Is the Milky Way special, or, at least, is it in a special place in the Universe? An international team of astronomers has found that the answer to that question is yes, in a way not previously appreciated. A new study shows that the Milky Way is too big for its “cosmological wall,” something yet to be seen in other galaxies. The new research is published in Monthly Notices of the Royal Astronomical Society.
A cosmological wall is a flattened arrangement of galaxies found surrounding other galaxies, characterized by particularly empty regions called ‘voids’ on either side of it. These voids seem to squash the galaxies together into a pancake-like shape to make the flattened arrangement. This wall environment, in this case, called the Local Sheet, influences how The Milky Way and nearby galaxies rotate around their axes, in a more organized way than if we were in a random place in the Universe, without a wall.
Typically, galaxies tend to be significantly smaller than this so-called wall. The Milky Way is found to be surprisingly massive in comparison to its cosmological wall, a rare cosmic occurrence.
The new findings are based on a state-of-the-art computer simulation, part of the IllustrisTNG project. The team simulated a volume of the Universe nearly a billion light-years across that contains millions of galaxies. Only a handful – about a millionth of all the galaxies in the simulation – were as “special” as the Milky Way, i.e. both embedded in a cosmological wall like the Local Sheet, and as massive as our home galaxy.
According to the team, it may be necessary to take into account the special environment around the Milky Way when running simulations, to avoid a so-called “Copernican bias” in making scientific inference from the galaxies around us. This bias, describing the successive removal of our special status in the nearly 500 years since Copernicus demoted the Earth from being at the center of the cosmos, would come from assuming that we reside in a completely average place in the Universe.
To simulate observations, astronomers sometimes assume that any point in a simulation such as IllustrisTNG is as good as any, but the team’s findings indicate that it may be important to use precise locations to make such measurements.
So, the Milky Way is, in a way, special,” said research lead Miguel Aragón. “The Earth is very obviously special, the only home of life that we know. But it’s not the center of the Universe, or even the Solar System. And the Sun is just an ordinary star among billions in the Milky Way. Even our galaxy seemed to be just another spiral galaxy among billions of others in the observable Universe.”
“The Milky Way doesn’t have a particularly special mass, or type. There are lots of spiral galaxies that look roughly like it,” Joe Silk, another of the researchers, said. “But it is rare if you take into account its surroundings. If you could see the nearest dozen or so large galaxies easily in the sky, you would see that they all nearly lie on a ring, embedded in the Local Sheet."
"That’s a little bit special in itself. What we newly found is that other walls of galaxies in the Universe like the Local Sheet very seldom seem to have a galaxy inside them that’s as massive as the Milky Way.”
WHY IS EARTH’S MOON SO DIFFERENT FROM OTHER MOONS??
Blog# 215
Wednesday, August 3rd, 2022
Welcome back,
There are lots of moons in the solar system, so in that sense, the Moon is not unique. Some planets, like Jupiter and Saturn, have dozens upon dozens of moons that come in nearly every shape and size. In fact, every planet that has a moon of some kind has more than one … except Earth!
Earth’s Moon isn’t the largest — not by a long shot. The largest planet boasts the largest moon as well, with Jupiter’s Ganymede coming in at over 5,200 kilometers in diameter. That’s a full ~67% larger than Mercury, the smallest planet. The Moon comes in at number 5 on the list of largest moons, between Io and Europa (two more of Jupiter’s moons). At 3,400 kilometers in diameter, the Moon’s is still larger than Mercury, though.
For the terrestrial planets (Mercury, Venus, Earth and Mars), our Moon stands alone. Sure, Mars has two moons but Phobos and Deimos are glorified space potatoes, captured asteroids that zip around the planet. Our Moon is spherical and circles us at a relatively leisurely 27 days per orbit. It is also the only astronomical object other than the Sun that influences processes on Earth (think: tides … but not earthquakes or volcanic eruptions).
Even our Moon’s birth sets it apart. A vast majority of the small chunks of rock and ice that orbit other planets were likely formed with the planet or captured by the planet’s gravitational well. Not our Moon. It was born of a cataclysm involving the early Earth. A massive collision with a Mars-size object jettisoned material off which coalesced into the current Earth-Moon system.
Speaking of which, now that Pluto is exiled to the land of dwarf planets, the Earth-Moon system is the closest we have to a twin planet in our solar system. The diameter of the Moon (~3,400 kilometers) is ~25 percent that of Earth’s (~12,700 kilometers) — a remarkably large ratio. Ganymede is only 6 percent the size of Jupiter.
The story of the Moon’s formation can also be seen by comparing the different densities of Earth and its partner. Grind up the Earth and measure the density of the material and you get ~5.5 grams per cubic centimeter. Do the same to the Moon and it’s only ~3.3 grams per cubic centimeter … which is actually pretty close to the density of the upper layers of Earth. This likely means that the Moon was made up of the outer parts of proto-Earth (crust and mantle) after the collision and not the heavy metallic innards.
Some of this turbulent history is visible on the surface of the Moon as well. Unlike the other big moons like Ganymede, Titan, Europa, Callisto and Triton, our Moon lacks any real atmosphere and isn’t covered in ice. Sure, it is a lot closer to the Sun, so maybe it lost much of its ice (there is some), but it is a much more desolate locale compared to the icy moons of the solar system.
That being said, there are some definite advantages to going back to our planetary companion.v The Moon is only around 240,000 miles (348,000 kilometers) from Earth (on average), yet we’ve only been there 6 times.
If we need a springboard to get out to the rest of the solar system, it is likely going to take getting back to the Moon and getting comfortable living down our celestial street, so to speak. There is a lot we don’t know about the Moon and its history, so let’s use this anniversary to get back into the mindset of voyage and discovery across the solar system.
I turned my back on the world
I wasn't always like this, girl
Do you know what loneliness does to a man?
Turn him into the walking dead
I may have died but your lovin' raised me
Milkyway rising over Mt.Rainier by Liang Li
Via Flickr:
This was a shot that took me a couple of years' attempt to nail and I am glad how it turns out. The volcano in this image is Mt.Rainier in Washington and with perfect timing and extensive scouting, I got this image with milkyway rising directly above the crater and white river leading your view to it.