An artist’s conception of our solar system. Image Source: NASA Science
As my freshman year of college draws to a bitter sweet and slightly chaotic close, I’ve been doing some major thinking about about the events of the past school year. I fell in love with astronomy from the very first lecture I attended, way back in August of last year. While my first semester gave me a basic understanding of stars and galaxies in ASTR-1010, this semester taught me about the solar system. I came into this course with a good deal of astronomical knowledge already; however, I could never have imagined how much more I would learn about this fascinating field. I absolutely loved taking such an in-depth look at each of the planets, the Sun, and the solar system as a whole. The study of our own star system gave way to the examination of other solar systems, and then we got into questions that will bring on existential crises like “What is life?” and “Where are the aliens?” This class has encouraged me to look at the universe in whole new ways. I’ve learned that the space between cosmic objects is almost unfathomable. I’ve learned why Venus rotates in the wrong direction and that my zodiac sign will be completely different in a couple thousand years or so. I’ve learned that while we haven’t met any aliens yet, other life forms may exist somewhere in our own solar system. The list goes on and on.
Astronomy is a complex science; while we have figured out so much about our universe already, there is still work to be done. I hope that one day, we might be able to answer some of humanity’s most burning questions.
To this day, one of humanity’s most loaded questions remains unanswered: are we alone in this universe? Though we have yet to detect the presence of any extraterrestrial civilizations, that has not stopped humanity from attempting to make contact with whatever else might be out there.
Globular Cluster Messier 13. Image Source: Wikipedia
The most famous attempt to communicate humanity’s existence to the cosmos was a 3-minute binary transmission made in 1974 from Puerto Rico, a United States territory in the Caribbean. The broadcast was made using a planetary radar transmitter on the Arecibo radio telescope, and it was directed towards a globular cluster called Messier 13 (M13). The cluster contains about a third of a million stars, and the message’s senders hoped that an interstellar civilization ready to accept our transmission might be among these many star systems. However, M13 is near the edge of the Milky Way and lies at a whopping distance of about 25,000 light years. This means that it will take around 25,000 years for our message to actually reach the cluster, never mind the time it might take for us to actually receive a reply.
The Arecibo message encoded. Image Source: SETI Institute
The Arecibo message was the most powerful transmission ever purposefully beamed into space; it was equivalent in strength to a 20 trillion watt broadcast in all directions. The message includes 1679 bits that are meant to be arranged into a rectangular grid of 73 rows and 23 columns. These numbers were chosen because they are both prime, and presumbably, the extraterrestrials would recognize the significance. Once the aliens deciphered the message, they would have discovered the above image (though it carried no color information). Among other things, the picture represents DNA structure, a human stick figure, our solar system, the Arecibo radio dish, and eight biochemicals that define life on Earth.
Though the chances of receiving a reply to the Arecibo message are very slim, the project allowed us to more fully understand the complexities of communicating across time, space, and cultures.
Recently, I have started work as an undergraduate research assistant in the Physics and Astronomy department at my university. The project I was assigned to is of a stellar nature; we are attempting to find evidence of extrasolar planets, or planets around other stars. Though we haven’t found any planets yet, I keep thinking about how crucial this type of work is to the future of the human race. Around 4,000 extrasolar planets have been confirmed by NASA, and it is amazing to think that one of these could be humanity’s next home. In this blog post, I will discuss the major approaches that astronomers use to detect planets outside of our solar system.
The Astrometric Method: Using the astrometric method, we can detect extrasolar planets by carefully monitoring small changes in the host star’s position in the sky. Planets orbit their host stars because of the enormous amount of gravitational force exerted on them by their stars. However, the laws of physics tell us that the planets also exert an equal amount of gravitational force on their host star. Though the star is much more massive, the planets can cause a star to “wobble” as it is gravitationally tugged at. Very few extrasolar planets have been identified using this method; the star studied must be relatively near to us, and observing positional changes may require long periods of observation.
The Doppler Method: Like the astrometric method, the Doppler method relies on the gravitational interactions and “tugs” between a host star and its planets. When a star is moving towards or away from us, we say that its light spectrum is blueshifted or redshifted, respectively. Alternating blueshifts and redshifts relative to average Doppler Shift can indicate a star’s motion or “wobble” due to its interactions with a planet or planets. The Doppler method has discovered many more extrasolar planets than the astrometric method, but it certainly does not come without its limitations. This technique is best suited for massive planets with close orbits, and because it calls for stellar spectra, large telescopes and long periods of observation are a must.
The Transit Method: When an extrasolar planet’s orbital plane is situated along our line of sight, the planet will appear to travel in front of its star once every orbit. Astronomers call this a transit event. Similarly, half an orbit later, the planet will become eclipsed by its host star as it passes behind. We can detect these events by monitoring changes in the star system’s brightness. During a transit event, the star undergoes a characteristic dip in brightness as the planet blocks some of its light. During an eclipse, there is a dip in the system’s infrared brightness as the star blocks the infrared light emanating from the planet. Most extrasolar planets have been discovered using this method, but it is only feasible for planets with orbital paths that are oriented in just the right way.
Direct Detection: This technique involves acquiring images or spectra of the planet. In theory, this is the best way to learn about an extrasolar planet; however, our current technology can only produce images with low resolution. There are other obstacles to directly detecting planets as well. For one, stars give off much more light than any planets that might be orbiting them. Because of diffraction, our telescopes also blur star light. Most extrasolar planets are too close to their bright host stars to be imaged directly from our vantage point, light years away. Second, compared to massive stars and the colossal distances between them, planets are miniscule.
An infrared image of a brown dwarf and an extrasolar planet (bottom left corner) located about 230 light years from Earth. Image Source: Slate
In his hit song “Rocket Man”, musical legend Elton John aptly remarks that “Mars ain’t the kind of place to raise your kids / In fact it’s cold as hell.” The average surface temperature of Mars is 220 Kelvin, or about -64 degrees Fahrenheit. Now, I’m not sure about hell, but that’s definitely too cold to be raising any kids.
For centuries, us humans have wondered about our next-door neighbor in the solar system: the red planet of Mars. Mars’ semi-major axis is about 1.5 times that of Earth, and Mars’ radius is only slightly more than half the size of Earth’s radius. Mars is the last of the four inner planets and the five terrestrial worlds; however, Earth is ten times more massive than Mars.
Mars is a truly fascinating world; from humongous volcanoes to polar caps of dry ice, the planet is a wonder to behold. Mars is too cold for liquid water today, but we have found evidence to suggest that Mars was a wet planet at some point in its history. Even if water was still around, human beings could not survive on Mars without a spacesuit due to low air pressure, freezing temperatures, little breathable oxygen, and the lack of an atmospheric ozone layer.
Though no human has step foot on Mars (yet), we have “visited” our neighboring planet with help from exploration rovers. There have been four successful rovers; in fact, the Opportunity mission was only recently terminated after working for 15 years and covering a total distance of just over 25 miles. As of now, the Curiosity rover is still active, and NASA is planning to launch a new mission in 2020.
Though we do not currently have the means to see directly inside the Earth (or any other planet), we can use clues to make inferences about what may be lying beneath their surfaces. On Earth and the Moon, our most helpful data stems from the analysis of seismic waves, or vibrations that travel along the world’s surface and through its interior after earthquakes. For other terrestrial worlds, we can use other measurements like average density and gravity to determine the distribution of mass in the world’s interior. In this blog post, I will be discussing the three major layers that are present inside the terrestrial worlds: the core, the mantle, and the crust.
The Core: The innermost layer of the terrestrial worlds also has the highest density. It is primarily composed of metals, including iron and nickel. Mercury has a very large core of iron that comprises around 85 percent of its interior. The cores of Earth and Venus are made up of a solid, inner core and a molten outer core. Tectonic activity is also caused by heat in the world’s core.
The Mantle: Thick, rocky, moderate-density mantles surround the cores of the terrestrial planets. They are composed of mostly minerals that contain oxygen, silicon, and other elements. With the exception of Mercury, the mantle makes up a large portion of a terrestrial world’s volume; Earth’s mantle makes up 84 percent of the planet’s total volume.
The Crust: The terrestrial planets have thin crusts composed of low-density rock that make up their outermost layer. The Earth’s crust contains a great assortment of metamorphic, sedimentary, and igneous rocks; however, it makes up less than 1 percent of the planet’s total volume. The crusts of the terrestrial planets were formed through various igneous processes, and they frequently change due to erosion, sedimentation, volcanism, and cratering.
This gif from DeLeoScience illustrates how the orbits of Earth and Mars around the Sun appear to make Mars move in a retrograde motion across our celestial sphere.
Over a single night, the planets behave much like the stars; they appear to rise in the east and set in the west. However, over the course of many nights, one will recognize that the movement of planets among the stars is quite intricate. The speeds and brightnesses of the planets fluctuate significantly, and while they typically travel eastward through the zodiac, they will periodically reverse course and move westward through the stellar background. This phenomenon is called apparent retrograde motion, and these periods can last anywhere from a few weeks to a few months.
For ancient astronomers who believed in a geocentric universe, this presented a problem. If planets supposedly moved in perfect circles around a stationary Earth, then what could be causing this peculiar backward motion? Greek astronomers like Ptolemy suggested that each planet traveled around Earth on a small circle, or epicycle, that simultaneously moved upon a larger circle, or deferent.
This animation from Kepler College depicts how a planet moving around Earth on an epicycle would show apparent retrograde motion.
Apparent retrograde motion can be explained much more simply with a heliocentric universe. Each of the planets orbits the Sun at a different rate; Mercury and Venus have shorter orbital periods than Earth since they are closer to the Sun, but Mars and the gas giants take a longer time to complete their revolutions. As the Earth passes or is passed by another planet in its orbit, the other planet appears to move back and forth relative to the stars in the distance. We know today that the heliocentric theory is the right one, but it would take almost 2,000 years from the time it was first suggested by Greek astronomer Aristarchus in 260 B.C. to be widely accepted. Nevertheless, the complexities of planetary motion would spur much of the debate over our planet’s place in the cosmos.
Nicholas Copernicus (February 19, 1473 – May 24, 1543) was a Polish scientist who mathematically calculated the details of a heliocentric, or sun-centered, solar system. He uncovered relationships that permitted him to calculate each planet’s orbital period and the distance from each planet to the sun in terms of the astronomical unit (AU), or the Earth-Sun distance. Copernicus also proposed that the Earth rotates on an axis, and this axis changes in direction very gradually to cause the precession of the equinoxes.
While Copernicus was making his discoveries, a lot was happening in the world. From 1508 to 1512, Michelangelo was painting the ceiling of the Sistine Chapel with several colorful and complex scenes from biblical scripture. On October 31, 1517, Martin Luther posted his “95 Theses” to denounce corrupt practices of the Catholic Church. This document sparked the Protestant Reformation. Italian Leonardo da Vinci (April 15, 1452 – May 2, 1519) was an artist, inventor, engineer, scientist, and architect whose work exemplified the humanist ideal of the Renaissance.
Having previously studied the cosmic calendar, it is already clear to me that on the grand scale of things, human civilization and scientific advancement has only comprised a short blip in the vast history of the universe. Copernicus was alive during the Renaissance, and it is very fascinating to see how other aspects of civilization were progressing alongside important astronomical discoveries. Around the same time that Copernicus was challenging the geocentric model, Martin Luther was denouncing the established church, and artists like Michelangelo and Leonardo da Vinci were creating masterpieces. It certainly was a time of great innovation and change.
An animation illustrating how the Earth precesses, or “wobbles,” on its axis. This gif was created by me using footage from a Youtube video published by Steven Sanders.
What if I told you that in a couple thousand years from now, your Zodiac sign would no longer be your Zodiac sign? It may be devastating to devout followers of astrology, but the relative positions of the Zodiac constellations are changing very, very slowly, at least from our viewpoint. This is due to a process called precession, the continuous, gradual wobble that changes Earth’s axial orientation in space. The Earth really is like a spinning top – just an extremely slow one. In fact, this top only makes a complete spin every 26,000 years.
Because Earth protrudes at its equator, the planet is not quite a perfect sphere. The equator is also tilted with respect to the ecliptic plane, and as a result, the gravitational attractions of the Sun and the Moon attempt to draw the equatorial bulge into the ecliptic plane. In simpler words, gravity from the Sun and the Moon tries to pull the Earth into straightness. However, because Earth tends to keep rotating, gravity fails to straighten out the Earth and instead causes the axis to precess.
It is not likely that we will see any major changes during our lifetimes, but the night sky will look a lot different thousands of years from now. The North Celestial Pole is pointed toward the star Polaris now, but in 3,000 BC, the North Star was actually Thuban, a star in the constellation Draco. In roughly 12,000 years, Vega, a star in the constellation Lyra, will be the new North Star. Precession also alters the points in Earth’s orbit at which equinoxes and solstices occur; this means that in 13,000 years, the seasons on Earth will have switched times of year.
An animation that illustrates how the North Celestial Pole (NCP) moves in relation to the North Ecliptic Pole (NEP) over a 26,000 year period. This gif was created by me using footage from a Youtube video published by Steven Sanders.
Our Vast Universe 
You must be logged in to post a comment.