Seen from the perspective of our own Solar System, most of the exoplanets discovered to date really seem out-of-place and outright strange. For instance, when we think of planets we conjure up images of celestial objects that revolve around their stars in tidy and fixed circular orbits. Even though this certainly holds true for our Solar System, many alien worlds in other exoplanetary systems have displayed a surprising variety in terms of orbital configurations and environmental extremes. This fact couldn’t be showcased more in the case of a hot Jupiter whose orbit is so elongated that it resembles that of comets in our Solar System. Not to be outdone in the weirdness department, a smaller “Super-Earth”-type world which is locked in a tight, close orbit to its host star exhibits such temperature differences between its day and night sides that astronomers speculate it may have an atmosphere that is only confined to one hemisphere. NASA’s Spitzer space telescope recently observed both of these bizarre worlds in detail, allowing astronomers to map their atmosphere and climate patterns in unprecedented precision.
Observing a Star-Grazing Planet
As implied by their name, hot Jupiters are large and massive, scorching-hot gas giant planets which lie very close to their respective stars, in orbits that are considerably smaller than that of Mercury around the Sun. Whereas Mercury’s mean distance from the Sun is 0.39 Astronomical Units (1 Astronomical Unit is equal to the Sun-Earth distance), hot Jupiters orbit around their host stars from approximately 0.015 to 0.5 A.U. away. The overall existence of hot Jupiters proved to be a major surprise for astronomers who had theorised that other planetary systems should exhibit a structure that would more or less resemble that of our own Solar System. After all, conventional wisdom for how planets generally form and evolve held that close to a star where temperatures are much higher, conditions are such that only small, terrestrial planets made up of rock and metal can form. Further out where temperatures drop below a certain point begins the realm of the gas and ice giants where all these hot Jupiter-type exoplanets should have been located.
In recent years, a series of computer simulations on the orbital dynamics of planetary systems have sought to address this discrepancy between theory and observation, by showing that planetary orbits could indeed be influenced and be thrown into eccentric or chaotic arrangements by the presence of neighboring planets or stellar companions. In these scenarios, multiple close passages from nearby planets could disrupt the orbits of gas giant exoplanets in such a way, forcing them to migrate inward toward their host star or even be thrown out of their planetary system altogether. A similar study undertaken last year and co-authored by Gregory Laughlin, a professor of astronomy and astrophysics at the University of California, Santa Cruz, shows that through a similar process Jupiter could have undertaken just such an inward-outward motion in the early Solar System. The study concluded that the results from a wandering Jupiter would have been widely catastrophic, wreaking havoc to the orbits of any pre-existing inner planets that could have formed prior to the Earth.
In an effort to better understand the processes that might drive such planetary migrations, Lauhlin participated in a team of astronomers in the U.S. led by Julien de Wit, a professor of planetary sciences at the Massachusetts Institute of Technology, who set out to study HD 80606b, a nearby hot Jupiter, with NASA’s Spitzer space telescope.
Discovered back in 2001 around a Sun-like star in a binary star system located approximately 190 light-years away in the constellation of Ursa Major, HD 80606b immediately stood out for having a very elongated 111-day orbit, one that is also inclined to its star’s rotation axis by a whopping 53 degrees. These orbital characteristics put HD 80606b in the same league with many comets in our Solar System, whose highly elliptical orbits bring them extremely close to the Sun for short periods of time during perihelion, before returning back to the outer reaches of the Solar System from which they originated. In a similar fashion, HD 80606b was found to spend most of its 111-day-long orbit far from its star, extending as far out as 0.85 A.U. away, before plunging inward again to as close as 0.03 A.U. during periastron.
HD 80606b was studied again with Spitzer back in 2009, in a series of observations which gave astronomers the chance to track the changes in the atmosphere of the star-grazing planet, as it experienced wild temperature swings between periastron and apastron. One key finding of these past observations was that HD 80606b reached a maximum temperature of 1,226 degrees Celsius in its star-facing side within a timeframe of approximately 20 hours before and after its closest passage to its host star. Computer simulations based on these data from Spitzer allowed astronomers to determine that the planet’s atmosphere develops immense shock wave storms as it tries to dissipate the extra heat, which gradually die down as HD 80606b moves farther away from its star and temperatures drop to a relatively “comfy” 526 °C. “We watched the development of one of the fiercest storms in the galaxy,” Laughlin had commented at the time, who had also taken part in these previous observations by Spitzer. “This is the first time that we’ve detected weather changes in real-time on a planet outside our Solar System.”
In their new study, which was published on 28 March in The Astrophysical Journal, de Wit’s team conducted a set of new observations of HD 80606b with Spitzer, in an effort to better understand the physical processes that take place in the planet’s atmosphere during its orbit. To that end, the researchers studied the planet for a total of 85 hours, much longer than the 30-hour timeframe of the original Spitzer pointings in 2009. Furthermore, de Wit’s team observed the planet at a wavelength of 4.5 µm in order to complement the previous studies that had been conducted at a longer wavelength of 8 µm. Taken together, both data sets show that HD 80606b’s atmosphere heats rapidly as it approaches periastron while receiving an 800-fold increase in stellar energy in the process, before cooling again just as fast as it moves away from the star. During this time, extreme shock wave storms rage in the atmosphere as the latter is heated abruptly to thousands of degrees. “As the planet gets closer to the star, it feels a burst of starlight, or radiation,” says Laughlin. “The atmosphere becomes a cauldron of chemical reactions, and the winds ramp up far beyond hurricane force.”
“This happens every 111 days,” adds de Wit. “The good thing is, the planet goes so far away that the process is a complete reset. It’s the same story over and over again.”
Analysis of the Spitzer observations also showed that the planet’s atmosphere is very inefficient in retaining the excess heat, making it visible to the space telescope’s instruments only a few hours before and after closest approach. According to the researchers this could mean that either the planet’s atmosphere has a significant amount of cloud covers which dampen its thermal emission, or that the latter is lesser than what Spitzer can detect. In addition, by studying the planet’s brightness as it varied with time, de Wit’s team was able to put a better constrain to its revolution around its axis, clocking it at approximately 90 hours—a much bigger estimate than the initial 35-hour period that had been previously calculated by the 2009 observations. Even though this newer estimate represents a much slower rotation period than what had been expected for hot Jupiters, it will nevertheless help astronomers to better model the interiors of these massive giants in the future. “Fifty years ago, we were measuring the rotation rates of planets in our own Solar System for the first time,” comments Laughlin at the significance of these calculations. “Now we are doing the same thing for planets orbiting other stars. That’s pretty amazing.”
By studying the way HD 80606b dissipates heat through the duration of its orbit, the researchers were also able to gain some insights to the mystery of the planet’s orbit. Theoretical simulations showed that such elongated orbits as in the case of HD 80606b, which are thought to be caused by planetary migration early on in a solar system’s history, tend to become circular with the passage of time through the loss of angular momentum. More specifically, as a planet in a very elliptic orbit swings around its star it is squeezed by the latter’s gravity, a process that slowly converts the planet’s orbital energy into heat. This in turn gradually shortens the planet’s orbit until it eventually becomes a more or less circular one, similar to that of most hot Jupiters that have been discovered to date. The amount of time it takes for a planet to settle from an elongated orbit to a circular one largely depends on the amount of heat it dissipates as it swings around its star. In the case of HD 80606b it turns out that the planet dissipates much less heat than the gas giants in our own Solar System, which suggests that it will retain its current orbital configuration for many billions of years more in the future.
“HD 80606 b is much less dissipative than any of the Solar System planets,” write the researchers in their study. “A comparatively low rate of tidal energy dissipation within HD 80606 b would then be consistent with it having retained a large eccentricity and a large rotation period in the face of billions of years of tidal evolution.” These results come in contrast to theoretical predictions for the migration of gas giant planets, which had posited that they should occur on much smaller timescales. This could mean that either planetary migration is not an efficient mechanism for hot Jupiters, or that alternative theories for their formation should be taken into account.
Video Credit: NASA/JPL-Caltech/MIT/Principia College
Despite the many mysteries that surround hot Jupiters, these intriguing and bizarre worlds are central to astronomers’ efforts in understanding the way with which exoplanets and exoplanet atmospheres evolve over time. “HD 80606 b is Earth’s opposite in almost every regard,” says Heather Knutson, an assistant professor of planetary sciences at the California Institute of Technology, who did not participate in the study by de Wit’s team. “It’s a massive gas giant planet on an extremely eccentric orbit around one star in a wide binary system, and experiences a more than 800-fold change in illumination over the course of each orbit. It’s a terrible place to search for life, but these same properties make it a great test case for atmospheric modelers.”
The Changing Face of a Super-Earth
Much closer to home lies a different specimen of the exoplanetary zoo. In the direction of the constellation of Cancer at a distance of 41 light-years away, a Sun-like star that belongs to a binary star system is home to a family of five exoplanets. The innermost member of this planetary system, named 55 Cancri e, is also the most massive, with a mass approximately eight times that of Jupiter. Located at a mean distance of 0.015 A.U. from its host star 55 Cancri e sweeps around its star in just under 18 hours. Despite these orbital similarities that 55 Cancri e shares with hot Jupiters, it belongs to a different breed of alien worlds. With an estimated size twice that of Earth, 55 Cancri e is a representative example of a class of exoplanets known as Super-Earths: planets that are slightly bigger and more massive than ours.
Since its discovery` in 2004, 55 Cancri e’s perceived identity has changed with time. Initially thought to be a gaseous mini-Neptune, the planet was later seen as a possible steamy water world, then a diamond planet, until its status was finally cemented as a solid, rocky world. As a consequence, for almost a decade after it was first detected, very little was known about the composition of this enigmatic planet. Furthermore, observing techniques hadn’t sufficiently advanced enough in order for astronomers to be able to study the characteristics of exoplanets smaller than hot Jupiters in any detail. The situation started to change in the last couple of years when observations with NASA’s Hubble and Spitzer space telescopes, coupled with a series of advancements in data analysis techniques, allowed astronomers to make the first detailed characterisation of Super-Earths. In 2015, a European team of astronomers, led by Brice Olivier Demory, an astrophysicist at the University of Cambridge, detected a significant variability in the planet’s thermal emission with the help of Spitzer. During the course of two years, the space telescope observed a 300-percent change in the intensity of the infrared light that was emitted from 55 Cancri e, indicating a wild temperature swing between 1,000 and 2,700 degrees Celsius on the planet’s day side. Independent observations with Hubble also revealed the presence of hydrogen and helium as well as traces of hydrogen cyanide in the planet’s atmosphere but no water vapor, which had been previously theorised to be present, indicating that 55 Cancri e was a dry, carbon-rich world.
An animation showing 55 Cancri e as it orbits around its star, based on data that were gathered with the Spitzer space telescope. The planet’s day side is significantly hotter than its night side, which could be explained by the presence of a thin atmosphere that only surrounds the day side, or the presence of extensive volcanic activity on the planet’ surface. Video Credit: NASA/JPL-Caltech
A new study, published yesterday in the journal Nature, a research team, also led by Demory, reports on the results of the most detailed observations of 55 Cancri e that have been conducted to date, while providing more insight into the possible environment that might dominate this enigmatic far-off world. The researchers utilised again the Spitzer space telescope in order to map 55 Cancri e’s temperature variations between its day and night sides. Contrary to the previous Spitzer observations that had been conducted only when the planet transited the face of its star every 18 hours, Demory’s team studied 55 Cancri e for a total of 80 hours, during several full revolutions of the planet around its host star. This enabled the researchers to observe how 55 Cancri e’s overall brightness varied while the planet was positioned in different points in its orbit as seen from our line of sight. “Spitzer observed the phases of 55 Cancri e, similar to the phases of the Moon as seen from the Earth,” explains Demory. “We were able to observe the first, last quarters, new and full phases of this small exoplanet. In return, these observations helped us build a map of the planet. This map informs us which regions are hot on the planet.”
What these observations revealed was a dramatic difference in temperature between 55 Cancri e’s day and night sides. Since the planet is located so close to its star, it is tidally locked, making one hemisphere constantly facing its host star—similar to the way the Moon is tidally locked to Earth and always presents the same face to our home planet. Theoretical predictions had posited that under such conditions, the heat from the star-facing hemisphere would be dissipated by the planet’s atmosphere into the night side, thus helping to maintain a thermal equilibrium between the two hemispheres. Yet what Demory’s team found was that the two sides of 55 Cancri e exhibited a temperature difference of a whopping 1,300 degrees Celsius, with temperatures on the day side reaching as high as 2,426 degrees and plummeting as low as 1,126 degrees on the night side. More surprisingly, Spitzer also detected that the hottest point on the planet’s day side was offset by 41 degrees from the point where the star was directly overhead.
In the absence of additional data, these temperature swings on 55 Cancri e can be explained by various different mechanisms. According to Demory’s team, the most likely explanations include either the presence of a thin atmosphere that only surrounds the day side or the presence of extensive volcanic activity on the planet’ surface in the form of widespread lava flows, volcanic plumes, and volcanic outgassing, similar to what has been observed on Jupiter’s moon Io. “Our view of this planet keeps evolving,” says Demory. “The latest findings tell us the planet has hot nights and significantly hotter days. This indicates the planet inefficiently transports heat around the planet. We propose this could be explained by an atmosphere that would exist only on the day side of the planet, or by lava flows at the planet surface.”
These observations by Spitzer were made possible through a combination of innovative design in the building of the space telescope and subsequent engineering enhancements, as explained by NASA. Even though Spitzer was never designed to deliver such cutting-edge results in the field of exoplanetary science, the space telescope has nevertheless been instrumental in important exoplanetary research, like the discovery of a distant exoplanet across the galaxy, as well as the characterisation of the atmospheres of dozens of hot Jupiters and hot Neptunes—to name just a few. Even so, a more detailed study of these alien worlds, which could help to settle some of the more pressing questions regarding their overall properties, will have to wait for the launch of NASA’s James Webb space telescope, scheduled for 2018. Until then, 55 Cancri e will probably remain somewhat of a mystery—a world full of fascinating possibilities. “We still don’t know exactly what this planet is made of – it’s still a riddle,” says Demorty. “These results are like adding another brick to the wall, but the exact nature of this planet is still not completely understood.”
And as remote as these distant exoplanets seem, they also have an important part to play in better understanding our own planet. According to Demorty, “Understanding the surface and climate properties of these other worlds will eventually allow us to put the Earth’s climate and habitability into context.”
As planetary exploration has showcased time and again, the study of other worlds is the also the study of our own.