unlike the other entries in this list, KIC 8462852 isn’t a planet. In fact, we’re not entirely sure what it is. The star first became well-known when analysis of Kepler data detected a intermittent, massive drop in the amount of light produced by the star- equivalent to covering up over half the star’s visible surface- something that had never been observed before. Furthermore, no dust or debris cloud has been detected around the star.
Initially, it was thought that the dimming could be due a mass of comets pulled inwards by a passing star- and, indeed, there’s another star in the local area that could’ve done such a thing. However, an examination of historical images showed that KIC 8462852 has been dimming for the last century- far too long a timescale for the comet explanation.
Lacking any other explanation, some researchers have begun speculating that the dimming could be due to the construction of megastructures in orbit around the star- perhaps a swarm of solar power satellites to capture the maximum amount of the star’s energy (popularly referred to as a Dyson sphere or Dyson swarm).
Admittedly, there are some problems with the aliens-did-it hypothesis- the laws of thermodynamics dictate that such structures would generate a large and detectable quantity of waste heat, which has yet to be observed. Observing campaigns by SETI also haven’t turned up any signs of intelligent life. Nonetheless, the sheer weirdness of the system means it will likely be a target of investigation for the foreseeable future. Whatever’s going on out there, it’s not like anything we’ve seen before.
These are just a handful of the potential living worlds that might be found throughout our galaxy. Undoubtedly more will be detected by upcoming missions, such as the James Webb Space Telescope, PLATO, and Kepler’s successor TESS. Get your travel itineraries ready- because the list of possible cosmic vacation hotspots is only going to keep growing!
Tessa is a 28 year old PhD student, and perhaps the world’s only queer trans astrobiologist. A nerd going way back, her interests include science fiction, space exploration, sustainability, science communication, and feminism and gender. Her hobbies also include horseback riding, playing the flute, social dancing, knitting, and occasional attempts at writing fiction. She currently resides in Tempe, AZ with her even nerdier fiancee and a mastiff mix who thinks he’s a lapdog. She tweets occasionally @spacermase.