Astronomers can’t stop debating about the definition of a planet (see: Pluto). But one thing is for sure — there are a lot of objects that skirt the line between two types of cosmic bodies.
Now, researchers are adding a new kind of boundary-bending world to the mix. In research set to be published in the Monthly Notices of the Royal Astronomical Society, astronomers named a new class of theorized objects that begin as moons around large planets, but eventually move out on their own. They call them “ploonets.” The scientists think these objects should exist in solitary orbits around their host stars and could even be discovered in observations from past and present exoplanet-hunting surveys, like Kepler and TESS.
And to put a cherry on top of the ploonet pie: Because Earth’s own moon is slowly spiraling away from our planet, the team says our only natural satellite may also end up as a ploonet in some 5 billion years. Unfortunately, that’s just in time for our moon to get fully baked by the sun as our star expands into a red giant, reaching a diameter roughly the size of Earth’s orbit.
Black holes have long served as fodder for science fiction — and for good reason. These unimaginably dense objects contain so much matter trapped in such a small volume that their gravity prevents even light from escaping their surfaces.
Although the first prediction of a black hole was made nearly 250 years ago by the English philosopher and clergyman John Michell, the first black hole candidate, Cygnus X-1, wasn’t discovered until 1971. Since then, astronomers have tirelessly chipped away at countless questions related to these once-mythical beasts. But one of the most basic and enduring questions remains: Do they come in all sizes?
Over the past few decades, astronomers have compiled loads of evidence for the existence of black holes at both ends of the mass spectrum. Researchers have uncovered small black holes that weigh just a few to 100 times the mass of the sun, as well as supermassive black holes that can reach billions of times the mass of their star-sized brethren.
Stellar-mass black holes are thought to form when a relatively massive star dies in spectacular fashion. As the exhausted star burns through its final traces of fuel, its immense gravity causes it to collapse in on itself. If the collapsing star isn’t too big, the infalling material rebounds off the star’s dense core. This causes a supernova explosion, often leaving behind a tiny white dwarf or neutron star. But if the surviving remnant is greater than about three solar masses, not even tightly packed neutrons can prevent the city-sized core from continuing to collapse into a stellar-mass black hole.
On the other hand, there’s another class of black holes known as supermassive black holes, which serve as the central gravitational anchors of most, if not all, large galaxies. Though supermassive black holes are anywhere from millions to billions of times the mass of the sun, they pack all that matter into a region roughly the size of a single star. There are many lines of evidence that indicate these cosmic behemoths are common throughout the universe, but exactly how and when they formed still remains a mystery.
But what about the in-betweeners? Shouldn’t there should be a class of mid-sized black holes that split the difference between stellar-mass and supermassive black holes? These cosmic middleweights, which would range from about 100 to 1 million solar masses — though the specific range varies depending on who you ask — are referred to as intermediate-mass black holes (IMBHs). And although astronomers have found several compelling IMBH candidates spread throughout the universe, the jury is still out on whether they truly exist. However, the evidence is beginning to pile up.
Though conclusive proof of IMBHs remains elusive, over the past few decades, there have been a number of studies that have uncovered intriguing evidence hinting at the existence of these not-so-big, not-so-small black holes.
For example, in 2003, researchers used the ESA’s XMM-Newton space observatory to identify two strong, distinct X-rays sources in the nearby starburst galaxy NGC 1313. Because black holes tend to ferociously gobble up material that gets too close and belch out high-energy radiation, they are some of the strongest known emitters of X-rays. And by pinpointing NGC 1313’s X-ray sources and studying how they periodically flash, in 2015, researchers were able to constrain the mass of one of the galaxy’s suspected black holes, known as NGC 1313 X-1. They calculated it’s about 5,000 times the mass of the Sun, give or take about 1,000 solar masses, which would put it firmly in the mass range of an intermediate-mass black hole.
Likewise, in 2009, researchers uncovered even stronger evidence for the existence of a medium-sized black hole . Located some 290 million light-years away near the edge of the galaxy ESO 243-49, the team observed an incredibly bright X-ray source called HLX-1 (Hyper-Luminous X-ray source 1) that did not have an optical counterpart. This suggests the object is not simply a star or background galaxy. Additionally, the researchers found HLX-1’s X-ray signature varied with time, suggesting a black hole is brightening every time a nearby star makes a close approach, feeding gas to the black hole and causing brief outbursts of X-rays that then slowly fade away. Based on the brightness of the observed flashes, the researchers calculated a minimum mass of the black hole of about 500 times the mass of the Sun, though some estimates put its weight closer to 20,000 solar masses.
“Such a detection is essential,” said lead author Sean Farrell of the University of Leicester after the discovery. “While it is already known that stellar-mass black holes are the remnants of massive stars, the formation mechanisms of supermassive black holes are still unknown.” Farrell went on to explain that “the identification of HLX-1 is therefore an important step towards a better understanding of the formation of the supermassive black holes that exist at the center of the Milky Way and other galaxies.
More recently, astronomers have started to uncover strong evidence of wandering intermediate-mass black holes lurking near the heart of the Milky Way. For example, in January 2019, astronomers used the Atacama Large Millimeter/submillimeter Array (ALMA) to trace streams of gas orbiting an invisible object, thought to be an IMBH, with an apparent mass of about 32,000 times the mass of the Sun.
Located a scant 23 light-years from the Milky Way’s supermassive black hole, Sagittarius A*, the discovery suggests the newfound IMBH could merge with the roughly 4-million-solar-mass Sagittarius A* in the not-too-distant future. To help bolster the case for IMBHs wandering through the Milky Way, the researchers hope to use other oddly-orbiting gas clouds to probe our galaxy for more mid-sized black holes tucked away in gas-dominated regions.
Moving forward, researchers will rely on a variety of methods to uncover a slew of more mid-sized black holes. By doing so, they not only hope to prove that IMBHs truly exist, but more importantly, they want to use IMBHs to help piece together how large black holes grow and evolve over time.
Fortunately, astronomers are now in a prime position to do just that. Thanks to the recent successes of the LIGO-Virgo gravitational-wave project — which has identified 20 stellar-mass black holes by probing the universe for gravitational waves that are produced when black holes merge — researchers have a new method for searching for small to mid-sized black holes.
Although the LIGO-Virgo collaboration has yet to uncover gravitational waves from mergers between black holes larger than about 40 solar masses, according to the LIGO website, “in [the] future, with improvement in [the] sensitivity of gravitational wave detector[s], we will have a better understanding of the frequency of IMBH mergers. The third observing run has started collecting data from April 1, 2019, and gravitational-wave scientists are very hopeful to observe these elusive sources soon!”
So stay tuned, because over the next few years, we may find definitive proof of the missing link between small and super-sized black holes. And if we do, it will finally put this cosmic conundrum to rest once and for all. Only then will we be able to stop debating the existence of IMBHs, and instead focus on unraveling their origin stories, as well as those of supermassive black holes.
By now, merging black holes and the gravitational waves they produce are a scientific surety. Astronomers have observed several black hole mergers, all between stellar-mass black holes less than 100 times the mass of our sun. But no mergers between supermassive black holes, those with masses millions or billions times that of our star, have ever been seen; and in fact, astronomers wonder how likely such a smash-up would be. Now, the discovery of two supermassive black holes headed right for each other could help scientists answer the question of what would happen if they were to meet.
This particular pair, each more massive than 800 million suns, lies in a galaxy 2.5 billion light-years away. The galaxy itself is a merger remnant — all that’s left after two galaxies, each hosting a supermassive black hole, combined. A team led by Andy Goulding at Princeton University made the find using the Hubble Space Telescope and published their discovery July 10 in The Astrophysical Journal Letters.Read More
All the black holes that astronomers have seen fall into one of three categories: stellar-mass black holes, intermediate-mass black holes, and supermassive black holes. Each is more massive than our Sun and formed at least hundreds of thousands of years after the Big Bang, as our universe grew and evolved.
But there is another type of black hole astronomers haven’t yet seen, but think could exist. These are primordial black holes.
As their name suggests, primordial black holes were born very early in the life of the universe, a mere fraction of a second after the Big Bang. It was a time long before stars or galaxies (and other types of black holes) could exist. But some theories predict that primordial black holes should have popped onto the scene anyway. That’s because in that fraction of a second after the universe itself began, space was not completely homogenous (the same at every point). Instead, some areas were denser and hotter than others, and these dense regions could have collapsed into black holes.
A Brief Window
There was only a small period of time — about 1 second — following the Big Bang when primordial black holes could have formed. But in the extreme world of our expanding early universe, a lot can happen in just one second. And the later in this window of time that primordial black holes formed, the more massive they would be. Depending on when exactly they formed, primordial black holes could have masses as low as 10-7 ounces (10-5 grams), or 100,000 times less than a paperclip, up to about 100,000 times greater than the Sun.
The idea of such tiny black holes intrigued astrophysicist Stephen Hawking, who explored their quantum mechanical properties. That work led to his 1974 discovery that black holes can evaporate over time. And while Hawking ultimately realized a large black hole would evaporate away in more time than the universe has been around so far, small black holes could have indeed evaporated away or currently be doing so, depending on their mass. Hawking calculated that any primordial black hole with a mass greater than 1012 pounds ([1012 kilograms]; that’s far less than the mass of any planet, dwarf planet, and most named asteroids and comets in our solar system) could still be around today, while those less massive would have already disappeared.
And depending on their mass (which, remember, depends on when they formed), any primordial black holes left today could neatly explain some of the outstanding problems in astronomy.
Dark Matter Candidates
One such problem is dark matter. Although it makes up about 30 percent of our universe, astronomers remain stumped as to what exactly dark matter is. Primordial black holes could be the answer — or, at least, part of it. Primordial black holes could be a type of dark matter called MACHOs, which stands for massive compact halo objects, because astronomers think they’re found in the halos, or outskirts, of galaxies. Such black holes would be difficult to see if they’re simply floating quietly in space and keeping to themselves.
One way to spot MACHOs is by looking for events called microlensing, which occur when a massive object (say, a black hole) passes in front of a more distant object, like a star or galaxy. The black hole bends the light from the distant source around it, brightening and magnifying the image. These events are infrequent and short lived, but catching enough of them could allow astronomers to determine what the objects doing the microlensing are and whether they could be primordial black holes.
However, several recent studies have determined that even if primordial black holes of this type exist, they probably can’t explain all or even most of the dark matter effects we see.
Another way to search for large primordial black holes is through mergers. Gravitational-wave observatories like LIGO and VIRGO have already seen several black hole mergers, and future projects like LISA will be detect mergers of different masses than the ones we can currently spot. Because astronomers can trace back the masses of the merging black holes, they could find that future events were caused by black holes with the right masses to make them primordial black holes.
Alternatively, primordial black holes could be tiny. Some theories hold that although black holes evaporate, there may be a size limit. So when an evaporating black hole reaches a certain mass, it stops evaporating and simply stays very small. If this is the case, primordial black holes could still account for dark matter, albeit in a different way, and searching them out would be more challenging. Perhaps astronomers could spot black holes that are still evaporating, which would give off energetic particles, which in turn give off gamma rays. If black holes do eventually pop out of existence without stopping, they could die in intense blasts of energy — equivalent to about one million 1-megaton hydrogen bombs, Hawking wrote — which we might also spot as bursts of gamma rays.
Even if they don’t account for dark matter, there is a second problem in astrophysics that primordial black holes could answer. Primordial black holes of a different — larger — size than those needed to explain dark matter might instead explain the supermassive black holes astronomers see in the centers of massive galaxies. These black holes, millions or billions of times the mass of the Sun, can’t be created by one or even several exploding stars. Astronomers don’t know how these black holes got there or what created them; perhaps they are built from primordial black holes that have been around since the first second of our universe, serving as seeds out of which supermassive black holes could grow.
This possibility, however, may also not be likely, because primordial black holes had to form by the time the universe was just 1 second old. Even primordial black holes that formed at the last possible instant possible would be, according to the physics, only about 100,000 times as massive as the Sun, which is not really in the supermassive black hole weight class. To get the even larger black holes we see today, they’d have to pull in a lot of material and grow very quickly. This isn’t impossible, but it may be less likely to explain the sheer number of supermassive black holes that exist today.
Regardless of where or how they’re found, primordial black holes could tell astronomers a lot about the universe we live in. Depending on their mass, they could serve as probes into galaxy evolution, high-energy physics, and even the earliest fractions of a second after the universe was birthed. But although primordial black holes could exist, they have yet to be seen, and currently remain one of astronomy’s great questions, rather than a tidy answer.
Last week, a new study in the journal Science highlighted the role forests could play in tackling climate change. Researchers estimated that by restoring forests to their maximum potential, we could cut down atmospheric carbon dioxide (CO2) by 25 percent — a move that would take us back to levels not seen in over a century. Though the study brings hope in the fight against climate change, other experts warn the solution is not that simple.
The study, led by scientists at ETH-Zürich, Switzerland, determined the planet has 0.9 billion hectares of land available to hold more trees — an area the size of the continental U.S. Converting those areas into forests would be a game-changer for climate change, the authors suggested.Read More
We know it best as a stringy slime dripping from noses and as viscous, discolored goop hacked up by sickened airways. But it’s so much more than that. Coating the surfaces of guts, eyes, mouth, nasal cavity and ears, mucus plays a range of important physiological roles — hydrating, cleaning, supporting good microbes and warding off foreign invaders.
“I like to call it the unsung hero of the body — it’s something that has such powerful effects over our health,” says Katharina Ribbeck, a biophysicist at MIT who with colleagues outlined the many roles of mucus in the 2018 Annual Review of Cell and Developmental Biology. Most of those functions come from the 5 percent of the substance that’s not water: various salts, lipids and proteins, most notably mucins, which give mucus its gel-like qualities — long, thread-like polypeptides coated in covalently bound chains of sugars called glycans.Read More
For the past ~40,000 years, Homo sapiens — modern humans — has been the only Homo species on Earth. But for most of our history, there were close evolutionary cousins of ours, human but not quite like us, coexisting and evolving at the same time in different regions.
Some of our now-extinct relatives, such as the Neanderthals, are well known. Others, like the recently-discovered Denisovans or Homo naledi have hardly made it into textbooks yet. And hints of even more human forms have been found in incomplete fossils and genetic patterns, although these relatives are poorly understood. Modern humans were just one of many variations on the Homo theme.Read More
What if the key to protecting our planet … was leaving it? Well, in part, at least. As worries about climate change mount, and the race to obtain resources from space heats up, some experts and über-rich CEOs are seriously considering moving our industry off-planet. That means using robots to build satellites and space stations by mining asteroids, the moon and other planets. A plot ripped from science fiction? Most definitely. But much of the technology to build this off-earth infrastructure already exists.
This contingency plan — known as in situ resource utilization — is not only necessary to reduce global warming, but could even be key to our continued growth as a species, according to Phil Metzger, a planetary scientist at the University of Central Florida. Before that, Metzger spent 30 years at NASA where he cofounded Swamp Works, a lab that develops tech for space mining and interplanetary living.Read More
(Inside Science) — Big, black wasplike things living in your toilet may sound more like a horror scene than a sanitation solution. That’s certainly what people in rural Louisiana thought in the summer of 1930, when black soldier flies infested a set of newly installed privies.
“[C]onsiderable consternation often resulted when a person lifted a privy lid and was greeted by a swarm of insects resembling wasps, or when upon leaving the privy he experienced a strange creeping and buzzing sensation due to flies being confined within his garments,” wrote researchers in an account published in 1930 in the Journal of Economic Entomology. To make matters worse, hungry local chickens tore down the privies’ foundations in search of larvae pupating in the surrounding dirt.Read More
Deepfake videos are hard for untrained eyes to detect because they can be quite realistic. Whether used as personal weapons of revenge, to manipulate financial markets or to destabilize international relations, videos depicting people doing and saying things they never did or said are a fundamental threat to the longstanding idea that “seeing is believing.” Not anymore.
Most deepfakes are made by showing a computer algorithm many images of a person, and then having it use what it saw to generate new face images. At the same time, their voice is synthesized, so it both looks and sounds like the person has said something new.Read More