To the stars: Solar Systems beyond our ownPosted on Oct 26, 2014
Originally Published October, 2014. Last updated 1/28/2016.
Table of Contents
- The discovery of new planet types
- The composition of other star systems
- Planets within their star’s habitable zone and the possibility of liquid water
In this essay, we present information about planets found outside of our own solar system. Our research touches on types of planets found, new types of solar system compositions that we have discovered. We also discuss planets that have been found that could contain liquid water. This essay’s aim is to expand upon our current knowledge of extrasolar planets (planets found outside of our solar system). Through space programs such as the Kepler program, we are able to find alien worlds and survey them. In the past two decades alone, scientists have been able to discovery over one hundred planets orbiting other stars.
As the late Carl Sagan (1980) once said, “the surface of the Earth is the shore of the cosmic ocean”. In the past two decades, pioneers in the space sciences community have been looking beyond our own galactic home – to view into other solar systems in a search for other planets. Research suggests that a variety of unfamiliar exoplanet types and solar systems will likely be discovered due to recent discoveries of new planet types, the identification of unexpected planets orbiting unlikely host stars, and newly found planets that orbit in other solar system’s habitable zones. The findings of scientists have been fantastic, once unimaginable planets and solar systems. They have found worlds that orbit closer to their stars than Mercury orbits our own sol. We have found rocky planets larger than any in our own solar system. We have even found possible hope for finding life outside of our solar system by finding planets where liquid water theoretically could exist.
2. The discovery of new planet types
Scientists have often wondered if planets are common in the universe;. Until the 1980s, our telescopes were not capable of detecting extrasolar planets. Exoplanets – planets that are beyond our solar system – are detected by measuring the light from a remote star. As a planet orbits (transits) around their host star, its gravity “tugs” on the star. Scientists then record stars for a long period of time (months to years) through a telescope. Next, we analyze the data that we record; this allows us to detect repeating gravitational anomalies that a planet exerts on a star. Each time an anomaly is detected it is representative of one orbit.
Our solar system is composed of eight planets – two Jovian gas giants (the gas giants Jupiter and Saturn), four rocky worlds (Mercury, Venus, Earth, and Mars), and two ice giants (Neptune and Uranus). Before we discovered exoplanets scientists were unsure if planets regularly formed around other stars. To our surprise, we have discovered hundreds of planets in just two decades worth of research. We were also unsure of what to results expect; many types of planets were theorized. Based on the results of scanning the night skies, the planets types that we have found are amazing. Scientists have discovered a variety of new planet types in the current sample (which contains 140 known exoplanets).
Out of the planet types we have found in our samples pool, we are starting to find enough that it is possible to begin categorizing them. Klahr. et al (2002) make a statement regarding the new types of planets we have discovered: “with more than 140 exoplanets known to date, some very interesting trends have already drawn attention in the past few years,” (p 132). Based on our observations, we have discovered that there are Chthonian planets (theorized to be the eroded core of gas giants), super Earths (rocky worlds with up to four times the mass of our own Earth), and “hot Jupiters” (gas giants that have a higher surface temperature than Saturn or Jupiter.
So called “Hot Jupiters” (also known as epistellar Jovians) seem to be abundant when searching for exoplanets. This could be attributed to the fact that they have such strong gravity that they remain within close orbit of their parent star. It could also due to a sampling bias – we do not yet have technology that could easily allows for us to find planets further from their parent stars (this should change with the upcoming launch of the James Webb Telescope). Regardless, we have found planets that orbit close enough to their stars that their atmospheres are nearly evaporating. Chthonian planets are theorized to be the remnants of these planets. Cole (2006) points out that data from NASA’s Spitzer Telescope has verified the existence of “hot Jupiters” and that “measurements confirm that each planet is indeed a hot Jupiter with surface temperatures of the order of 1,000 K” (p370).
It is believed that gas giants are common throughout the universe based on the sample data from our experiments. “For now, however, all that can be said is that bloated planets are not that unusual (Bakos, 2007)”. After sorting through our available data, many planets with one and a half times the radius of Earth (or more) appear to become gas giants. Based on our data it is theorized that any planet beyond that range forms into a gas giant. As Weiss et al note, “above 1.5 R (Earth), the average planet density rapidly decreases with increasing radius, indicating that these planets have a large fraction of volatiles (Weiss et al, 2014)”. There is good news for Earth-like planets, however. We have found what we suspect to be a multi-planet system orbiting around the star Gliese 581. Gliese 581d is notable in that as a planet, “Gliese 581d, can be considered the first confirmed exoplanet that could support Earth-like life, according to a team of scientists in France (CNRS, 2011).
3. The composition of other star systems
Many planets have been found orbiting host stars that we did not expect to be capable of containing planets. These solar systems are nothing like our own. Imagine a solar system where the only rocky planetoids orbit enormous gas giants. Our closest galactic neighbor – Alpha Centauri – is a binary star system with an orbiting red dwarf (creating an elongated triad of stars). The system is our closest galactic neighbor, it is still 4.39 light years away. It is hard to imagine how planets would form in such an unstable environment. While our own solar system is neat and organized (all of the planets orbit on near the same elliptical plane in our solar system), others star systems may not be. Extrasolar planetary systems are not mirrors of our own; instead we have found wild, unexpected star systems beyond our home.
With the “recent discovery of a planetary system with five Neptune-mass planets, as well as others including one mass of about 1.4 times that of Earth,” we are now able to fully realize how different alien solar systems are compared to our own (Committee on the Planetary Science Decadal Survey, 2003, p.175). We have found other planets similar in size to Venus and Earth; however they aren’t likely lush, watery worlds like our own planet. Some of them appear to have unstable planetary orbits, others orbit too close to their parent star to support life as we know it. Even though the Alpha Centauri contains “the first planet with a mass similar to Earth ever found around a star like the sun (Dumusque et al, 2012),” don’t get too excited: it is orbiting closer to its own star than Mercury is to our sun!
There are a few star systems that do look like our own. We have found similar planets to our own in configurations similar to our own solar system. Crowell points out it is possible that there are solar systems like ours; in fact “…similar to our own is Epsilon Eridani,” as it contains a Jupiter sized gas giant (Crowell, 2007). Unfortunately, smaller planets are harder to detect around their stars because of the smaller gravitational “tug” that they exert on their parent stars. It is difficult for current generation telescopes to detect planets further away from their stars. This currently limits us to what we can see; there could be entire worlds that we are missing in our surveys of other stars. Based on the small sample size that we have, it is likely that we will still find solar systems that continue to defy our expectations. Consider that “Kepler observed just 0.28 percent of the sky and the telescope was able to peer out to only 3000 light years away, studying less than 5 percent of the stars in its field of view (Becker, 2013)”.
4. Planets within their star’s habitable zone and the possibility of liquid water
Even when we discover rocky worlds (planets that might be as small as Mercury to planets with 4 times the mass as Earth) they may not have suitable conditions for life as we know it. That life requires liquid water; for liquid water to exist on the surface of a planet, the planet must orbit close enough to the star to maintain liquid water on the surface of the planet. If you look at our own solar system, Mercury and the Moon both have ice in craters that sunlight never reaches that is locked away inside of ice. Mars’ northern ice cap is composed of liquid water (the southern ice cap contains dry ice). Europa and Enceladus – moons of the great Jovian planets in our solar system – have plumes of water jetting from their frozen surfaces into space; water is trapped underneath ice on these worlds and is spraying into space through what could be oceanic vents.
It is easy to classify types of worlds that could be habitable based on the planets found in our own solar system. There are “garden worlds” – worlds capable of maintaining liquid water on their surface at all times (like Earth). Ocean worlds are another possibility – worlds where either surface water freezes and allows for seas below the surface (such as Europa and Enceladus). There are finally barren worlds – worlds that could hold liquid water given certain conditions (and where it is found in ice) like Mars.
In our search for planets in other solar systems, we have found a handful that theoretically could be capable of maintaining liquid water due to their orbits resting within their host star’s habitable zones. First, there are planets like Mars – planets that under the right conditions could host liquid water. They might require a thicker atmosphere, plate tectonics, or even a stable orbit – but it is possible based on our observations that they could harbor liquid water. This is important if we want to find life (or to find a suitable planet to migrate to in the future of our species). We have also found planets that could be similar to our own Earth if geological conditions are right. “The geophysics of GJ581d is unknown, but if a similar mechanism were present there, its atmospheric CO2 would stabilize above the level needed to maintain a liquid water cycle by negative feedback (Wordsworth, 2011)”.
It is also possible that planets may only have a window where they can support liquid water. Perhaps as their star begins to heat up and expand, the planet begins drying up and the water on the world evaporates away. Or perhaps the gravitational tug of another planet displaces it further away from its star, freezing its surface. Based on these possibilities, it is possible that some planets in their habitable zones may have a shorter life span than our own Earth’s for liquid water (and life) to exist. “Compared to larger and more massive CO2-rich planets which may evolve to class II type habitable planets, smaller size planets like Mars cool down faster and water may condense earlier compared to an Earth-size and mass planet (Lammer, 2009)”. Looking at our own solar system, it is theorized that Venus may have once had liquid water (as it is in Sol, our star’s, habitable zone). Some speculate that it evaporated and caused the runaway greenhouse effect that is evident today.
In just a few decades, the science community has gone from knowing nothing about extrasolar planets to realizing how abundant planets are in our own galaxy. We have found that there are solar systems that contain more ice giants (like Neptune and Uranus) than our own. We know that there are planets as large as Jupiter orbiting so close to their host stars that their atmospheres are close to evaporating away. We even know that there is a chance that a few planets could support liquid water – a requirement for the only known types of life in the universe. Our research into the unknown has allowed us to find solar systems that are nothing like our own, planets that orbit within their star’s habitable zones, and discovered planet types that we never knew existed. As scientific techniques (and technology) improve over the years to come, we are sure to find even more amazing things in other star systems.
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Cole, George H. A. (2006). Wandering Stars: About Planets and Exo-Planets : An Introductory Notebook. London, GBR: Imperial College Press, 2006.
Klahr, Hubert, Brandner, Wolfgang, Jakosky, Bruce. Planet Formation: Observations, Experiments, and Theory. West Nyack, NY: Cambridge University Press, 2002.
Committee on the Planetary Science Decadal Survey. Vision and Voyages for Planetary Science in the Decade 2013-2022. Washington, DC: National Academies Press, 2003.
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