First published October 3, 2018.
Earth (marked by the arrow) beside the other planets of the solar system. Even the giants Jupiter and Saturn are tiny next to the Sun, and the Sun is one star among hundreds of billions in our galaxy.
How small is Earth, really? A three-and-a-half-minute BuzzFeed video called "209 Seconds That Changed Our Perspective of the World" tries to answer that by stacking up facts about our planet, the Sun, the Milky Way and the wider universe. Its message is easy to grasp: Earth is a speck, everything in space is moving, and the numbers involved are almost impossible to picture.
The video is worth watching, but some of the figures that circulate with it are muddled or out of date. Below, each big idea is explained in plain language, with current numbers from astronomers.
The short answer: Earth is very small, and nothing in space stands still
If you want one takeaway, it is this. Earth spins on its axis, circles the Sun, travels with the Sun around the centre of the Milky Way, and the Milky Way itself is drifting through a cosmic neighbourhood of galaxies that pull and push on each other. On top of all that, space between distant galaxy groups is stretching. None of this is felt in daily life because we move together with everything around us, just as you do not feel a smooth flight at cruising speed.
The rest of this article walks outward step by step, from a single spacecraft to the scale of the observable universe.
How fast is the universe expanding?
The figure often quoted with the video is that the universe expands at about 43 miles per second per megaparsec. That is a fair approximation of the Hubble constant, the number that describes cosmic expansion. In the units astronomers use, it is about 70 kilometres per second per megaparsec.
The strange unit is the key to understanding it. A megaparsec is about 3.26 million light-years. The Hubble constant says that a galaxy one megaparsec away appears to move away from us at roughly 70 km/s, a galaxy two megaparsecs away at about 140 km/s, and so on. It is not a single speed; it is a rate that grows with distance. That is why comparing it with the speed of a rocket or a probe, as some versions of this story do, mixes two different kinds of quantity.
There is also a live scientific puzzle here. Measurements based on the early universe, from the Planck satellite's map of the cosmic microwave background, give about 67.4 km/s per megaparsec. Measurements based on nearby stars and supernovae, such as the SH0ES project, give about 73. The gap is larger than chance can explain, and astronomers call it the Hubble tension. It may point to measurement errors or to new physics, and it is one of the most debated questions in cosmology today.
Voyager 1: our most distant messenger
To feel how big even our own solar system is, consider NASA's Voyager 1. It launched on September 5, 1977, and is the most distant human-made object. It travels at roughly 61,000 km/h, about 17 kilometres every second, which works out to nearly 1.5 million kilometres (around 900,000 miles) a day.
Even at that speed it took about 35 years to cross the heliopause, the boundary where the Sun's wind gives way to interstellar space. Voyager 1 crossed it on August 25, 2012, and NASA confirmed the crossing in 2013. By 2026 it was more than 170 times farther from the Sun than Earth is, and only a couple of its instruments were still running on its slowly fading power supply. Yet it has not truly left the solar system in the widest sense: the distant Oort Cloud of icy bodies lies far beyond it, and the nearest star is thousands of years away at its pace.
The motions we cannot feel
Every one of us is riding several motions at once:
- Earth's spin: about 1,670 km/h at the equator, less as you move towards the poles.
- Earth's orbit: around 107,000 km/h as it circles the Sun once a year.
- The Sun's orbit around the galaxy: commonly estimated at roughly 800,000 km/h (figures between about 790,000 and 830,000 km/h are quoted, depending on the measurement). One lap takes around 230 million years.
- Our galaxy's motion through the cosmos: measured against the cosmic microwave background, the afterglow of the Big Bang, our Local Group of galaxies moves at about 630 km/s, well over 2 million km/h.
One correction to the popular version: it is sometimes said that "the universe moves" at around 2.1 million km/h. The universe as a whole does not travel through anything. That speed describes the motion of our galaxy and its neighbours relative to the cosmic background radiation.
Pushed by a void: the Dipole Repeller
Why does our galaxy move in that particular direction? For decades the answer centred on gravity pulling us towards dense regions such as the Great Attractor and, farther away, the Shapley Supercluster. In January 2017 a team led by Yehuda Hoffman, with Daniel Pomarède, R. Brent Tully and Hélène Courtois, published a study in Nature Astronomy that added the other half of the picture.
By mapping the motions of thousands of galaxies, they found a large region nearly empty of galaxies on the opposite side of the sky. Because it contains less matter than average, it effectively pushes matter away, and they named it the Dipole Repeller. The researchers concluded that the pull of the Shapley Attractor and the push of this underdense region contribute roughly equally to our motion. It is less a repulsive force than a gravitational tug-of-war in which emptiness on one side means the other side wins.
The speed is often reported as about 1.2 million miles per hour. The figure in the study, around 630 km/s, is closer to 1.4 million mph. Either way, the point stands: we are being carried through space faster than any rocket humans have built.
Laniakea: our cosmic home address
In September 2014, Tully and colleagues published a paper in Nature that redrew the map of our neighbourhood. Instead of drawing superclusters by how galaxies cluster on the sky, they defined them by where galaxies are flowing, like rivers draining into a basin. The basin that contains the Milky Way was named Laniakea, Hawaiian for "immense heaven", a name suggested to honour Polynesian navigators who crossed the Pacific by the stars.
Laniakea is enormous. It holds roughly 100,000 galaxies, has a mass of about 1017 times that of the Sun (around 100,000 times the Milky Way), and spans hundreds of millions of light-years; estimates of its width range from about 400 to 520 million light-years depending on how the edge is drawn. Its gravitational centre is the Great Attractor, in the direction of the southern constellation Norma. Several groupings once treated as separate superclusters, including our own Virgo Supercluster, turned out to be parts of it.
How many galaxies and stars are there?
Here the older numbers have changed the most. Claims that the observable universe holds about ten billion galaxies are far too low by current estimates. Surveys suggest the true number is in the hundreds of billions, and some estimates go as high as around two trillion, although a 2021 study using light measured by the New Horizons spacecraft argued for a figure at the lower end of that range.
Stars are even harder to count. The European Space Agency has given an estimate of roughly 1024 stars in the observable universe, a one followed by 24 zeros. That is far more than the "billion trillion" (1021) sometimes quoted, which assumed too few galaxies. The observable universe itself is about 93 billion light-years across, and the whole universe may be much larger still.
Why scale can be oddly comforting
Many people find that thinking about these numbers puts a bad day into perspective. Our worries feel smaller beside a supercluster, but the more striking point may be the opposite: on a tiny planet, among a trillion trillion stars, matter has arranged itself into beings who can measure the expansion of the universe and argue about the result to the third decimal place.
If you watch the video, try pausing at each figure and asking what it is measured against. That one habit turns a stream of big numbers into a real understanding of where we are.
Sources: Video: "209 Seconds That Changed Our Perspective of the World" by BuzzFeed (via YouTube); Wikipedia: Hubble's law; Wikipedia: Voyager 1; Wikipedia: Dipole Repeller (Hoffman et al., Nature Astronomy, 2017); Wikipedia: Laniakea Supercluster (Tully et al., Nature, 2014); Wikipedia: Observable universe. Image: solar system planets to scale, widely shared illustration.
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