The Color of Habitable Worlds

“This is where we live. On a Blue Dot.” said Carl Sagan when the now famous Pale Blue Dot photo was released. Captured February 14, 1990 by the Voyager 1 Space Probe, Pale Blue Dot remains the most distant photograph of the Earth ever taken at 6 billion kilometers. This past February marked the 30th anniversary of Pale Blue Dot which was reprocessed using modern digital photo techniques creating an even more remarkable image.

This updated version of the iconic “Pale Blue Dot” image taken by the Voyager 1 spacecraft uses modern image-processing software and techniques to revisit the well-known Voyager view while attempting to respect the original data and intent of those who planned the images. Credit: NASA/JPL-Caltech

Whether Pale Blue Dot, or Blue Marble, our planet is associated with the color blue. As Earth is the only inhabited world we know of, it might stand to reason that other habitable planets in space will also be blue. But it’s a little more complicated than that.

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There’s No Chemical Difference Between Stars With or Without Planets

Strange New Worlds

Imagine if a star could tell you it had planets. That would be really helpful because finding planets orbiting distant stars – exoplanets – is hard. We found Neptune, the most distant planet in our own solar system, in 1846. But we didn’t have direct evidence of a planet around ANOTHER star until….1995.…149 years later. Think about that. Any science fiction you watched or read that was written before 1995 which depicted travel to exoplanets assumed that other planets even existed. Star Trek: The Next Generation aired its last season in 1994. We didn’t even know if Vulcan was out there. (Now we do!…sortof)

Jupiter (right bright point) and Saturn (left bright point) seen here against the Milky Way were the most distant planets we could see before inventing telescopes – C. Matthew Cimone
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Extremely Hot Exoplanets Can Have Extreme Weather, Like Clouds of Aluminum Oxide and Titanium Rain

Thanks to the success of the Kepler mission, we know that there are multitudes of exoplanets of a type called “Hot Jupiters.” These are gas giants that orbit so close to their stars that they reach extremely high temperatures. They also have exotic atmospheres, and those atmospheres contain a lot of strangeness, like clouds made of aluminum oxide, and titanium rain.

A team of astronomers has created a cloud atlas for Hot Jupiters, detailing which type of clouds and atmospheres we’ll see when we observe different Hot Jupiters.

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Astronomers Detect Water in the Atmosphere of a Planet 179 Light-Years Away

The HR 8799 system contains the first exoplanet be directly imaged. Image Credit: NRC-HIA/C. MAROIS/W. M. KECK OBSERVATORY

Gathering detailed information on exoplanets is extremely difficult. The light from their host star overwhelms the light from the exoplanet, making it difficult for telescopes to see them. But now a team using cutting-edge technology at the Keck Observatory has taken a big leap in exoplanet observation and has detected water in the atmosphere of a planet 179 light years away.

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The Power of the Wobble: Finding Exoplanets in the Shifting of Starlight

They say there’s more than one way to skin an interstellar cat, and in astronomy there’s more than one way to find alien exoplanets orbiting a distant star. With the recent shut-down of NASA’s prolific Kepler mission and its windfall of discoveries, it’s time to look towards the future, and towards alternatives.

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Astronomy Cast Ep. 502: No Touching: Determining Composition of Worlds Remotely

We’re not learning that the vast majority of potentially habitable worlds out there are actually icy moons like Europa and Enceladus. Good news, there are hundreds, if not thousands of times more of them than worlds like Earth. Bad news, they’re locked in ice. What have we learned about water worlds and their potential for habitability?

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Farthest Galaxy Ever Seen Viewed By Hubble Telescope

Since it was first launched in 1990, the Hubble Space Telescope has provided people all over the world with breathtaking views of the Universe. Using its high-tech suite of instruments, Hubble has helped resolve some long-standing problems in astronomy, and helped to raise new questions. And always, its operators have been pushing it to the limit, hoping to gaze farther and farther into the great beyond and see what’s lurking there.

And as NASA announced with a recent press release, using the HST, an international team of astronomers just shattered the cosmic distance record by measuring the farthest galaxy ever seen in the universe. In so doing, they have not only looked deeper into the cosmos than ever before, but deeper into it’s past. And what they have seen could tell us much about the early Universe and its formation.

Due to the effects of special relativity, astronomers know that when they are viewing objects in deep space, they are seeing them as they were millions or even billions of years ago. Ergo, an objects that is located 13.4 billions of light-years away will appear to us as it was 13.4 billion years ago, when its light first began to make the trip to our little corner of the Universe.

An international team of scientists has used the Hubble Space Telescope to spectroscopically confirm the farthest galaxy to date. Credits: NASA/ESA/B. Robertson (University of California, Santa Cruz)/A. Feild (STScI)
An international team of scientists has used the Hubble Space Telescope to spectroscopically confirm the farthest galaxy to date. Credits: NASA/ESA/B. Robertson (University of California, Santa Cruz)/A. Feild (STScI)

This is precisely what the team of astronomers witnessed when they gazed upon GN-z11, a distant galaxy located in the direction of the constellation of Ursa Major. With this one galaxy, the team of astronomers – which includes scientists from Yale University, the Space Telescope Science Institute (STScI), and the University of California – were able to see what a galaxy in our Universe looked like just 400 million years after the Big Bang.

Prior to this, the most distant galaxy ever viewed by astronomers was located 13.2 billion light years away. Using the same spectroscopic techniques, the Hubble team confirmed that GN-z11 was nearly 200 million light years more distant. This was a big surprise, as it took astronomers into a region of the Universe that was thought to be unreachable using the Hubble Space Telescope.

In fact, astronomers did not suspect that they would be able to probe this deep into space and time without using Spitzer, or until the deployment the James Webb Space Telescope – which is scheduled to launch in October 2018. As Pascal Oesch of Yale University, the principal investigator of the study, explained:

“We’ve taken a major step back in time, beyond what we’d ever expected to be able to do with Hubble. We see GN-z11 at a time when the universe was only three percent of its current age. Hubble and Spitzer are already reaching into Webb territory.”

The Hubble Space Telescope in 1997, after its first servicing mission. It's about 552 km (343m) above Earth. Image: NASA
The Hubble Space Telescope in 1997, after its first servicing mission. Credit: NASA

In addition, the findings also have some implications for previous distance estimates. In the past, astronomers had estimated the distance of GN-z11 by relying on Hubble and Spitzer’s color imaging techniques. This time, they relied on Hubble’s Wide Field Camera 3 to spectroscopically measure the galaxies redshift for the first time. In so doing, they determined that GN-z11 was farther way than they thought, which could mean that some particularly bright galaxies who’s distanced have been measured using Hubble could also be farther away.

The results also reveal surprising new clues about the nature of the very early universe. For starters, the Hubble images (combined with data from Spitzer) showed that GN-z11 is 25 times smaller than the Milky Way is today, and has just one percent of our galaxy’s mass in stars. At the same time, it is forming stars at a rate that is 20 times greater than that of our own galaxy.

As Garth Illingworth – one of the team’s investigator’s from the University of California, Santa Cruz – explained:

“It’s amazing that a galaxy so massive existed only 200 million to 300 million years after the very first stars started to form. It takes really fast growth, producing stars at a huge rate, to have formed a galaxy that is a billion solar masses so soon. This new record will likely stand until the launch of the James Webb Space Telescope.”

Last, but not least, they provide a tantalizing clue as to what future missions – like the James Webb Space Telescope – will be finding. Once deployed, astronomers will likely be looking ever farther into space, and farther into the past. With every step, we are closing in on seeing what the very first galaxies that formed in our Universe looked like.

Further Reading: NASA

How Do We Know How Old Everything Is?

We hear that rocks are a certain age, and stars are another age. And the Universe itself is 13.7 billion years old. But how do astronomers figure this out?

I know it’s impolite to ask, but, how old are you? And how do you know? And doesn’t comparing your drivers license to your beautiful and informative “Year In Space” calendar feel somewhat arbitrary? How do we know old how everything is when what we observe was around long before calendars, or the Earth, or even the stars?

Scientists have pondered about the age of things since the beginning of science. When did that rock formation appear? When did that dinosaur die? How long has the Earth been around? When did the Moon form? What about the Universe? How long has that party been going on? Can I drink this beer yet, or will I go blind? How long can Spam remain edible past its expiration date?

As with distance, scientists have developed a range of tools to measure the age of stuff in the Universe. From rocks, to stars, to the Universe itself. Just like distance, it works like a ladder, where certain tools work for the youngest objects, and other tools take over for middle aged stuff, and other tools help to date the most ancient.

Let’s start with the things you can actually get your hands on, like plants, rocks, dinosaur bones and meteorites. Scientists use a technique known as radiometric dating. The nuclear age taught us how to blow up stuff real good, but it also helped understand how elements transform from one element to another through radioactive decay.

For example, there’s a version of carbon, called carbon-14. If you started with a kilo of it, after about 5,730 years, half of it would have turned into carbon-12. And then by 5,730 more years, you’d have about ¼ carbon-14 and ¾ carbon-12.

A list of the elements with their corresponding visible light emission spectra. Image Credit: MIT Wavelength Tables, NIST Atomic Spectrum Database, umop.net
A list of the elements with their corresponding visible light emission spectra. Image Credit: MIT Wavelength Tables, NIST Atomic Spectrum Database, umop.net

This is known as an element’s half-life. And so, if you measure the ratio of carbon-12 to carbon-14 in a dead tree, for example, you can calculate how long ago it lived. Different elements work for different ages. Carbon-14 works for the last 50,000 years or so, while Uranium-238 has a half-life of 4.5 billion years, and will let you date the most ancient of rocks. But what about the stuff we can’t touch, like stars?

When you use a telescope to view a star, you can break up its light into different colors, like a rainbow. This is known as a star’s spectra, and if you look carefully, you can see black lines, or gaps, which correspond to certain elements. Since they can measure the ratios of different elements, astronomers can just look at a star to see how old it is. They can measure the ratio of uranium-238 to lead-206, and know how long that star has been around. How astronomers know the age of the Universe itself is one of my favorites, and we did a whole episode on this.

Artist's conception of Planck, a space observatory operated by the European Space Agency, and the cosmic microwave background. Credit: ESA and the Planck Collaboration - D. Ducros
Artist’s conception of Planck, a space observatory operated by the European Space Agency, and the cosmic microwave background. Credit: ESA and the Planck Collaboration – D. Ducros

The short answer is, they measure the wavelength of the Cosmic Microwave Background Radiation. Since they know this used to be visible light, and has been stretched out by the expansion of the Universe, they can extrapolate back from its current wavelength to what it was at the beginning of the Universe. This tells them the age is about 13.8 billion years. Radiometric dating was a revolution for science. It finally gave us a dependable method to calculate the age of anything and everything, and finally figure out how long everything has been around.

So, fan of our videos. How old are you? Tell us in the comments below.
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Spectroscopy: The Key to Humanity’s Future in Space

Imagine, if you would, a potential future for humanity… Imagine massive space-elevators lifting groups of men, women, and children skyward off Earth’s surface. These passengers are then loaded onto shuttles and ferried to the Moon where interstellar starships are docked, waiting to rocket to the stars. These humans are about to begin the greatest journey humanity has ever embarked upon, as they will be the first interstellar colonists to leave our home Solar System in order to begin populating other worlds around alien stars.

There are many things we must tackle first before we can make this type of science-fiction scene a reality. Obviously much faster methods of travel are needed, as well as some sort of incredible material that can serve to anchor the aforementioned space elevators. These are all scientific and engineering questions that humanity will need to overcome in the face of such a journey into the cosmos.

But there is one particular important feature that we can begin to tackle today: where do we point these starships? Towards which system of exoplanets are we to send our brave colonists?

Of all of the amazing things we need to discover or invent to make this scene a reality, discovering which worlds to aim our ships at is something that is actually being worked on today.

Artistic view of a possible space elevator. Image Credit: NASA
Artistic view of a possible space elevator. Image Credit: NASA

It’s an exciting era in astronomy, as astronomers are currently discovering that many of the stars that we view in the night sky have their own planets in orbit around them. Many of them are massive worlds, all orbiting at varying distances from their parent star. It is no surprise that we are discovering a vast majority of these Jupiter-sized worlds first; larger worlds are much easier to detect than the smaller worlds would be. Imagine a bright spotlight pointing at you some 500 yards away (5 football fields). Your job is to detect something the size of a period on this page that is orbiting around it that emits no light of its own. As you can see, the task would be daunting. But nevertheless, our planet hunters have been utilizing methods that enable us to accurately find these tiny specks of gas and rock despite their rather large and luminous companion suns.

However, it is not the method of finding these planets that this article is about; but rather what we do to figure out which of these worlds are worthy of our limited resources and attention. We very well cannot point those starships in random directions and just hope that they happen across an earth-sized planet that has a nitrogen-oxygen rich atmosphere with drinkable water. We need to identify which planets appear to have these mentioned characteristics before we go launching ourselves into the vast universe.

How can we do this? How is it possible that we are able to say with any level of certainty what a planet’s atmosphere is composed of when this planet is so small and so very far away? Spectroscopy is the answer, and it just might be the key to our future in the cosmos.

Artistic impression of what Kepler-186f may look like. Image Credit:  NASA Ames/SETI Institute/JPL-CalTech
Artistic impression of what Kepler-186f may look like. Image Credit: NASA Ames/SETI Institute/JPL-CalTech

Just so I may illustrate how remarkable our scientific methods are for this very field of research, I will first need to show you the distances we are talking about. Let’s take Kepler 186f. This is the first planet we have discovered that is very similar to Earth. It is around 1.1 times larger than Earth and orbits within the habitable zone of its star which is very similar to our own star.

Let’s do the math, to show you just how distant this planet is. Kepler 186f is around 490 lightyears from Earth.

Kepler 186f = 490 lightyears away

Light moves at 186,282 miles/ 1 second.

186,282 mi/s x 60s/1min x 60min/1hr x 24hrs/1day x 356days/1year = 5.87 x 1012 mi/yr

Kepler 186f: 490 Lyrs x 5.87 x 1012miles/ 1 Lyr = 2.88 x 1015 miles or 2.9 QUADRILLION MILES from Earth.

Just to put this distance into perspective, let’s suppose we utilize the fastest spacecraft we have to get there. The Voyager 1 spacecraft is moving at around 38,500 mi/hr. If we left on that craft today and headed towards this possible future Earth, it would take us roughly 8.5 MILLION YEARS to get there. That’s around 34 times longer than the time between when the first proto-humans began to appear on earth 250,000 years ago until today. So the entire history of human evolution from then till now replayed 34 times BEFORE you would arrive at this planet. Knowing these numbers, how is it even possible that we can know what this planet’s atmosphere, and others like it, are made of?

First, here’s a bit of chemistry in order for you to understand the field that is spectroscopy, and then how we apply it to the astronomical sciences. Different elements are composed of a differing number of protons, neutrons, and electrons. These varying numbers are what set the elements apart from one another on the periodic table. It is the electrons, however, that are of particular interest in the majority of what chemistry studies. These different electron configurations allow for what we call spectral signatures to exist among the elements. This means that since every single element has a specific electron configuration, the light that it both absorbs and emits acts as a sort of photon fingerprint; a unique identifier to that element.

A list of the elements with their corresponding visible light emission spectra. Image Credit: MIT Wavelength Tables, NIST Atomic Spectrum Database, umop.net
A list of the elements with their corresponding visible light emission spectra. Image Credit: MIT Wavelength Tables, NIST Atomic Spectrum Database, umop.net

 

The standard equation for determining the characteristics of light is:

c= v λ

c is the speed of light in a vacuum (3.00 x 108 m/s)

v  is the frequency of the light wave (in Hertz)

λ (lambda) represents the wavelength (in meters, but will usually be converted to nanometers) which will determine what color of light will be emitted from the element(s), or simply where the wavelength of light falls on the electromagnetic spectrum (infrared, visible, ultraviolet, etc.)

If you have either the frequency or the wavelength, you can determine the rest. You can even start with the energy of the light being detected by your instruments and then work backwards with the following equations:

The energy of a photon can be described mathematically as this:

Ephoton = h v
OR
Ephoton = h c / λ

What these mean is that the energy of a photon is the product of the frequency (v) of the light wave emitted multiplied by Planck’s Constant (h), which is 6.63 x 10-34 Joules x seconds. Or in the case of the second equation, the energy of the photon is equal to Planck’s Constant x the speed of light divided by the wavelength. This will give you the amount of energy that a specific wavelength of light contains. This equation is also known as the Planck-Einstein Relation. So, if you take a measurement and you are given a specific energy reading of the light coming from a distant star, you can then deduce what information you need about said light and determine which element(s) are either emitting or absorbing these wavelengths. It’s all mathematical detective work.

So, the electrons that orbit around the nucleus of atoms exist in what we call orbitals. Depending on the atom (and the electrons associated with it), there are many different orbitals. You have the “ground” orbital for the electron, which means that the electron(s) there are closest to the nucleus. They are “non-excited”. However, there are “higher” quantum orbitals that exist that the electron(s) can “jump” to when the atom is excited. Each orbital can have different quantum number values associated with it. The main value we will use is the Principle Quantum Number. This is denoted by the letter “n”, and has an assigned integer value of 1, 2, 3, etc. The higher the number, the further from the nucleus the electron resides, and the more energy is associated with it. This is best described with an example:

A hydrogen atom has 1 electron. That electron is whipping around its 1 proton nucleus in its ground state orbital. Suddenly, a burst of high energy light hits the hydrogen. This energy is transferred throughout the hydrogen atom, and the electron reacts. The electron will instantaneously “vanish” from the n1 orbital and then reappear on a higher quantum orbital (say n4). This means that as that light wave passed over this hydrogen atom, a specific wavelength was absorbed by the hydrogen (this is an important feature to remember for later).

Diagram of an electron dropping from a higher orbital to a lower one and emitting a photon. Image Credit: Wikicommons
Diagram of an electron dropping from a higher orbital to a lower one and emitting a photon. Image Credit: Wikimedia Commons

Eventually, the “excited” electron will drop from its higher quantum orbital (n4) back down to the n1 orbital. When this happens, a specific wavelength of light is emitted by the hydrogen atom. When the electron “drops”, it emits a photon of specific energy or wavelength (dependent upon many factors, including the state the electron was in prior to its “excitement”, the amount of levels the electron dropped, etc.) We can then measure this energy (or wavelength, or frequency,) to determine what element the photon is coming from (in this case, hydrogen). It is in this feature that each element has its own light signature. Each atom can absorb and emit specific wavelengths of light, and they are all tied together by the equations listed above.

So how does this all work? Well, in reality, there are many factors that go into this sort of astronomical study. I am simply describing the basic principle behind the work. I say this so that the many scientists that are doing this sort of work do not feel as though I have discredited their research and hard work; I promise you, it is painstakingly difficult and tedious and involves many more details that I am not mentioning here. That being said, the basic concept works like this:

We find a star that gives off the telltale signs that it has a planet orbiting around it. We do this with a few methods, but how it all first started was by detecting a “wobble” in the star’s apparent position. This “wobble” is caused by a planet orbiting around its parent star. You see, when a planet orbits a star (and when anything orbits anything else), the planet isn’t really orbiting the star, the planet AND the star are orbiting a common focal point. Usually with this type of orbital system, that common focal point is fairly close to the center of the star, and thus it’s safe to say that the planet orbits the star. However, this causes the star to move ever so slightly. We can measure this.

Once we determine that there are planets orbiting the star in question, we can study it more closely. When we do, we turn our instruments towards it and begin taking highly detailed measurements, and then we wait. What we are waiting for is a dimming of the star at a regular interval. What we are hoping for is this newly-found exoplanet to transit our selected star. When a planet transits a star, it moves in front of the star relative to us (this also means we are incredibly lucky, as not all planets will orbit “in front” of the star relative to our view). This will cause the star’s brightness to dip ever so slightly at a regular interval. Now we have identified a prime exoplanet candidate for study.

Diagram of how we can use aborbstion specral reading to determine the atmosphere of an exoplanet. Image Credit: A. Feild, STScl, NASA
Diagram of how we can use absorption spectral reading to determine the atmosphere of an exoplanet. Image Credit: A. Feild, STScl, NASA

We can now introduce the spectroscopic principles to this hunt. We can take all sorts of measurements of the light that is coming from this star. Its brightness, the energy it’s kicking out per second, and even what that star is made of (the emission spectrum I discussed earlier). Then what we do is wait for the planet to transit the start, and begin taking readings. What we are doing is reading the light passing THROUGH the exoplanet’s atmosphere, and then studying what we can call an Absorption Spectrum reading. As I mentioned earlier, specific elements will absorb specific wavelengths of light. What we get back is a spectral reading of the star’s light signature (the emission spectra of the star), but with missing wavelengths that show up as very tiny black lines where there used to be color. These are called Fraunhofer lines, named after the “father” of astrophysics Joseph Fraunhofer, who discovered these lines in the 19th century.

The dark lines represent the light frequencies that were absorbed by specific chemicals that this particular light passed through. Image Credit: Wikimedia Commons
The dark lines represent the light frequencies that were absorbed by specific chemicals that this particular light passed through. Image Credit: Wikimedia Commons

What we now have in our possession is a chemical fingerprint of what this exoplanet’s atmosphere is composed of. The star’s spectrum is splayed out before us, but the barcode of the planet’s atmospheric composition lay within the light. We can then take those wavelengths that are missing and compare them to the already established absorption/emission spectra of all of the known elements. In this way, we can begin to piece together what this planet has to offer us. If we get high readings of sulfur and hydrogen, we have probably just discovered a gas giant. However if we discover a good amount of nitrogen and oxygen, we may have found a world that has liquid water on its surface (provided that this planet resides within its host star’s “habitable” zone: a distance that is just far enough from the star to allow for liquid water). If we find a planet that has carbon dioxide in its atmosphere, we may just have discovered alien life (CO2 being a waste product of both cellular respiration and a lot of industrial processes, but it can also be a product of volcanism and other non-organic phenomena).

What this all means is that by being able to read the light from any given object, we can narrow our search for the next Earth. Regardless of distance, if we can obtain an accurate measurement of the light moving through an exoplanet’s atmosphere, we can tell what it is made of.

We have discovered some 2000 exoplanets thus far, and that number will only increase in the coming decades. With so many candidates, it will be a wonder if we do not find a planet that we humans can live on without the help of technology. Obviously our techniques will further be refined, and as new technologies, methods, and instruments become available, our ability to pinpoint planets that we can someday colonize will become increasingly more accurate.

With such telescopes like the James Webb Space Telescope launching soon, we will be able to image these exoplanets and get even better spectroscopic readings from them. This type of science is on the leading edge of humanity’s journey into the cosmos. Astrophysicists and astrochemists that work in this field are the necessary precursors to the brave men and women who will one day board those interstellar spacecraft and launch our civilization into the Universe to truly become an interstellar species.

Possible glimpse into our future... Image Credit: Battlestar Wiki Media
Possible glimpse into our future… Image Credit: Battlestar Wiki Media