Direct Imaging: Up Close and Personal with Extrasolar Planets (Kind of)

Ever wonder how scientists know about the existence of so many planets out there in the galaxy? Well they use a combination of indirect and direct methods, but the method that allows the scientists to actually capture an image of the planet is called Direct Imaging.

When you're dealing with stars and planets thousands of lightyears away from us, you might think that it's impossible to snap a picture. Even if a camera could reach such massive distances, the bright rays from the orbited star would block out the light waves coming from any planet around it. It was this same problem that made Direct Imaging so seemingly impossible for so long. 

It wasn't until 2004 that there was really a true breakthrough in the pursuit of getting actual images of planets. A group of scientists in Europe using the Very Large Telescope Array were able to get what many believe to be the first direct image of an extrasolar planet. The planet, which is many times larger than Jupiter, orbits a brown dwarf star much dimmer than our own Sun. This small detail of brightness may seem small but its implications paved the way for the method of Direct Imaging most commonly used today (The Planetary Society). 

This idea of trying to detect planets around dimmer stars was strengthened further when a group of astronomers were able to get a direct image of another planet in 2008. The planet was found to be orbiting the star called Fomalhout which is surrounded by a veil of gas and dust. The reason they were able to get a direct image is because the light from the star was partially blocked out (The Planetary Society).

Nowadays, advances in technology have allowed astronomers to manipulate the amount of light received by telescopes from bright stars. This makes the task of getting direct images of extrasolar planets easier. The reduced light can either be blocked out further or just completely removed from the image. This process, while massively further advanced than before, still works best with the largest, and brightest planets, or planets that are quite far from the star in their orbit. This last detail makes Direct Imaging very special, because all of the indirect methods of extrasolar planet detection work better with planets that are close to their stars. This sets Direct Detection apart and makes it an invaluable method for the future of extrasolar planet detection. But that's not the only reason that Direct Ditection is important. We can get information from Direct Detection that is impossible to attain through the indirect methods. A spectrum can only be gotten through the Direct Imaging method, so if we want to continue to search for planets with similar chemical makeups as our own, we must continue using Direct Imaging. The idea of using the light of the planet is also crucial in getting an accurate measurement of the temperature of a planet, using the emission of light that falls under infrared on the spectrum. 

Direct Imaging has advanced the possibilities of exploring our galaxy to new heights, heights that would have seemed impossible just a decade ago. Looking forward, into trying to get more Direct Images of planetary systems, the leader in the industry and the next big leap is the Gemini Planet Imager. The new set of optics was added to the Gemini Telescope located in the Andes in 2013, and allows the telescope to obtain Direct Images of planets that are ten million times fainter than the star they are orbitting (gpi Exoplanet Survey). A huge leap for the technology. Below is a planetary system found by Gemini in its first year of use.

(gpi Extrasolar Survey)

Sources:
http://planetimager.org/about/#1440146237271-4e488616-b6fd
http://www.planetary.org/explore/space-topics/exoplanets/direct-imaging.html

The Transit Technique-- Why It's Important!

With the near-daily announcements of newly discovered planets, one may wonder: How do astronomers find new planets? Intuitively it may seem that planet detection is simply a matter of viewing distant stars and trying to observe orbiting objects around them. And while there are processes that require actually seeing the planet to classify it-- known as direct detection methods-- there are other means of detecting planets where astronomers infer the existence of a planet by monitoring other features of distant stars.

One of these indirect methods is know as the Planetary Transit Technique. This process generally consists of observing slight changes in a star's brightness and determining whether or not the those changes were caused by the motion of an orbiting planet. In order to execute this process, one must set up a detection system that takes a spectrum of the star during the transit--when the planet is in front of the star--and again when the star is not transiting. By comparing the two spectra, one can discover whether or not certain elements are present during the transit that are not present when the star is not in transit. If such elements are discovered, then the change in brightness is likely caused by a planet in orbit.

Shifts in the star’s light emission are reflected in the following graph, with relative brightness on the y-axis and time on the x-axis. The visible dip in the brightness of the star results in a trough that shows when the planet is blocking light from the object.

Source: The Essential Perspective pg. 265 (1)

It is important to note that not all dips in the light received from a distant star are the result of a planet in orbit. Stars experience what are called “sunspots,” temporary dark spots on the surface of the star, which also block the light received from the star. This is why the periodicity of the light fluctuations are important. If the change in recorded light is a single occurrence and is not observed again, then the change in light was likely the result of a sunspot. However, if there is a pattern to the changes in relative brightness, then that is likely explained by the presence of a planet in orbit.  

Other than providing us with the data needed to determine whether or not a slight light shift is the result of a planet, the transit method can provide us with some fairly important information about the planet-- if, of course, it is determined to be such.

For instance, the transit method can provide us the planet’s size, orbital period, and distance from Earth. And using the Kepler’s 3rd law, one can determine the star’s distance from its parent star, thereby giving us critical information as to whether or not the planet may be suitable for life. Compared to other methods, what is unique about the transit technique is that it is the only extrasolar detection method that can provide us with the size, or radius, of the planet.

There are, however, drawbacks to the transit technique. Most notably, in order for the planet to be detected, the relevant solar systems need to be aligned in such a way that one or more of the star’s planets need to pass between us and the parent star. Only about 1% of known systems are aligned in this way. (2) Therefore the transit technique is vastly limited in the number of systems it can analyze. Luckily, 1% of the solar systems in the universe is still a large number, so the transit technique is still widely used!

In fact, the transit technique is the primary method used by the Kepler telescope, which has contributed immensely in the search for Earth-sized planets. The number of detected planets has increased drastically with the addition of Kepler, and the transit method is instrumental in the satellite's process. And if you’re curious to see some of the fascinating data and amazing images from Kepler, visit https://kepler.nasa.gov/index.cfm

Lastly, the stars that Kepler searches for are tremendous in size when compared to their orbiting planets. Without the advanced technology of telescopes like Kepler, it is quite difficult to discern a planet in transit. See for yourself! The following image is of a transit of Mercury. Can you find the planet?

Mercury in Transit

Source: Colby’s 8″ Schmidt-Cassegrain telescopes. (3)

That's right! It's at the bottom right of the Sun!

If you enjoyed that and want to contribute to the planetary detection cause, there are many ways that we can put on our lab coats and be citizen-scientists. Visit planethunters.org to help discover more exoplanets!!

Citations: (1) Bennett, Donahue, Schneider, and Voit. The Essential Cosmic Perspective. Pearson Education, 2015: pg. 265. (2) Ibid.  

(3) Colby’s 8″ Schmidt-Cassegrain telescopes. Mercury in Transit.   5/19/16 http://www.colby.edu/physicsastronomy/astronomy-events/

Detection of Exoplanets using the Radial Velocity Method

In 1995, a team of Swiss astronomers detected the first known exoplanet orbiting a main sequence star, 51 Pegasi (previously a planet was discovered orbiting a pulsar, but that's a very different system). The method with which they detected this planet is called the Doppler Technique or the Radial Velocity Method. This technique combines gravity, Newton's 3rd Law, and the Doppler effect to provide information about exoplanets.

First of all we need to think about the gravitational forces acting on a star and it's planet. We all know that the sun exerts a gravitational pull on the Earth, which keeps it in orbit, but we often forget that the Earth also exerts a force on the sun due to Newton's third law. This force is modeled by the equation:

,

where G is the gravitational constant, m1 and m2 are the masses of the star and planet (order does not matter), and r is the distance from the star to the planet.

Using Newton's second law, we can remember that forces cause accelerations (F=ma). This means that stars are pulled around by the planets that orbit them! Stars orbit around the center of mass of their solar system, which usually is actually within the radius of the star itself. You can see what this would look like for a one planet solar system below.

(Wikimedia Commons)

There are 2 detection methods that result from this motion of the star one is the Astrometric Method, which directly measures the change in position of a star over time, the other is the Radial Velocity Method.

The Radial Velocity Method uses the Doppler effect to measure the velocity at which a star is moving over the course of its orbit. As per the Doppler effect, when the Star is moving towards us, the light waves appear compressed and thus more blue. When the star is moving away, the light it emits appears to shift to longer (more red) wavelengths.

(Wikimedia Commons)

The video below shows the absorption lines from a star shifting to blue as the star moves towards the observer, then to red as the star moves away.

http://www.eso.org/public/videos/eso1035g/

Now that we have the change in wavelength, we can use the following equation to determine the actual velocity of the star:

,

where vrad is the radial velocity of the star, c is the speed of light (3.00 x 10^8 m/s), λshift is the wavelength at a given time, and λemit is the wavelength emitted at no radial velocity.

Plotting the velocity over time produces a sine wave where the amplitude is the maximum radial velocity.

What if there are multiple planets?

This is where it gets really intricate, graphically. The graph becomes sine waves on sine waves...

This would be the velocity of a star over time in a hypothetical 3 planet system...Wanna see a 10 planet system?

As you can see, it gets very messy very quickly...

The radial velocity method can get us an approximate mass of the planet because the larger the planet, the larger the amplitude. Additionally, we can use the period of the sine wave to find the period of the planet. Then, using Newton's variation of Kepler's third law, we can determine the distance at which the planet orbits. If you combine this information with the radius obtained through the transit method, you can obtain the density of the planet, which provides some insight to the planet's composition.

The downside of the radial velocity method is that you can only get mass, period, and average orbital distance, which tell you very little about the planet by themselves. To get any substantial information, you need to combine this with the transit method.  Additionally, the assumption must be made that we are observing the doppler shift in the plane of the planets orbit. The chances are, we are actually looking at an inclination to the orbital plane, and therefore can only calculate a minimum planet mass.

While this method was once only good for finding "Hot Jupiters" (massive planets orbiting very close to their stars), it has been refined and is starting to find planets closer to Earth's mass. In fact, our satellites are now picking up changes in a star's velocity of as little as 50 cm/s! This is less than the maximum speed of a mosquito! As we increase the precision of our instruments, we will be able to find smaller planets at greater distances from stars.

Solar System Formation: The Frost Line and Composition of Planets

Solar System Formation: 

The Frost Line and Composition of Planets 

(picture credit to GnosticWarrior.com)

It is crucial to remember that “we are all star stuff!” In other words, the planets form as a result of galactic recycling.  Galactic recycling is the phenomena where the elements that form our planets are created within starts and recycled through space as the stars shed their layers.

As a result of galactic recycling, we know that all the elements that makeup our planets can be found within the Sun.

To understand the formation of our planets, we must go back to the very beginning!  During the formation of our solar system as the solar nebula collapsed, gravity was able to pull together the material to create the Sun, however, the other planets were not immediately formed.   Instead, seed formation occurred as a result of condensation out of the nebular gas. 

For those who don't know what the solar nebula is, check out Katie's blog accessed here --> 

In short, the solar nebula is the spinning molecular cloud in which our solar system formed.

However, the condensation of different materials depends on temperature.  The materials that make up our solar nebula can be placed into four broad categories:

1)   Hydrogen and Helium Gas: these gases do not condense in the solar nebula

2)   Hydrogen Compounds: these compounds can condense into ices below 150 K

3)   Rock: these turn from gas form into solid rock between 500 K and 1300 K.

4)   Metals: these turn from gas form into solid metals between 1000 K and 1600 K

Based on these differing temperatures needed for condensation, we know that distance from the Sun, which determines temperature, must be the deciding factor.  Here is where the frost line becomes crucial.

(picture credit to Pearson Education and Addison Wesley, accessed through www.cfa.harvard.edu)

What is the frost line?

The frost line is the boundary where ices can freeze.  It is the divider between the warmer inner region of our solar system, closest to the Sun where terrestrial planets form and the cooler regions further away from the Sun where jovian planets form.

The Formation of Planets:

Within the frost line bits of metal and rock began to condense into "seeds" which were small solid particles that acted as a building piece for gravity to build planets around.  Outside of the frost line "seeds" began to form from metal, rock and ice.  The formation of these seeds is the basis for the forming of the two types of planets.  

The Formation of Terrestrial Planets:

The seeds made up of metal and rock developed into planets through accretion.  Accretion is the process where the seeds began joining together as a result of electrostatic forces, and once they were large enough, their gravitational pull towards one another joined the pieces into large rocks which can be called planetesimals.  Once the planetesimals reached a certain large size, more growth was rare as collisions would just lead to rocky pieces breaking off.  From those planetesimals that did survive the collisions, terrestrial planets formed.  However, terrestrial planets tend to be quite small as they have less material to be built from since ice cannot form within the frost line.  This smaller size leads to less gravitational pull, therefore, less ability to capture gases and therefore, less frequent formation of moons.

The Formation of Jovian Planets:

A similar process occurred to form icy planetesimals, however, from there, the biggest of those icy planetesimals were so massive that their gravity could capture increasing amounts of helium and hydrogen gas.  These two gases were extremely plentiful.  Eventually, these gaseous planets had so much gas that their disks of gas ended up spinning, heating and flattening like the solar nebula which resulted in accretion of moons from the icy planetesimals that were already present earlier. 

References: "The Essential Cosmic Perspective" 7th Edition

Light: Properties of Thermal Radiation

           We all know that a light bulb emits visible light. However, did you know that a light bulb also produces invisible light that still falls under the category of radiation? When one looks at a light bulb, or the sun, they observe a blinding white light, which in reality is the superposition of the seven colors of the rainbow.  In 1800, Sir William Herschel used Newton’s research and discovery of the visible light spectrum to test for the influence of light on temperature. In doing so, Herschel discovered that not only does light carry warming energy, but it also emits radiation in both visible spectrum forms and invisible infrared and ultraviolet forms. By determining that light emits radiation scientists were able to further expand their knowledge of energy and light and did so by analyzing wavelengths.

Thermal radiation can be defined as the electromagnetic energy emitted by a surface. This radiation travels in all directions and continues to travel until it reaches its point of absorption. To fully grasp the concept of thermal radiation one must understand electromagnetic energy which is when electric and magnetic fields, making up electromagnetic waves, flow through space at the speed of light until they are absorbed when the energy of these electromagnetic waves are transferred to the matter that they pass through. Thermal radiation is measured in wavelengths and lives within the light spectrum defined by Newton and evolved by Herschel. 

            Radiation is measured in terms of wavelengths (See:  Let’s Shine A Little Light!). Colors are then defined by their corresponding wavelength. For example infrared (invisible light at the exterior of the red portion of the visible spectrum) has a wavelength of approximately 0.001mm, red light has a wavelength of 0.0007mm, blue light has a wavelength of 0.0004 mm, and ultraviolet light (invisible light at the exterior of the blue portion of the visible spectrum) has a wavelength that is far too short to detect with the naked eye. Based on these numbers and our knowledge of intensities linked to color, one can note that the intensity of the radiation increases significantly as the wavelengths shorten. These wavelengths correspond with the energy of photons and the energy level of individual radiative objects impacts the color of these objects. Higher energy concentrations per surface area result in higher temperatures, than lower energy concentrations within the same surface area. Principally, regardless of the surface area, hotter object appear bluer than cooler objects, which appear red.

 

 https://upload.wikimedia.org/wikipedia/commons/1/19/Black_body.svg

            After having fully examined the facts regarding radiation and its link to light and wavelengths, we turn our attention to four principles of thermal radiation. 1) The thermal radiation emitted by an object at any temperature consists of a wide range of frequencies. This rule means that the frequency or intensity of an object is distributed throughout the visible spectrum, which is referred to in Planck’s Law of Black-Body Radiation. 2) The dominant frequency range of the emitted radiation shifts to higher frequencies as the temperature of the emitter increases. Wein’s Displacement Law can explain this phenomenon and argues that the larger the kinetic energy the shorter the peak wavelengths. This principal is based on the fact that all atoms and molecules remain in a constant motion and that dense objects, or blackbody objects, constantly emit thermal radiation. 3) The total amount of radiation of all frequencies increases as the temperature rises. Stefan-Boltzmann’s Law proves this principal and shows that the total radiative density of a blackbody rises to the fourth power of the absolute temperature. This shows that radiative density and the objects temperature are proportional to each other. 4) The rate of electromagnetic radiation emitted at a given frequency is proportional to the amount of potential absorption of a source. This means that objects that emit high frequencies also absorb high frequencies of energy and vise versa. Blackbody sources, such as the sun, are objects that absorb all radiant energy and also emit radiation at all wavelengths. Since blackbody objects can emit all wavelengths they tend to emit the highest intensity radiative waves and thus shine the brightest.

https://upload.wikimedia.org/wikipedia/commons/a/a2/Wiens_law.svg

     https://upload.wikimedia.org/wikipedia/commons/b/b0/Emissive_Power.svg

These aforementioned four properties of thermal radiation help to describe influences of temperature, energy, and size on radiation. Light radiation is measured by wavelengths in terms of the light spectrum.

Sources:

http://www.teachastronomy.com/astropedia/article/Thermal-Radiation

http://www.kidsgeo.com/geography-for-kids/0062-radiation.php

http://www.britannica.com/science/thermal-radiation

AAAAAAAAAAAAEEEEEEOOOOOOOOO

Have you ever wondered why the note of a train's horn moving past you goes from sharp to flat, like a pubescent choir boy struggling with a plagal cadence? (Sorry Ms. Pecko, I should have stuck to the drums)

Turns out, it has a much more succinct name: The Doppler effect.

Neat! So, what is the Doppler effect exactly?
Here's how Sheldon from TBBT explains it:


(eerie when it stops before the canned laughter, eh?)

"It's the apparent change in the frequency of a wave caused by relative motion between the source of the wave and the observer."

Those were a lot of words in 6 seconds. Let's break it down by going back to the train example. 

Imagine standing at a railroad crossing and listening to the train's horn. As the train gets closer to you, the pitch of the horn is higher. As it passes you and moves further away, the pitch becomes lower. From this, we can deduce that there is a relationship between the velocity of the wave source (with respect to you) and the frequency of the sound waves. 

This is the Doppler effect in a nutshell, and it applies to all waves. (Including light!)

     

Here are three really neat gifs that demonstrate the principle. (From Wikimedia Commons)

Referring to the three animations above, we can see that waves are compressed in front of a wave emitting object when it is in motion. In the opposite direction of the objects movement, we see that the pulses are spaced further apart, resulting in longer wavelengths. The faster an object is moving, the more compressed/elongated the emitted waves become. The rightmost image depicts an object breaking the sound-barrier, where all waves become bunched up in front of the object. This means that an observer would not hear any activity until the object passes them. 

This principle becomes especially useful when dealing with light. You see, light is wave too. On the longer end of our visible spectrum are the colors orange and red, while blue and violet sit on the shorter (higher energy) side of the spectrum. We can use this information to our advantage to determine whether something is moving towards or away from us, if we know what wavelength of light the object is supposed to be emitting. 

Two common terms used to describe such observations are redshift and blueshift.
As the following diagram demonstrates, blueshift occurs in the compressed area in the direction of motion of the light source. Redshift occurs in the elongated area following a light source in motion.

This nifty diagram shows the Doppler Effect in relation to light 

(From physics.ucr.edu - Jose Wudka)

In astronomy, the Doppler effect is extremely useful for finding the velocity of an object. If the frequency at the source is known, the observed frequency can be used to find the velocity of the source. 

We can use the following formula to calculate the speed at which something is moving if we know the resting wavelength and the observed wavelength of an object.

where lambda represents the wavelength emitted by the object

& c is the speed of light. 

Gravity

Gravity

If you’ve ever wondered why your back hurts after a long car ride, why we see phases of the moon, or how our earth stays intact the answer is that it has to do with gravity. When thinking of gravity, people often think of throwing a ball up and watching it fall. The idea that gravity is a force that brings objects “down” is a misconception. While gravity is the force that brings a basketball back down after a shot, it is, more specifically, the phenomenon that all objects with mass attract each other. The more mass an object has and the closer two objects are the stronger gravitational pull between them. When someone shoots a basketball it is attracted towards the center of the earth, and comes back down to the ground. In a system with two objects, both apply a force on each other. If a system contains a star and a pebble, the pebble still applies a gravitational force on the star, but the star moves so little that it is negligible. The forces are equal, but since the star is much more massive the acceleration induced by the force is small, so we don’t see it move.

  From: http://www.rri.wvu.edu/WebBook/Goetz/slides/figuresII/sld002.htm

When an object enters the gravitational pull of a relatively extremely massive object, the smaller object can orbit the larger one. An example of this is when an asteroid passes a nearby planet. It is possible the gravitational force towards the center of a planet alters the path of the asteroid so that it makes a full loop around the planet. As the distance between objects decreases the force of gravity becomes stronger. When the asteroid gets closer to the planet, the asteroid is attracted towards its center, and the result is that the asteroid orbits the planet. Although if the conditions are not right like the asteroid is moving too fast it will pass the planet. If the asteroid moves too slowly it will collide with the planet because the gravitational pull is too strong.

From: http://www.qrg.northwestern.edu/projects/vss/docs/space-environment/1-what-causes-an-orbit.html

The force of gravity effects the weight we measure with a scale. Weight is the force of gravity acting upon a scale, while mass is determined by the amount of mass in one’s body. This means that if one were to weigh themselves on the moon they would weigh significantly less because the moon’s gravity is significantly less than the earth’s. The person’s mass would remain unchanged on the moon.

From: https://www.emaze.com/@AIZFQCI/Gravity-and-Motion

Galileo famously hypothesized that if one drops an object of different weights on earth the acceleration of gravity on them is the same. This shocked many people because the common belief was that weight determined how fast an object fell. Legend has it that Galileo went to the top of the leaning tower of Pisa and dropped two balls, one much larger than the other, and they fell at the same rate. The acceleration of gravity on earth is 9.8 meters per second squared. On earth every second the force of gravity acts upon something it applies an acceleration of about 10 meters per second. The reason a feather takes longer to fall is because the air on earth acts as a force against it. On the moon where there is no air the feather and the brick fall at the same rate.

From: http://lannyland.blogspot.com/2012/12/10-famous-thought-experiments-that-just.html

Like Galileo, Isaac Newton further revolutionized the idea of gravity. Isaac Newton was said to be sitting under a tree when he watched an apple fall. At this moment he had an epiphany, that what causes an apple to fall from a tree, and the moon to orbit the earth is the same force. This was revolutionary because Newton introduced the idea that some physical laws that apply to us on earth apply across the universe. Newton’s theories were progressive, but later revised 250 years later by Albert Einstein. Isaac Newton believed that gravitational effects occurred instantly, and if the sun were to vaporize the earth would immediately stop its orbit around the sun. Einstein disproved this notion after his study his light. Einstein found that light acts as cosmic speed limit, and gravity does not take effect instantaneously. He hypothesized that if the sun were to vaporize, it would take eight seconds for any change in gravitational effects on the earth to occur. It is eight seconds because that is the time it takes for light to reach the earth from the sun.

Gravity is a unique property of our universe. The attraction between objects with mass shapes our universe into what it is today. This law causes orbits, the formation of planets and stars, ocean tides, and many more phenomenon we witness every day.