How Many Years Have There Been?

I recently got nerd-sniped (exposed to an interesting question that ‘took over’ my thinking for a while) by the question “How many years have there been anyway?”

Heading into the New Year, the first answer that comes to mind is “2,019,” but this is one of the few we can eliminate right off the bat. There have been 2,019 years since Jesus of Nazareth was 4-6 years old. This method of counting up the years was begun in 525 by a Christian Monk named Dionysius Exiguus, but we know there were years before that, so 2,019 years is not our answer.

If you ask people from around the world you’ll get answers ranging from 13 (Mayan Long Count), 227 (French Republican Calendar), 1397 (Kurds and Afghans), 1440 (Islamic), 1940 (Indian Civil Calendar), 4716 (Chinese year according to an online converter), 5779 (Hebrew calendar) – or if you ask your computer it’ll say “1546300800” (unix).

There are groups that think we ought to base the year system on something that acts as a common heritage to all humans. Italian scientist, geologist, and field-founding badass Cesare Emiliani created an idea called the Holocene Calendar that would add 10,000 years to the current Julian date, making the upcoming year 12,019. Moving the ‘year 0’ to 10(ish) millennia ago acknowledges a large part of human history and eliminates the need to count backward when looking at cultures like the Romans, Norte Chico, Indus River Valley civilizations, or Egyptians. This is an interesting place to start a calendar, but does nothing to further our quest for an answer to our question.

The next method I thought of was counting the number of years since the very beginning. The universe formed about 13.8 billion years ago, so let’s see how that answer fits. One year (roughly) measures how many times the planet Earth has gone around the star Sol, so it doesn’t really make sense to say that there have been 13.8 billion years.* The Earth hasn’t been around for quite that long.

The universe's baby photo

<sidenote> Futuristic societies are often imagined to measure their time in years even though they have either left the planet Earth long ago, or never knew of Earth at all. An Earth-year in that context seems like a really poor choice of timekeeping unit. </sidenote>

Looking at the Earth, then – that has been around for about 4.543 billion years. This is the first answer to the question that fits. Let’s go ahead and make our math teachers proud and circle our answer:

At this point I thought I was done, but then another question started to bother me: if we consider a year to be the time it takes for a particular ball of rock to float around a particular ball of gas one time, 4,543,000,000 is looking good, but if we consider a year as a concept devised by human beings, then that number is far too large.

Humans have been around for the past 300,000 years or so, and they have probably kept at least a crude track of the handful of years they are alive through all of it, so our next answer that fits the question is:

If you want to get really pedantic, and trust me – I do – then you can consider when we started calling this shifting of seasons and shadows a “year.” The word “year” goes all the way back to Proto-Indo-European language family, believed to have been spoken from about 4500 BCE to 2500 BCE.  This word “year” has alternatively been spelled “gear,” “gar,” “yar,” etc… in different languages throughout the years, but has stayed remarkably similar for millennia. So our next number answers the question: “how long have we been calling this phenomenon a “year?”

For our next answer we’re asking how many years have been lived by humans? If you ask me and Paul Manafort how our year has been, you’ll get two very different answers… so how many years have been lived by individual humans? The Population Reference Bureau has a bizarrely precise 'estimate' of the human population at 108,470,690,115, and each of them lived through about 40 years on average.** That means that humans have cumulatively seen about 4.3 trillion years go by – in other words, if each human lived one at a time, we’d have seen the universe go by more than thirty times. This is another answer to our question:

Until now I’ve only been considering Earth years, but when someone wants to know how long it takes Mercury to go around the sun, they ask “how long is a year on Mercury?” So if we ask “how many lengths of time are the ‘year’ for a particular planet (which is another way of asking how many planets there are), we can get yet another answer.

There are a few a few different ways we can find exoplanets. To try to find out how many planets there are we can look at the the data. The Kepler and K2 missions have been hunting planets to help us find out how many stars have planets, and if they do, how many. It turns out, that our solar system is probably about average. Taking a look at the number of stars in the Milky Way and figuring on an average of 5 planets/star, we see that there are about a trillion planets in our galaxy (a safe estimate).

With 100 billion galaxies, each with a trillion planets, it stars getting easier to just count the zeros after the number rather try to say the number.

My final number for “How many years are there?” is the number of planets, each with its own unique year:

Written out: 100,000,000,000,000,000,000,000

Cheers,

    -Scott

*An amount of time equal to 13.8 billion years has elapsed, but we are counting the number of years, not counting how much time has passed in units of years. A "year" doesn't make any sense if the planet Earth isn't orbiting the Sun.

** There is an oft-quoted statistic that people used to die by age forty, but that is an average, not a maximum. Socrates, Ben Franklin and Saint Anthony were not freaks who managed to double the normal lifespan and live to see 80, but just regular old people. The 35-40 life expectancy figure includes a large infant mortality rate – if you lived past 5, you could expect to see 60.


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What Can We Learn From Twinkling Starlight?

When you look up at the night sky, the stars twinkle. This is interesting, but not what I am talking about today. If you are curious why the stars twinkle from here on Earth check out this video:

There ya go. Now, the 'twinkling starlight' I'll be mainly talking about is related to exoplanets, or planets orbiting stars other than our own. If you look carefully at stars from outside our atmosphere, or correct for atmospheric effects, the stars still twinkle, but for what I think is a much more interesting reason.

Do you want to know something interesting about starlight?

Great!

For the longest time, it was thought that our planet and our solar system were pretty unique. Aristotle laid down the thinking about many topics including astronomy for many centuries, and as it turns out he was wrong about a fair bit of it. While understandable for his time, by the 1600's, times were changing. In the early 1600's Galileo looked up at the moon with a telescope he made (didn't invent) and observed the moon's terminator, the area where light met dark. In the shadows he saw craters, bumps, and ridges; the moon wasn't a perfect celestial orb, it was its own world with its own unique features. Couple that with his discovery of moons orbiting Jupiter, and we were on our way to discovering other worlds, inferred from points of light.

*IC6.G1333.610s, Houghton Library, Harvard University

Both the cratering of the moon and the motion of the Jovian
moons were published by Galileo in 1610 in this pamphlet.

We have now sent spacecraft to nearly all of the thirty or so largest bodies in the solar system. With missions visiting the asteroids Vesta and Ceres, and the upcoming mission to Pluto, New Horizons, our curiosities about other worlds just took steps much farther afield.

Just as we could see other worlds in our own solar system, we can now look for worlds orbiting other stars using several methods of analyzing flickering starlight from their home star. With few exceptions, we cannot just take pictures of the planets because their star outshines them by many orders of magnitude, what we can see it the influence they have on their star.

Transit method -

One way to detect exoplanets is too find a planet that passes directly between its home star and us here on earth. A bit like a solar eclipse. When this happens, the planet blocks a little bit of the light, and the star dims. We can track the stars brightness and if it dims consistently and periodically we can tell that there is probably a planet orbiting that star. Here is what one of these dips looks like:

Image: Nikola Smolenski

The transit method is currently by far the most common way to detect exoplanets, but it has its drawbacks. Due to the fact that the planet has to pass between the star it is orbiting and the observer here on earth, it biased toward planets that orbit "edge - on" to us here on earth. Imagine flipping a coin, and taking a picture when the coin is exactly edge-on. Most of the pictures are going to show at least some of either the heads side or the tails side. This is roughly the same probability as a particular star system appearing exactly edge on to ours so the planet passes in front of the sun.

This method tends to be biased in finding large planets orbiting close to their stars. The larger and closer to its star that a planet is, the more likely it is to cross in front of the star and dim the light we see. These planets are known as "hot Jupiter" because they tend to be larger than Jupiter and closer to their star than Mercury is to our sun. This flies in the face of how we think planets developed, suggesting that hot Jupiters are quite rare. If this is the case, there could many, many more planets out there than we can currently find using this method.

It is important to note at this point that we cannot see the outline of the planet in front of the star. The only thing we can detect from here on earth is the slight dimming from a distant point of light.

Other Methods

Two more ways I'll briefly touch on on the radial velocity method and something called astrometry.

To describe the radial velocity method I first have to talk about the Doppler effect.

<sidenote>
I always thought "Christain Doppler and the Effects" would make a great band name
</sidenote>

There are plenty of great video about how this works, so I'll only go into it very briefly here. When a noise-making object approaches you, the sound waves "stack up" and compress on their way to your ears. This registers as a higher pitch. When the noise-making object moves away from you the sound waves "stretch out" and you register this as a lower pitch. This is why cars passing you make the characteristic "weeeee-yahhhhhh" sound.
The same goes for light. When an object is moving toward you, you register the compression of the waves as a "blueshift," the object literally looks a bit bluer. When the object travels away, the light looks redder, a "redshift."

TL;DR: Stars look bluer moving toward you, redder when they're moving away.

Alright, on to radial velocity. A large planet orbiting a star will cause the star to wobble a little bit, as seen below:

This is because the planet gravitationally tugs on the star, just as the star tugs on the planet. Notice how the star moves up and down. If we look at this star from earth, we can see it getting redder and bluer as it travels farther and closer to us, and from that, infer the presence of a planet by looking at the rate of the wobble.

On to Astrometry!

If you imagine looking at the system above from earth just as it is portrayed, you would see the star travelling in a little circle. If you look at both the foreground star as well as background objects, you can see the motion of the star and from that find out characteristics of the planet orbiting it.


This topic is difficult to convey through writing alone, so if you're interested check out YouTube for some great videos about exoplanets and exoplanet detection. Here are a few of my favorites:

Overall, it is truly amazing what we can discover merely by looking at twinkling starlight.


Cheers,

   - Scott



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