The Solar System formed from a cloud of gas, which gave rise to a proto-star, a proto-planetary disk, and eventually the seeds of what would become planets. This story of planetary formation occurred not only hundreds of billions of times in our own galaxy, once for each of the stars that exists, but all throughout the Universe over our cosmic history as well as in systems that never grew massive enough to form a successful star. As a result, most estimates of the number of planets in the Universe grossly underestimate the total number that are out there. (Credit: NASA/Dana Berry)

For most of history, our Solar System contained the only known planets.

At any epoch in our cosmic history, any observer will experience a uniform “bath” of omnidirectional radiation that originated back at the Big Bang. Today, from our perspective, it’s just 2.725 K above absolute zero, and hence is observed as the cosmic microwave background, peaking in microwave frequencies. At present, in most locations of space, it’s this leftover radiation that determines the temperature of the Universe. (Credit: Earth: NASA/BlueEarth; Milky Way: ESO/S. Brunier; CMB: NASA/WMAP)

Whenever we want to understand what’s going to happen to an object when we place it in an environment that’s unfamiliar, there are a few properties we need to know about that environment itself. One of them, unflaggingly, is temperature. Whether something becomes solid, liquid, gas, or plasma depends on…

Apollo 11 brought humans onto the surface of the Moon for the first time in 1969. Shown here is Buzz Aldrin setting up the Solar Wind experiment as part of Apollo 11, with Neil Armstrong snapping the photograph. (Credit: NASA/Apollo 11)

In all of human history, only 24 people have ever flown to the vicinity of the Moon, traveling hundreds of thousands of miles from Earth to do so. Twelve of those people, on six independent missions, actually set foot on the lunar surface. …

The ‘raisin bread’ model of the expanding Universe, where relative distances increase as the space (dough) expands. The farther away any two raisin are from one another, the greater the observed redshift will be by time the light is received. The redshift-distance relation predicted by the expanding Universe is borne out in observations, and has been consistent with what’s been known all the way back since the 1920s. (Credit: NASA/WMAP Science Team)

Whenever you have a puzzle, you have every right to expect that any and all correct methods should lead to you to same solution. This applies not only to the puzzles we create for our fellow humans here on Earth, but to even the deepest puzzles that nature itself has…

An event like AT2018cow, now known as either FBOTs or Cow-like events, is thought to be the result of a breakout shock from a cocooned supernova. With five such events now discovered, the hunt is on to uncover precisely what causes them, as well as what makes them so unique. (Credit: Shanghai Astronomical Observatory, China)

Every once in a while, a stellar cataclysm occurs in our Universe, bringing the life of a star to an end. The most common type of cataclysm is a core-collapse supernova, where a massive star’s interior implodes, leading to a runaway fusion reaction and a tremendous explosion, where the energy…

Planet Earth’s motion through space isn’t just defined by our axial rotation or our motion around the Sun, but the Solar System’s motion through the galaxy, the Milky Way’s motion through the Local Group, and the Local Group’s motion through intergalactic space. Only with everything combined, and by comparing to the Big Bang’s leftover glow, can we arrive at a meaningful answer. (Credit: Jim slater307/Wikimedia Commons; background: ESO/S. Brunier)

Planet Earth isn’t at rest, but continuously moves through space.

This image shows the supernova remnant of SN 1987a in six different wavelengths of light. By monitoring the same object across wavelengths and across time, we can gain unparalleled insight into what this object’s history, properties, and evolution are. (Credit: Alak Ray, Nature Astronomy, 2017; ACTA/ALMA/ESO/Hubble/Chandra composite)

Some stars die of natural causes. Others meet a grizzlier fate. As they go through their life cycles, most stars will die on time, right when they’re supposed to, as determined by their initial masses. They’ll burn through their fuel until they can fuse elements no longer, and then…

This image shows a depiction of a solar sail, and in particular of the sail used by the Japanese IKAROS mission. The idea of a thin, light, large-area surface has traditionally been based off of “sailing” on particles and radiation emitted by the Sun. However, a similar concept would leverage a highly reflective surface to reflect directed laser light 180 degrees from the surface, enabling direct propulsion and large, continuous accelerations, with the goal of completing an interstellar journey. (Credit: Andrzej Mirecki/Wikimedia Commons)

For all of human history, embarking upon an interstellar journey has been a seemingly unreachable dream, made practically impossible by the enormous distances separating our Sun from any of our stellar neighbors. Even with the most powerful rocket technology ever developed, it would take tens of thousands of years to…

While many unstable particles, both fundamental and composite, can be produced in particle physics, only protons, neutrons (bound in nuclei) and the electron are stable, along with their antimatter counterparts and the photon. Everything else is short-lived, but if muons can be kept at high enough speeds, they might live long enough to forge a next-generation particle collider out of. (Credit: Contemporary Physics Education Project/CPEP, DOE/NSF/LBNL)

If you want to uncover all the particles that fundamentally exist, your best bet is to smash particles together, under controlled, laboratory conditions, at extremely high energies. Whenever two particles collide, they have to conserve both energy and momentum, as well as other quantum properties that have associated conservation laws…

An illustration of our cosmic history, from the Big Bang until the present, within the context of the expanding Universe. We cannot be certain, despite what many have contended, that the Universe began from a singularity. We can, however, break the illustration you see into the different eras based on properties the Universe had at those particular times. We are already in the Universe’s 6th and final era. (Credit: NASA/WMAP science team)

The Universe is not the same today as it was yesterday. With each moment that goes by, a number of subtle but important changes occur, even if many of them are imperceptible on measurable, human timescales. …

Ethan Siegel

The Universe is: Expanding, cooling, and dark. It starts with a bang! #Cosmology Science writer, astrophysicist, science communicator & NASA columnist.

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