We live in a cosmic Wonderland filled with weird worlds that perform a fantastic celestial ballet within the distant systems of stars beyond our own Sun. Some of these strange worlds are exotic, unlike anything previously imagined, not even in the wildest dreams of astronomers–while others are eerily familiar–bizarre because they are so similar to the planets of our own Solar System. At approximately 40 light-years from our planet–or 235 trillion miles away–there are seven newly discovered, rocky alien worlds that are nevertheless considered to be our near neighbors in Space. This richly endowed “nearby” planetary system, that has been named TRAPPIST-1, for The Transiting Planets and Planetesimals Small Telescope (TRAPPIST) in Chile, has been singing a sirens’ song to astronomers ever since its discovery was announced in May 2016. In August 2017, scientists announced that they now have a good estimate for the age of this system–one of the most intriguing planetary systems discovered to date–and it is quite ancient, at twice the age of our own 4.56 billion year old Solar System.
This new estimate of the ancient age of the TRAPPIST-1 system is important because if astronomers want to know whether life can survive on worlds beyond our own Solar System, it’s necessary for them to know the age of the targeted exoplanet’s parent-star. Active young stars have frequent eruptions of high energy radiation, called flares, that can brutally assault the surfaces of their system of baby planets. If the planets are newly formed, their orbits might be unstable. In contrast, planets circling older, more mature, parent-stars, have managed to survive the assault of these flares from their star’s flaming youth. However, the bad news is that they have also been exposed to the destructive effects of radiation, pouring out from their parent-stars, for a longer period of time.
Similar in size to Earth, the intriguing TRAPPIST-1 planets circle an ultra-cool dwarf star that is quite old at between 5.4 and 9.8 billion years old. This distant family of seven wonder-worlds was revealed early in 2017 at a NASA news conference. The team of astronomers making the discovery used a combination of results derived from the TRAPPIST telescope and NASA’s infrared Spitzer Space Telescope, as well as other ground-based telescopes. A trio of TRAPPIST-1’s seven planets are located in their parent-star’s “habitable zone”, which is that “Goldilocks” region surrounding a star where the temperatures are not too hot, not too cold, but just right for water to exist in its life-sustaining liquid phase. All seven of the TRAPPIST-1 planets are thought to be tidally locked to their stellar-parent–meaning that each planet has a perpetual dayside and nightside.
The Seven Wonder-Worlds Of The TRAPPIST-1 System
The astronomers who discovered the TRAPPIST-1 system, using the TRAPPIST telescope, were also assisted by several ground-based telescopes, including the European Southern Observatory’s (ESO’s) Very Large Telescope (VLT). Spitzer confirmed the existence of two of the three planets that were the first of the bunch to be discovered–and then went on to discover the quintet of additional rocky planets.
By making use of the newly acquired data derived from Spitzer, the team of astronomers made a precise measurement of the sizes of the seven exoplanets, and then made the first estimates of the masses of six of them. This helped the team calculate their density.
Based on their densities, all seven of the TRAPPIST-1 planets are generally thought to be rocky–like our own planet. Future observations will help astronomers determine if liquid water flows and pools on their surfaces.
The seven TRAPPIST-1 planets are the first batch of Earth-size worlds that have been found in orbit around a small, cool dwarf star. It is also the best target known that astronomers can use to study the atmospheres of potentially habitable Earth-size planets.
In March 2017, a different team of astronomers, using NASA’s planet-hunting Kepler Space Telescope, announced that they had also been studying the TRAPPIST-1 system since December 2016. Between December 15, 2016 and March 4, 2017, Kepler–currently operating as the K2 mission–collected revealing information about the elderly, ultracool dwarf parent-star. The new data acquired from K2 showed minuscule changes in stellar brightness occurring as the result of transiting exoplanets floating in front of the glaring face of their star. Transiting events occur when a planet passes in front of its star, causing a very tiny drop in its apparent stellar brightness.
The observation period, named K2 Campaign 12, includes 74 days of monitoring the TRAPPIST-1 system. This represents the longest, nearly continuous collection of observations of this system yet, and it also gives astronomers the opportunity to further observe the gravitational ballet being performed by the seven alien worlds–and, in addition, go on the hunt for as yet undiscovered exoplanets that may still be hiding somewhere in this very fertile system.
The observing field for K2 Campaign 12 was set when the detection of the first planets circling TRAPPIST-1 were announced, and the astronomical community had already submitted proposals for specific targets of great interest in that field. The unexpected chance to further observe the TRAPPIST-1 system was quickly recognized, and the K2 team and science community prevailed.
Additional refinements of the earlier measurements of the known TRAPPIST-1 planets, as well as any additional planets that may be detected in the K2 data, will help astronomers plan follow-up observations of the TRAPPIST-1 wonder worlds using NASA’s upcoming James Webb Space Telescope.
The Ancient TRAPPIST-1 System
At the time of its discovery, astronomers thought that the TRAPPIST-1 system was at least 500 million years old. This is because it takes a star of TRAPPIST-1’s low mass (a mere 8% of the mass of our small Sun) approximately that long to contract to its minimum size–which would be only a tiny bit larger than that of our Solar System’s banded behemoth, Jupiter. However, even this theoretical lower age limit is uncertain, and the low mass star could potentially be almost as old as our almost 14 billion year old Universe. There are many questions that are yet to be answered. For example, are the orbits of this compact system of planets stable? Could life have had sufficient time to emerge and evolve on any of these seven worlds?
“Our results really help constrain the evolution of the TRAPPIST-1 system, because the system has to have persisted for billion of years. This means the planets had to evolve together, otherwise the system would have fallen apart long ago,” commented Dr. Adam Burgasser in an August 11, 2017 NASA Jet Propulsion Labortory (JPL) Press Release. Dr. Burgasser is an astronomer at the University of California, San Diego, and the lead author of a paper calculating TRAPPIST 1’s true age. This new paper is to be published in The Astrophysical Journal. Dr. Burgasser teamed with Dr. Eric Mamajek, deputy program scientist for NASA’s Exoplanet Exploration Program, based at the JPL.
It is not precisely known what this older age signifies for the planets’ potential habitability. For one thing, old stars emit their powerful flares much less frequently than their youthful counterparts. Dr. Burgasser and Dr. Mamajek confirmed that the elderly TRAPPIST-1 is, indeed, a quiet old star in comparison to other ultra-cool dwarf stars. However, because the seven planets hug their parent-star in close orbits, they have been showered for billions of years by its deadly high-energy radiation–which could have boiled off atmospheres and whatever large quantities of water that they may have once had. Liquid water is necessary for life as we know it to emerge. In fact, the equivalent of an Earth ocean may have evaporated from each of the TRAPPIST-1 worlds–with the exception of the duo that are the farthest away from the star (planets g and h). The planet Mars, in our own Sun’s family, can serve as an example. In the early Solar System, Mars is likely to have had liquid water sloshing around on its surface. Alas, Mars lost its plentiful supply of liquid water, as well as its atmosphere, as a result of our Sun’s high-energy radiation showering it for billions of years.
But, just because a parent-star is elderly, does not necessarily indicate that a planet’s atmosphere has been dissipated. Since the TRAPPIST-1 planets sport lower densities than Earth, it is a possibility that large pools of volatile molecules such as water could create thick atmospheres that would serve as a protective barrier, shielding the planetary surfaces from deadly radiation. A heavy atmosphere could additionally help redistribute heat to the dark sides of these tidally locked planets. This could increase the habitable area of a planetary surface. However, the down side of this scenario is that it could also result in a “runaway greenhouse” effect, in which the atmosphere grows so extremely thick that the planet’s surface overheats. In our Solar System, the hell-like surface of Venus illustrates this tragic effect. Venus, a victim of the “runaway greenhouse” effect, is much hotter than it should be. This is because the “runaway greenhouse” process has gone completely wild on this inhospitable planet. Indeed, Venus has a surface so extraordinarily hot that it could melt lead, and because of this intense heat, the Venusian rocks emit an eerie red glow, similar to the coils of a toaster.
“If there is life on these planets, I would speculate that it has to be hardy life, because it has to be able to survive some potentially dire scenarios for billions of years,” Dr. Burgasser commented in the August 11, 2017 JPL Press Release.
The good news is that low-mass stars like cool little TRAPPIST-1 have brightness and temperatures that are relatively constant over trillions of years–and they only occasionally shoot out a destructive magnetic flare. Small stars lead long, peaceful lives, and take their time burning their necessary supply of hydrogen fuel. In dramatic contrast, massive stars live fast and die young, because they have hastily burned their necessary supply of fuel. Indeed, many of the most massive stellar inhabitants of the Universe live only for millions of years. Even though our Sun is a small star, it is more massive than tiny, cool dwarf stars, like TRAPPIST-1. Our Sun will “live” to the ripe old age of approximately 10 billion years, and it is currently in mid-life at 4.56 billion years of age. Our still active Star has approximately 5 billion years to go before its lights go out. But little TRAPPIST-1, and stars of similar mass, are predicted to have lifetimes that are much, much longer than our almost 14 billion year old Universe.
“Stars much more massive than the Sun consume their fuel quickly, brightening over millions of years and exploding as supernovae. But TRAPPIST-1 is like a slow-burning candle that will shine for about 900 times longer than the current age of the Universe,” Dr. Mamajek explained in the August 11, 2017 JPL Press Release.
Some of the information that Dr. Burgasser and Dr. Mamajek used to calculate the age of TRAPPIST-1 includes the rate that the star is traveling in its orbit around our Milky Way Galaxy. This is because stars that travel more quickly tend to be elderly. Another clue that Dr. Burgasser and Dr. Mamajek took into consideration is the composition of the tiny star’s atmosphere. The two astronomers also counted the number of flares TRAPPIST-1 shot out during their observational periods. These variables all indicate that the star is quite a bit older than our Sun.
Future observations with NASA’S Hubble Space Telescope and the upcoming James Webb Space Telescope may show whether or not these seven alien worlds possess atmospheres, and whether these atmospheres are like Earth’s.
Additional observations with Spitzer could help astronomers refine their estimates of the TRAPPIST-1 planets’ densities, which would help them gain a better understanding of their compositions.
Dr. Tiffany Kataria, an exoplanet scientist at JPL, who was not involved in the study, noted in the August 11, 2017 JPL Press Release that “These new results provide useful context for future observations of the TRAPPIST-1 planets, which could give us great insight into how planetary atmospheres form and evolve, and persist or not.”