With new exoplanet discoveries announced almost at a monthly basis, it is no surprise that only those that involve potentially Earth-like, habitable worlds mostly manage to grab the headlines. Yet, as exoplanetary research has shown, even the ones that do not fit that bill are fascinating in their own right, offering a great insight into the processes that drive planetary formation and evolution. Such is the case with the discovery of the first Uranus-sized exoplanet candidate in a stable long-period orbit that was announced earlier this week, which could be similar to the ice giants of our own Solar System.
Contrary to most of the exoplanetary systems that have been discovered to date, our Solar System can be described as a tidy and well-organized place, with all the eight major planets neatly grouped together in two different and distinct categories: four small rocky, terrestrial planets orbiting close to the Sun are followed by four Jovians, or gas giants that are lying further out, with a large asteroid belt in between separating these two planetary groups. This assortment is not a product of chance, however, but of physics and chemistry. In the inner part of the protoplanetary disk of material that gave rise to the planets, temperatures were such that all the volatile elements, like water, ammonia, methane, hydrogen, nitrogen, etc., boiled off due to the Sun’s intense heat, leaving behind only the microscopic solid particles of rock and metal that later clumped together to form the terrestrial planets. Farther out in the protoplanetary disk, beyond a certain point which is known as the frost or “snow line,” temperatures were low enough for all the volatile compounds to condense into solid ice grains, forming the building blocks from which the gas giant planets later emerged.
Traditional planetary formation models had indicated that the structure of other exoplanetary systems should closely follow that of our own Solar System as well. Yet the advent of exoplanetary research during the last two decades revealed the presence of a new class of planets that was completely unexpected—that of “hot Jupiters.” As implied by their name, these strange alien worlds share similar characteristics with Jupiter, but are found in extremely close orbits to their parent stars between approximately 0.015 and 0.5 Astronomical Units (much closer than Mercury’s 0.39 AU-wide orbit around the Sun). As an effect, these planets experience scorching-hot temperatures that can reach up to a few thousand Kelvins, leading to exotic weather conditions on their atmospheres that are unlike anything seen on the gas giant planets of our Solar System.
Even though the discovery of “hot Jupiters” came as a great surprise to astronomers, it was long suspected that their apparent abundance in the galaxy has been the result of observational bias. The two main techniques widely used for exoplanet discovery have been the radial velocity and the transit method. More specifically, astronomers use the radial velocity method to detect any gentle, periodic “wobbles” in a star’s motion that might be caused by the gravitational tug of an unseen orbiting planet, while with the transit method they look for the dimming of a star’s light that would be caused by the passage of a planet across the star’s face. The closer a planet orbits its star, the greater its gravitational tug on it would be, and if aligned properly with our line of sight here on Earth, the more times it would transit in front of it. Since “hot Jupiters” lie so close to their stars, their orbital periods range from a few hours to a few days, making them the easiest to detect through both radial velocity and transit observations, which are best suited for detecting such short-period orbits.
NASA’s Kepler space telescope, which hunted for exoplanets between 2009 and 2013, proved pivotal in revealing the wider diversity and abundance of different types of exoplanets in the galaxy. Throughout its operational four-year mission, Kepler observed many thousands of transits within a fixed field of view which contained more than 150,000 stars, allowing astronomers to reveal the presence of 977 confirmed exoplanets in 452 different planetary systems and another 4,234 exoplanet candidates as of date, ranging from supermassive hot Jupiters to sub-terrestrial worlds with masses considerably lower than Earth’s. Moreover, most of these worlds were found to be not bigger than 2.5 times the size of Earth and to have stable, almost circular orbits akin to those of the terrestrial planets in our own Solar System.
Yet, even though smaller-sized planets were found to be the norm in the galaxy, one thing that continued to puzzle astronomers was the apparent lack of any “cold Neptunes” located in wide, circular orbits similar to those of the ice giants in our Solar System. Now, a new study by an international team of astronomers that has been accepted for publication in The Astrophysical Journal comes to fill that void by announcing the discovery of the first Uranus-sized exoplanet candidate in a long-period orbit, making it the first such planet ever to be discovered with the transit method.
The team, led by Dr. David Kipping, an astronomer at the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Mass., used publicly available Kepler data to study a transit event that had been observed around an orange dwarf star named Kepler-421, located approximately 1,000 light-years away in the constellation Lyra. Although this transit had a high signal-to-noise ratio, it nevertheless represented an insufficient amount of data for making any accurate estimates regarding the nature of the transiting body, called Kepler-421b. Yet, while conducting a routine systematic error correction analysis on the Kepler data, Kipping’s team discovered a second transit event for Kepler-421 that was almost identical to the first one, leading astronomers to conclude that what they were observing, was an actual planetary companion to the star that had an orbital period of approximately 704 days. “Inspection of the two transits reveals remarkable similarity,” write the researchers in their study. “The second transit aligns nearly perfectly with the first, indicating that this event is indeed due to the same transiting object.” To further validate their results, Kipping’s team conducted an extensive spectroscopic analysis of a series of follow-up observations of Kepler 421 that were made with ground-based telescopes, in order to exclude the possibility of the two transit events being false positives due to the presence of a transiting dim stellar companion, or some other astrophysical phenomenon. The results of the their analysis showed that Kepler-421b was indeed an orbiting planet, to a high level of statistical significance. “The probability of a planetary nature for [Kepler-421b] is approximately 28,000 greater than that of a false positive, corresponding to a confidence level for the validation, of 4.1 sigma,” writes Kipping’s team. (A confidence level of 5 sigma in physics corresponds to a formal discovery.)
The discovery of Kepler-421b is a significant one, not only because it represents the first exoplanet in a long-period orbit ever to be discovered with the transit method, but also because of its similarity to the ice giants in our own Solar System. “We discovered the planet by detecting the tell-tale decrease in brightness of the parent star as Kepler-421b passed in front – a ‘transit’ event,” explains Kipping at his personal webpage. “The amount of light blocked by the planet betrays her size and we find that Kepler-421b is nearly the same size as Uranus, which is about four times larger than the Earth. Using a special technique I developed called ‘Asterodensity Profiling’, we can also exploit the transit light curve shape to measure the orbital eccentricity of Kepler-421b, which works out to be 0.04, almost exactly the same as Uranus as well.” Furthermore, Kepler-421b’s distance from its host star was found to be approximately 110 million miles, or 1.2 Astronomical Units. Since orange dwarf stars like Kepler-421 are approximately 30 percent smaller and less bright than the Sun, that means that at its current distance Kepler-421 orbits outside of its planetary system’s “frost line”—the boundary beyond which ice grains condense to become solid, leading to the formation of gas and ice giant planets. “With a semi-major axis of 1.22 AU, Kepler-421b orbits closer to its parent star than the orbit of Mars (1.52 AU) around the Sun,” write the researchers in their study. “Despite this smaller orbit, the lower luminosity of Kepler-421 causes Kepler-421b to receive just 64% of the insolation received by Mars. Comparing the incident insolation to the habitable-zone boundaries of [previous studies], Kepler-421b lies firmly outside the maximum greenhouse outer edge … Thus, Kepler-421b has an insolation roughly midway between the insolations of Mars and the asteroid Vesta.” Under these conditions, Kipping’s team estimated the possible temperature on Kepler-421b to be a chilling -135 degrees Fahrenheit (-93 degrees Celsius).
Scientists had long speculated about the existence of Uranus-sized ice giant exoplanets in other planetary systems. Yet the chances for discovering such worlds were always considered to be very low, due to their long-period orbits coupled with the small likelihood of these orbits being aligned in such a way as to be detectable with the transit method. “Finding Kepler-421b was a stroke of luck,” says Kipping. “The farther a planet is from its star, the less likely it is to transit the star from Earth’s point of view. It has to line up just right … In general, planets further away from their host star have a lower probability that their orbital inclination is small enough that they appear to pass in front of their parent star. A planet like Kepler-421b has a minuscule 0.3% chance of transiting, and yet here she is.”
The fact that, despite the odds, a world like Kepler-421b was discovered at all hints at the possibility of these worlds being more common in the galaxy, filling a gap in our understanding of the processes behind planetary formation and evolution. The next generation of ground- and space-based observatories that are scheduled to come online in the following years might uncover the presence of more such ice giant worlds that, although present in our Solar System, seem to be absent from the planetary population of the rest of the galaxy. In the end, such discoveries could allow us to finally put our own Solar System in its proper context to the rest of the Milky Way.