Our starlit spiral Milky Way Galaxy is literally filled with brave new worlds–distant planets that were born to stars beyond our own Sun. Some of these planetary children of distant stars are similar to the familiar planets dwelling within our own Solar System, while others are genuine “oddballs” that are unlike anything astronomers believed could exist–that is, until they were discovered. Even though the discovery of new exoplanets has become routine, surprising revelations about their often exotic attributes still keep pouring in. In August 2019, a team of astronomers announced that they have gotten a rare sneak peek at the conditions existing on the surface of a distant rocky planet in orbit around another star. The exoplanet very likely has little, if any, atmosphere, according to data from the IRAC camera aboard NASA’s infrared Spitzer Space Telescope.
This exoplanet study, published in the August 19, 2019 issue of the journal Nature, is just one of a treasure trove of nearly 700 publications using IRAC since 2009. This is when Spitzer’s Warm Mission began, and IRAC became its only operating instrument. The IRAC camera’s team is based at the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts, and is led by Dr. Giovanni Fazio.
The distant rocky planet, named LHS 3844b, may also be well-coated with the same cooled volcanic material that creates the dark lunar regions known as mare (Latin for sea). This means that the exoplanet might be similar to Mercury, or to Earth’s Moon.
LHS 3844b was discovered in 2018 by NASA’s Transiting Exoplanet Satellite Survey (TESS) mission. It is located 48.6 light-years from Earth, and has a radius 1.3 times that of our own planet. It orbits a small, cool type of star called an M dwarf. This is especially important because, as one of the most common and long-lived types of stars in our Galaxy, M dwarfs may be the stellar parents of a high percentage of the total number of planets in our Milky Way. The smaller the star, the longer its “life” on the hydrogen-burnng main-sequence of the Herttzsprung-Russell Diagram. This is because relatively cool stars burn their supply of hydrogen fuel more slowly than their much hotter and more massive stellar kin, who “live” fast, and “die” young.
TESS discovered the remote rocky world using the transit method. This method spots the existence of an exoplanet when it floats in front of the glaring face of its parent star, thus dimming its light across the line-of-sight between the star and Earth.
During follow-up observations, IRAC detected light originating from the surface of LHS 3844b. The planet completes one full orbit around its stellar parent in only 11 hours. With such a close-in orbit to the searing-hot surface of its star, LHS 3844b is probably “tidally locked” with one side of the planet perpetually facing its star. The star-facing side, referred to as the “dayside”, is approximately a roasting 1,410 degrees Fahrenheit. Being quite hot, the planet radiates an abundant amount of infrared light which IRAC, an infrared camera, is capable of measuring. This observation is important because it marks the first time IRAC data have been able to provide information about the atmosphere of a terrestrial-sized alien world in orbit around an M dwarf star.
Brave New Distant Worlds
Exoplanets are planets outside of our own Solar System. The first possible evidence of the existence of an exoplanet was back in 1917 but, at that time, it was not recognized for being what it was. The first validated discovery of an exoplanet occurred in 1992, and this was followed by the confirmation of an alien world that had first been detected in 1988. As of October 2019, there are 4,118 confirmed exoplanets inhabiting 3,063 distant planetary systems belonging to stars beyond our Sun. Six hundred and sixty-nine of these systems sport more than one planet.
In addition to the transit method, there are several other ways for astronomers to detect exoplanets. Transit photometry and Doppler spectroscopy have spotted the greatest number of exoplanets, but both of these methods have an observational bias favoring the discovery of giant planets that circle their stars fast and close in searing-hot orbits. Indeed, 85% of the exoplanets found so far are inside the tidal locking zone around their stars. Approximately 1 in 5 Sun-like stars may have an “Earth-sized” planet in the habitable zone, which is that “Goldilocks” region around a star where water can exist in its life-sustaining liquid phase. The presence of liquid water indicates the possibility–though by no means the promise–that life as we know it is present on that distant world. Assuming that there are 200 billion stars within our Galaxy, it can be estimated that there are 11 billion potentially habitable Earth-sized planets in our Milky Way. This great number rises to 40 billion if planets orbiting red dwarf stars are included. Red dwarf stars are both the smallest, as well as the most numerous, true hydrogen-burning stars in our Galaxy. Because red dwarfs are very small and cool, they can “live” for a very long time. In fact, it is thought that no red dwarf star has had enough time–since the Big Bang birth of the Universe 13.8 billion years ago–to run out of its nuclear-fusing fuel, and “die”. Red dwarf stars can “live” for trillions of years. By comparison, our own small Sun–which is nevertheless much more massive than a red dwarf star–is still enjoying its stellar mid-life. Our Sun is about 4.56 billion years old, and stars of its mass can “live” for about 10 billion years.
Exoplanets are a diverse collection. There are planets that hug their parent-star in roasting orbits that are so close to the roiling, broiling heat of their star that it takes them only a few hours to complete a single orbit. In contrast, there are exoplanets that are so far from their star that they take thousands of years to complete an orbit. Indeed, some exoplanets are so far out that it is extremely difficult for astronomers to determine whether they are gravitationally bound to the host star. Almost all of the distant worlds discovered so far dwell within our Milky Way. Nevertheless, there is evidence indicating that exoplanets exist beyond our Galaxy. The closest exoplanet to our own Solar System is Proxima Centauri b, which is located only 4.2 light-years from Earth. Its parent-star, Proxima Centauri, is the closest star to our Sun.
The most massive planet listed on the NASA Exoplanet Archive is HR 2562, which weighs-in at approximately 30 times Jupiter’s mass. However, according to some definitions–based on the nuclear fusion of deuterium (heavy hydrogen)–this world is too hefty to be classified as a “planet”, and may really be a brown dwarf. Brown dwarfs, referred to as “failed stars”, probably formed the same way as true hydrogen-burning stars. Unfortunately, they never managed to acquire enough mass to light their nuclear-fusing stellar fires. Red dwarf stars are the least massive true stars in our Galaxy.
The least massive exoplanet has been dubbed Draugr (also known as PSR B1257+12 A or PSR B1257+12b), which sports the puny mass of only twice that of Earth’s Moon.
The discovery of planets beyond our own Solar System intensified interest in the scientific hunt for extraterrestrial life. There is a special interest in planets that circle within a star’s habitable zone. While the existence of liquid water is a prerequisite for life as we know it, there are other indicators that need to be considered.
In addition to exoplanets that orbit a parent-star, there are also rogue planets. These unfortunate worlds do not orbit any stellar parent at all, and wander through interstellar space as orphans. Once rogue planets probably circled a parent-star, but were unceremoniously evicted from their systems as a result of gravitational interactions with other planetary siblings or by the jostling of a nearby star that floated too close to their own planetary systems, wreaking havoc. Rogue planets tend to be classified as a separate category–especially if they are gas-giants like our own Solar System’s Jupiter and Saturn. In this case they are frequently categorized as sub-brown dwarfs–like WISE 0855-0714. There are probably billions (or more) rogue planets wandering around our Milky Way.
Even though the discovery of new exoplanets has become almost routine, historically the quest to find worlds in orbit around stars beyond our Sun proved to be difficult. For centuries scientists, philosophers, and science fiction writers considered the possibility that exoplanets could exist. However, there was no way for them to find out, nor was there a method available to determine how common exoplanets might be–or even how similar they could be to the planets dwelling in our own Solar System. There were numerous claims of discovery during the nineteenth century, but these various detections were ultimately rejected as invalid by astronomers.
The first confirmation of an exoplanet in orbit around a Sun-like star didn’t come until 1995, when a giant planet was detected in a four-day orbit around the nearby star 51 Pegasi. This historic discovery was made by the Swiss astronomers Michel Mayor and Didier Queloz, who were awarded the 2019 Nobel Prize in Physics for their work. The behemoth world that they found, dubbed 51 Pegasi b, proved to be a surprise because it was a giant planet that hugged its parent-star fast and close in a hell-like, broiling orbit. Up until its discovery, astronomers thought that gas-giant planets, like 51 Pegasi b, could only exist in more distant orbits around their parent-stars–at approximately the same distance that Jupiter is from our Sun. 51 Pegasi b was the first of its exotic kind to be discovered–the first exoplanet to be classified as a hot Jupiter. Many hot Jupiters have been discovered since then, but they constitute only a small percentage of exoplanets.
A Sneak Peek
By calculating the differences in temperature between the day and night sides of LHS 3844b, the team of astronomers concluded that there is only a small amount of heat being exchanged between the two sides. If an atmosphere existed, hot air on the dayside would naturally expand. This would unleash strong winds that would transfer heat around the planet. On a rocky world sporting little or no atmosphere, like Earth’s Moon, there is no air present to transfer heat.
“The temperature contrast on this planet is about as big as it can possibly be. That matches beautifully with our model of a bare rock with no atmosphere,” commented Dr. Laura Kreidberg in an August 19, 2019 CfA Press Release. Dr. Kreidberg is a researcher at the CfA and lead author of the study.
By gaining a new understanding of the conditions that could either preserve or destroy a planet’s atmosphere, astronomers can then plan how they will hunt for habitable worlds around distant stars. For example, our own planet’s atmosphere enabled water, in its life-sustaining liquid phase, to exist on Earth’s surface–enabling life to emerge, evolve, and thrive on our planet. In contrast, the atmospheric pressure of Mars is currently less than 1% that of Earth’s. For this reason, the oceans and rivers that once existed on the Martian surface have vanished.
“We’ve got lots of theories about how planetary atmospheres fare around M dwarfs, but we haven’t been able to study them empirically. Now, with LHS 3844b, we have a terrestrial planet outside our Solar System where for the first time we can determine observationally that at atmosphere is not present,” Dr. Kreidberg continued to explain in the CfA Press Release.
M dwarfs, when compared to small (but, nevertheless, much more massive) stars like our Sun, emit relatively high levels of ultraviolet light. Ultraviolet light can endanger lifeforms, as well as erode a planet’s atmosphere. This ultraviolet radiation is particularly violent when M dwarfs are young, as the youthful star hurls out a large number of flares. Flares are blasts of radiation and particles that can tear away newly-formed planetary atmospheres.
The IRAC observations rule out an atmosphere sporting more than 10 times the pressure of Earth’s on LHS 3844b, and an atmosphere between 1 and 10 bars on this exoplanet has also been almost entirely ruled out. According to measurements in units termed bars, Earth’s atmospheric pressure at sea level is about 1 bar.
However, the researchers point out that there remains a slight chance that an atmosphere could exist if the stellar and planetary properties possess some improbable characteristics. The astronomers also note that with the planet hugging its parent-star so closely, a thin atmosphere would be ripped away by the star’s powerful radiation and violent winds. “I’m still hopeful that other planets around M dwarfs could keep their atmospheres. The terrestrial planets in our Solar System are enormously diverse, and I expect the same will be true for exoplanet systems,” Dr. Kreidberg added.
The terrestrial planets of our own Solar System are the four inner, solid planets: Mercury, Venus, Earth, and Mars. In contrast, the quartet of outer, gaseous planets, inhabiting our Solar System’s colder regions, are Jupiter, Saturn, Uranus, and Neptune.
The authors of this new study delved deeper into the mystery by using LHS 3844b’s albedo (reflectiveness) to determine its composition. The Nature study shows that LHS 3844b is “quite dark”, according to co-author, Dr. Renyu Hu. Dr. Hu is an exoplanet scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California, which manages Spitzer. Both Dr. Hu and his co-authors propose that the distant planet is coated with basalt, which is a form of volcanic rock. “We know that the mare of the Moon are formed by ancient volcanism, and we postulate that this might be what has happened on this planet,” Dr. Hu explained in the August 19, 2019 CfA Press Release.
IRAC/Spitzer and NASA’s Hubble Space Telescope have previously collected information about the atmospheres of multiple gas planets. However, LHS 3844b stands out in the crowd because it is the smallest planet for which astromers used the light flowing out from its surface to learn about its atmosphere–or lack of one. IRAC previously used the transit method to study the seven rocky worlds that orbit the TRAPPIST-1 star–which is also an M dwarf–in order to learn more about their possible overall composition. For example, some of the planets probably contain water ice. NASA is planning to end the Spitzer/IRAC operations in February 2020, as a cost savings measure.