Archive for 2019

DISCOVERY OF OUMUAMUA: First Interstellar Object 2019

SOURCE BY - NASA DEPARTEMENT

Oumuamua — a curious, cigar-shaped rock — was the first interstellar space rock ever discovered in our solar system, though scientists debate whether it should be classified as an asteroid or comet. Now, a new study suggests that the strange object could actually be an alien light sail or solar-powered spacecraft.

Abraham Loeb and Shmuel Bialy from the Harvard Smithsonian Center for Astrophysics (CfA) conducted the study, titled “Could Solar Radiation Pressure Explain ‘Oumuamua’s Peculiar Acceleration?” The paper was posted on the preprint website, the arXiv. In it, the astronomers offer a unique, new explanation for some strange behavior ‘Oumuamua has displayed.

The asteroid was first detected by the Pan-STARRS-1 survey in 2017. Astronomers saw it traveling out of our solar system and noted that it was spinning rapidly and likely had a rocky and metallic composition. Further research showed that ‘Oumuamua was also surprisingly icy, despite it not being a comet.

An alternative explanation

Researchers were in for another surprise when an international team found that the asteroid’s velocity had increased when they were expecting it to slow down.

That team concluded that, most likely, this was because of a process called outgassing, where material on the object’s surface is heated by the sun and vaporized. This process creates a comet’s tail, and the researchers thought that ‘Oumuamua it could be pushing the object and increasing its velocity.

Oumuamua

Small object against a background of stars.

This very deep combined image shows the interstellar object ‘Oumuamua at the center of the image. It is surrounded by the trails of faint stars that are smeared as the telescopes tracked the moving comet.

Credit: NASA AGENCY ESO/K. Meech et al.

Discovery of OUMUAMUA: First Interstellar object 2019

The first known interstellar object to visit our solar system, 1I/2017 U1 ‘Oumuamua, was discovered Oct. 19, 2017, by the University of Hawaii’s Pan-STARRS1 telescope, funded by NASA’s Near-Earth Object Observations (NEOO) Program, which finds and tracks asteroids and comets in Earth’s neighborhood. While originally classified as a comet, observations revealed no signs of cometary activity after it slingshotted past the Sun on Sept. 9, 2017 at a blistering speed of 196,000 miles per hour (87.3 kilometers per second). It was briefly classified as an asteroid until new measurements found it was accelerating slightly, a sign it behaves more like a comet.

Artist's concept of cigar-shaped space rock.

Artist's concept of interstellar object1I/2017 U1 ('Oumuamua) as it passed through the solar system after its discovery in October 2017. The aspect ratio of up to 10:1 is unlike that of any object seen in our own solar system. Image Credit: European Southern Observatory / M. Kornmesser.

ALTERNATIVE EXPLANATIONS

The first confirmed object from another star to visit our solar system, this interstellar interloper appears to be a rocky, cigar-shaped object with a somewhat reddish hue. The object, named ‘Oumuamua by its discoverers, is up to one-quarter mile (400 meters) long and highly-elongated—perhaps 10 times as long as it is wide. That aspect ratio is greater than that of any asteroid or comet observed in our solar system to date. While its elongated shape is quite surprising, and unlike objects seen in our solar system, it may provide new clues into how other solar systems formed.

The observations suggest this unusual object had been wandering through the Milky Way, unattached to any star system, for hundreds of millions of years before its chance encounters with our star system.

“For decades we’ve theorized that such interstellar objects are out there, and now―for the first time―we have direct evidence they exist,” said Thomas Zurbuchen, associate administrator for NASA’s Science Mission Directorate in Washington, in November 2017.

Animated view of the interstellar object's orbit.

NASA DEPARTEMENT

Immediately after its discovery, telescopes around the world, including ESO’s Very Large Telescope in Chile, were called into action to measure the object’s orbit, brightness, and color. The urgency for viewing from ground-based telescopes was vital to get the best data.

Combining the images from the FORS instrument on the ESO telescope using four different filters with those of other large telescopes, a team of astronomers led by Karen Meech of the Institute for Astronomy in Hawaii found that ‘Oumuamua varies in brightness by a factor of 10 as it spins on its axis every 7.3 hours. No known asteroid or comet from our solar system varies so widely in brightness, with such a large ratio between length and width. The most elongated objects we have seen to date are no more than three times longer than they are wide.

“This unusually big variation in brightness means that the object is highly elongated: about ten times as long as it is wide, with a complex, convoluted shape,” said Meech. “We also found that it had a reddish color, similar to objects in the outer solar system, and confirmed that it is completely inert, without the faintest hint of dust around it.”

These properties suggest that ‘Oumuamua is dense, composed of rock and possibly metals, has no water or ice, and that its surface was reddened due to the effects of irradiation from cosmic rays over hundreds of millions of years.

A few large ground-based telescopes continued to track the fading object as it receded from our planet. Two of NASA’s space telescopes (Hubble and Spitzer) tracked the object traveling about 85,700 miles per hour (38.3 kilometers per second) relative to the Sun. Its outbound path is about 20 degrees above the plane of planets that orbit the Sun. The object passed Mars’s orbit around Nov. 1 and will pass Jupiter’s orbit in May of 2018. It will travel beyond Saturn’s orbit in January 2019; as it leaves our solar system, ‘Oumuamua will head for the constellation Pegasus.

Preliminary orbital calculations suggest that the object came from the approximate direction of the bright star Vega, in the northern constellation of Lyra. However, it took so long for the interstellar object to make the journey―even at the speed of about 59,000 miles per hour (26.4 kilometers per second)―that Vega was not near that position when the ‘Oumuamua was there about 300,000 years ago.

Astronomers estimate that an interstellar object similar to ‘Oumuamua passes through the inner solar system about once per year, but they are faint and hard to spot and have been missed until now. It is only recently that survey telescopes, such as Pan-STARRS1, are powerful enough to have a chance to discover them.

“What a fascinating discovery this is!” said Paul Chodas, manager of the Center for Near-Earth Object Studies at NASA’s Jet Propulsion Laboratory, Pasadena, California. “It’s a strange visitor from a faraway star system, shaped like nothing we’ve ever seen in our own solar system neighborhood.”

The predicted position of `Oumuamua versus observed position

NASA DEPARTEMENT - HUBBLE TELESCOPE

The predicted position of `Oumuamua versus observed position

This diagram shows the orbit of the interstellar object ‘Oumuamua as it passes through the Solar System. It shows the predicted path of ‘Oumuamua and the new course, taking the new measured velocity of the object into account.

‘Oumuamua passed the distance of Jupiter’s orbit in early May 2018 and will pass Saturn’s orbit in January 2019. It will reach a distance corresponding to Uranus’ orbit in August 2020 and of Neptune in late June 2024. In late 2025 ‘Oumuamua will reach the outer edge of the Kuiper Belt, and then the heliopause — the edge of the Solar System — in November 2038.

How to Give Oumuamua Name

The object was officially named 1I/2017 U1 by the International Astronomical Union (IAU), which is responsible for granting official names to bodies in the solar system and beyond. In addition to the technical name, the Pan-STARRS team dubbed it ‘Oumuamua (pronounced oh MOO-uh MOO-uh), which is Hawaiian for “a messenger from afar arriving first.”

SOURCE BY - NASA DEPARTMENT

DISCOVERY OF OUMUAMUA: First Interstellar Object 2019

Eternally Collapsing Object (ECO)

An Eternally Collapsing Object (ECO) is a compact star that resembles a ball of fire, and it is so hot that its radiation helps it stay put despite its intense pull of gravity. Being extremely compact, ECOs mimic mathematical “Black Holes” in many ways, but there are observational reasons to believe that the so-called astrophysical “Black Holes” are really ECOs:

The eruptions and jet formations from the black hole candidates are better understood if they are indeed hot balls of fire rather than a cold piece of vacuum with an imaginary surface from which “nothing, not even light can escape”.

ECOs however asymptotically shrink towards the mathematical Black Hole state of infinite compactness.

Suppose a massive spherical star started collapsing due to its own weight. Then by Newtonian physics, sooner or later, in a FINITE time, the entire star material should collapse to a geometrical point forming a SINGULARITY where the density of matter and strength of / are infinite. It must be so because in Newtonian physical space-time has an absolute meaning and the (initial) actual/locally measured radius of the star, say R0, is Fixed & Finite:

In contrast, in Einstein’s “General Relativity” (GR), in the presence of gravity, the actual interior or PROPER radius (L) is always larger than the externally perceived radius (R);

i.e., L = a*R > R, because of a>1. Furthermore, the value of “a” will always increase if the strength of self-gravity will increase; i.e., if the star would get more compact. In fact, in strong gravity, similar stretching (dilation) happens for time intervals too.

In this sense, in GR, space-time is like a rubber membrane which gets stretched with the increase of gravity:

Therefore, there is no a-prior certainty that a Newtonian type singularity must occur for GR collapse. Despite this, even after the introduction of GR, most general relativists believe in essentially the same Newtonian picture of gravitational collapse by which collapsing matter must converge in a point-like singularity in a finite ( proper/ physically measured) time. This is, unfortunately, the case though not only Einstein himself but also many great physicists like.

Sir Arthur Eddington, the best theoretical astrophysicist ever, Sir Paul Dirac (Nobel Laureate), one of the finest theoretical physicists ever,

Nathan Rosen and many others argued that this picture must be incorrect and the star must not collapse beyond what is known as its “Schwarzschild Radius”:

Here G is the gravitational constant and c is the speed of the light. For a star

like the Sun,~3 km and for star ten times more massive, Rs ~30 km.

In particular, Dirac’s comment made in Proc. Royal Soc. (London) A 270, 354 (1962), was:

“The mathematics can go beyond the Schwarzschild Radius and get inside, but I would maintain that this inside region is not physical space, because to send a signal inside and get it out again would take an infinite time, so I feel that the space inside the Schwarzschild radius must belong to a different universe and should not be taken into account in any physical theory”.

Such objections, however, got ignored because of two misleading mathematical developments, and effectively mathematics beat physics:

1. In 1939, two American physicists J.R. Oppenheimer & H. Snyder APPARENTLY showed that for general relativistic collapse of a DUST (a fictitious form of matter having no pressure at all!), not only does the DUST collapse to a singularity but a Black Hole (BH) is formed whose imaginary surface lies at Rs and nothing can escape from within this surface, now called, an Event Horizon .

2. Further, in 1965, while studying generic gravitational collapse, Sir Roger Penrose, a highly regarded mathematical physicist, ASSUMED that for continued gravitational collapse, sooner or later, a one way trap – door will form, and once the collapsing matter would enter this trap-door it will be doomed to hurtle inwards no matter how much powerful rocket it would employ to escape . He essentially ASSUMED that at the advanced stages of collapse, any section of the star having mass M and radius R must plunge within its running Schwarzschild radius:

R(t) =2 G M(t)/c2 ----------------- (1)

Obviously, this idea was a sort of generalization of the concept of the formation of an “Event Horizon” and got known as a “TRAPPED SURFACE”. Once this ASSUMPTION was made, then under reasonable physical conditions, the collapsing matter must again end up in a singularity, as intended. Ideally, Penrose should have proved the inevitability of the formation of a trapped surface.

But instead, he just ASSUMED the crucial Eq.(1) which he was expected to prove or at least attempt to prove! Nevertheless, post-1965, most of the mathematical relativists adopted the same ASSUMPTION. If a singularity will indeed form, the space-time membrane, after the initial dip, would suddenly be pinched off and the stellar matter would hit the BOTTOM of the pit:

Black Hole Singularity

It appears that only one humble mathematical physicist Kriele offered proof that at least for a homogeneous sphere, there cannot be any trapped surface.

http://seminariomatematico.dm.unito.it/rendiconti/cartaceo/50-1/147.pdf

Unfortunately, Kriele could publish his result only in an obscure journal and maybe sensing trouble, he himself NEVER cited his important result!

Given this backdrop, in 1998, completely unaware of Kriele’s work, I showed that spherical GR collapse cannot form “trapped surfaces” irrespective of whether the sphere is homogeneous or not. I published my proof in a peer-reviewed journal first in 2000 [4] and in a broader context later [5,6]. Note, if a trapped surface did not exist, then in principle, collapsing matter could form.

(i) Quasi-static Objects (Chandrasekhar mass limit becomes irrelevant for very hot stars),

(ii) Rebound due to heat, radiation or pressure, or

(iii) Simply keep on contracting indefinitely!

As already mentioned, while the last option is absurd in Newtonian physics, it is not so in GR. As gravity increases, the space-time membrane elongates/dips and forms a pit; a particle undergoing collapse has to slide down this membrane into the pit. And if indeed the strength of gravity would grow indefinitely (a–> Infinity) during the continued collapse, then the

GRAVITATIONAL PIT TOO COULD BECOME ENDLESS & BOTTOMLESS

If so, continued gravitational collapse should indeed become ETERNAL without the formation of a true singularity. Consequently, objects undergoing continued general relativistic collapse should end up as “Eternally Collapsing/Contracting Objects” rather than as Black Holes or “Naked Singularities” (Singularities not covered by any Event Horizon).

Later as I interacted with several American astrophysicists like Darryl Leiter, Stanley Robertson [7] and Norman Glendenning, it became clear that as the contracting object would become hotter and hotter under gravitational compression, the Radiation Pressure would become so strong that it would almost counterbalance the inward pull of gravity. And it is in this way that ECOs conceived in 1998 could be realized in real life [8, 9, 10, 11, 12].

ECO-Ball of Radiation

Thus ECOs are Extremely Hot and Relativistic Radiation Pressure Supported Stars

whose radius hovers just above the INSTANTANEOUS “Schwarzschild Radius” as conceived by Einstein, Eddington, Dirac, Rosen and many other great physicists.

These quasi-static ECOs are however always radiating (unlike true black holes) and losing mass-energy by the E= Mc2 formula. Suppose as a given ECO radiates and it's mass decreases, it contracts by 1cm. But as it contracts, its gravity becomes a bit stronger and dips the rubber membrane of internal space maybe by 1.1 cm. Thus it becomes a hopeless unending chase for singularity.

Recall that the Acceleration Due to Gravity as measured by any static observer on the Event Horizon of a black hole is g=Infinity (limitless).

(And this is the reason, why nothing can escape from within the EH)

Similarly, gravitational red-shift of the EH, as seen by a distant observer too is infinite: z=Infinity (unbounded, limitless)

This means that, even if an extremely energetic gamma-ray photon would start its journey near the EH, its energy would reduce by a factor of (1+z) as it would reach the distant astronomer. And since z=∞, no energy would reach the astronomer, and he would see the EH as black.

In contrast, on the surface of an ECO, ” g’” and “z” are extremely large, maybe thousands or even million times larger than the corresponding values for a Pulsar/Neutron Star which is supposed to be an extremely compact star (object). Accordingly, to a distant observer, an ECO appears almost as a “black hole” though it has a physical surface made of hot plasma (fire) unlike the fictitious mathematical surface of a black hole (Event Horizon). As mentioned, it is the relentless heating due to extreme gravitational compression which makes the ECO essentially a “Ball of Fire”.

- ABSOLUTE GROUND STATE OF COLLAPSE

In classical physics, an absolute ground state is characterized by E=0 . So, if a star would really become “dead” and the “singularity” would be its “dead body” or the absolute ground state, then the singularity too should have

E=Mc2= 0; i.e., M=0:: Expected Mass of Singularity

Now recall that, the interior of an ASSUMED (static) BH is swept clear of all matter/radiation because nothing can stay put inside its ”trapped region”, and the “singularity” is the only source of matter/energy. Thus it is expected that a (neutral) BH may have unique mass M=0! And indeed it was found that though the mathematical “black hole” solution is a correct one, BHs nevertheless have unique gravitational mass
M=0 [13, 14].

Note, in GR, an occurrence of M=0 need not mean the absence of matter! This is so because gravitational mass is the sum total of all sources of energy including the NEGATIVE self-gravitational energy. Thus,
an occurrence of M=0 may imply that the (negative) self-gravitation has been so extreme that it has off-set the all other sources of energy like rest mass-energy of protons, neutrons constituting the star and all internal energies (heat, pressure etc).

And such proof confirms that the so-called BLACK HOLE CANDIDATES having huge masses cannot be true black holes.

In effect, the ECOs strive to become true mathematical black holes that have the unique mass M=0 [13]. However, an ECO never succeeds to attain this state. As an ECO continues to slide down the self-created pit, the pit gets deeper and deeper, i.e. the depth increases eternally :

Actually, near the singularity,” a” tends to blow up making interior radius L too blow up! Simultaneously, both “g” and “z” –> Infinity. Thus as if every sphere would internally be stretched to Infinity if self-gravitation would push them towards the gravitational singularity. Although such a scenario is weird, absurd from the point of view of Newtonian gravity, it is plausible in GR; as if all objects contain an Infinity within themselves!

Note, in principle, one should solve Einstein’s equations analytically to see whether singularities are indeed formed or not. Unfortunately, even the infinitely simplified Newtonian collapse equations cannot be solved analytically (or numerically, without many simplified assumptions) because one does not know the exact equation of state of matter and radiation transportation properties at extremely high density and temperature. Thus, so far no-body has analytically solved Einstein’s equations for truly realistic cases (without making various assumptions) to show that singularities are formed! And the only way one can solve collapse equations analytically and without making convenient simplifications is by setting pressure=0! Of course, one must not hope that such an extremely unrealistic assumption would ever lead to a true physical picture.

In hindsight, Penrose overlooked that

(a) Gravitational collapse must be radiative, and the value of M and Rs must keep on decreasing [15], and there is no fixed goal post which collapsing matter can target

(b) Also, unlike what he ASSUMED, pressure always decreases effective gravity and continues to oppose collapse in general relativity as it does so in Newtonian gravity [16].

Effectively, at least for a spherical case, Penrose assumed the gravitational free-fall of the star by ignoring resistive effects like pressure gradient and radiation/ heat flow, etc.

In fact, what Roger Penrose, John Wheeler, and many GR experts forgot was the intuitive comments by none other than Oppenheimer & Snyder THEMSELVES before they embarked on the idealistic, unrealistic computation about gravitational collapse:

“Physically such a singularity would mean that the expressions used for the energy-momentum tensor do not take into account some essential physical fact which would really smooth the singularity out. Further, a star in its early stages of development would not possess a singular density or pressure, it is impossible for a singularity to develop in a finite time.”

And maybe research works cited here have indeed substantiated the above comment made in 1939 by the persons who are (incorrectly) credited with proving the inevitability of formations of “Black Holes’’ and “Singularities’’ in GR!.

Incidentally, in 2011, it was shown that even the apparent formation of “Trapped Surface” and “Black Hole” in Oppenheimer & Snyder Collapse was only a mathematical illusion and physically it does not correspond to any gravitational collapse at all as because when pressure is strictly zero, density too is zero, and there is no matter, no gravity [17]!

RADIATION PRESSURE SUPPORTED STARS (RPSS)

Most of the stars we know of, resist their gravity by means of gas pressure, i.e., the pressure due to electrons, ions and atoms. But, in the 1960s, Sir Fred Hoyle, who many believe should have got the Nobel not once but twice, and Nobel Laureate William Fowler first pointed out that stars can be supported by radiation pressure alone if they would be sufficiently hot:
WillFowler

There is NO UPPER MASS LIMIT FOR RPSSs
They went on to suggest that QUASARS contain such hot RPSSs rather than true BHs. Similarly, ECOs too are hot RPSSs. But there are an important difference

The RPSSs conceived by Hoyle & Fowler were strictly static Newtonian stars (z <<1 1="" an="" and="" ascribed="" central="" efficiency="" energy="" generation="" having="" nbsp="" power="" source="" span="" their="" thermonuclear="" to="" was="">a la Sun; and the possibility of power generation by slow gravitational contraction was ignored. Thus the RPSSs of Hoyle & Fowler were perceived to run out of nuclear fuel.

As the star contracts and becomes more compact, its negative gravitational potential energy becomes even more negative. Then, in order to conserve total energy, the star creates additional positive energy. Part of this positive energy makes the star even hotter, and the rest of it is radiated away, a process explained independently by Lord Kelvin & Hermann von Helmholtz approx. 150 years ago:

The net energy which can be tapped in this way is 100%:
E0=M0 c2

the initial mass-energy of the star. In this case, there is no need for invoking any specific fuel! Furthermore, ECOs are dynamic solutions and they are extremely relativistic with z>>1.

As the exterior radius of the contracting star R(t) shrinks, Rs = 2GM(t)/c2 too shrinks (very slowly) because of loss of mass by radiation. So as if there is a perennial chase R(t) –>Rs = 2GM(t) /c2 , and by the time R(t) would catch up with Rs, both would attain nil values as the final ground state must correspond to E=Mc2 =0. Yet, it is likely that
L(t)= a(t) R(t)–>;∞, because of a(t) –>;∞
infinitely fast!

Eternally Collapsing Object (ECO) Explained 2019

PLUTO PLANET – WHY PLUTO IS NOT PLANET ?

Pluto

Pluto was discovered in 1930 and assigned as the ninth planet in our solar system. However, in 2006, the status was withdrawn and reassigned as a dwarf planet.
The name Pluto was proposed by Venetia Burney, an 11-year-old girl from England. The Student Dust Counter was named in her honour.
By the time we discovered and stripped Pluto of its planet status, the icy object had not completed an entire orbit around the Sun.
Pluto is the largest KBO and the second most massive.
One-third of Pluto is water ice-almost equal to three times the water in all our oceans put together. The remaining two-thirds is rock.
It takes 248 earth years for Pluto to orbit around the Sun.
The sunlight on Pluto is as intense as moonlight on Earth.
It takes five hours for the sunlight to reach Pluto (as against 8 mins to Earth).
Pluto has a retrograde rotation (opposite to that of the Earth). So, on Pluto the Sun rises in the west and sets in the east.
Pluto is a cold planet with an average temperature of about-230oC.
The feeble light on Pluto makes the days appear dark, and one can see stars doing the day.
on Pluto, sunrise to sunset is about a week long.
Pluto has a thin atmosphere mostly of nitrogen ice, carbon monoxide and methane.
The surface is rocky and believed to be having cryovolcanoes and geysers.

Why is Pluto not a planet & Where is

Pluto planet?

Pluto has only two of the three necessary characteristics to be called a planet. Pluto has not cleared its neighborhood or orbit. But, of course, there are additional details.

The simplest reason that Pluto is not a planet is that planet experts say so, and this is their job. But you may be looking for a more detailed explanation.

Let's look at what defines a planet. This could be a very long and tedious discussion, because "planet" is an ancient concept used long before scientists knew very much about them. Also, frankly, in many areas of science the definition of a thing, perhaps counter-intuitively to non-scientists, is often pretty irrelevant to its study.

Definitions that change over time that are never quite in line with the phenomenon being observed etc. may seem like an impediment to science, but they often are not. The definition of a "gene" has changed dramatically as we've learned more about them, but this shifting description has not hampered genetic research. To some extent, this may be the same with planets. A "planetologist" who studied Pluto back when it was still counted as a planet would not have to find a new job when our solar system went from 9 to 8.

The International Astronomical Union has settled on a set of definitions of solar system bodies, which includes planets, dwarf planets (which are mostly minor planets), small solar system bodies, trans-Neptunian objects which also might be called plutoids (those are also minor planets) and some small solar system bodies (including some comets) and satellites, and satellites are, of course, things that go around things that are not the Sun. Confused? Probably, but that is not a big problem because while these various identified flying objects have complex overlapping categorical status, one type of object does not. Planets are planets and they are not anything other than planets.

To be a planet, you have to be in orbit around the Sun. This would rule out the Moon, which, if it was in orbit around the Sun instead of the Earth, could well be a planet.

To be a planet the object has to have sufficient mass to have been shaped by gravity to be (mostly) a globe. This depends on various things so at the low end of the mass spectrum there will be different masses and different sizes of things that don't quite make it to globular status.

To be a planet the object has to have cleared its orbit. In other words, as an object orbits around the sun, it is likely to bump into other objects. Over a period of time, the object has finished bumping into everything it is likely to bump into and thereafter has only a low probability of bumping into something. That does not rule out something bumping into the object, of course.

A globe-shaped object that goes around the sun but that has not cleared its orbit is classified as a "dwarf planet." This is of course historically contingent. In the early days of a solar system, perhaps there would be large star-circling round things that have not yet cleared their orbit. This speaks to the strangeness of definitions alluded to above. The definition we use today to classify our solar system's objects applies to a solar system developed to the extent ours is developed. The IAU nomenclature would probably need revisions if applied to all planetary-star systems in the Universe.

This scheme is not without its critics and there is indeed debate. Some of that debate is a bit nitpicky but still interesting. For example, Alan Stern, with NASA, notes that many planets have not really cleared their orbit, noting in relation to the Pluto controversy, "If Neptune had cleared its zone, Pluto wouldn't be there." Yes, apparently heavenly bodies have irony.

Anyway, as implied, Pluto is not classed as a planet because it has not cleared its orbit. Therefore it is a Dwarf Planet. Since it is far away (farther than Neptune) it also gets classed as a Trans-Neptunian Object. Furthermore, it is a Plutoid. That is simply a newer term applied to Trans-Neptuina dwarf planets.

The term Plutoid, then, refers to a dwarf planet, which for various reasons is apparently always specifically an ice dwarf, which is a trans-Neptunian body (orbiting most of the time beyond Neptune) that is sufficiently massive to be shaped like a globe. This term, plutoid, is officially adopted A plutoid or ice dwarf is a trans-Neptunian dwarf planet, i.e. a body orbiting beyond Neptune that is large enough to be rounded in shape. The term plutoid was adopted by the International Astronomical Union's Committee on Small Bodies Nomenclature, but not by the working group on Planetary System Nomenclature. So you can use Plutoid or Dwarf Planet, or Ice Dwarf, depending on whom you wish to annoy.

Pluto, Eris, Haumea, and Makemake are the only known Plutoids. They are small enough and far enough away that more could be discovered.

I would say, "the only known Plutoids that are accepted as dwarf planets by the IAU". More already have been discovered, even recommended for classification as dwarf planets, but aren't officially there yet:

Orcus, Quaoar, Sedna, Salacia, Varuna, Ixion -- and those are just one that have non-numerical names...

Then there is the first dwarf planet to be discovered -- and it's not Pluto: Ceres, the planetino whose namesake gave us "cereal". It was discovered in 1801, before even Neptune, and was initially regarded as a planet; it's now officially just another dwarf planet. (And it was visited with a spacecraft ahead of Pluto.)

The practical problem that led to the need to come up with this definition of a planet is one of sheer number: If we were to accept Pluto as "The Ninth Planet", it would immediately have to take 10th place, as Ceres would be forced to return to planetary status as #5, displacing Jupiter. Then, joining Pluto as #10, would be the above-mentioned plutinos, plus, eventually, the other existing discoveries, and then future discoveries.

Anyone up to memorizing the names of the 537 "planets"?? No, me neither. We need a manageable number, and 8 is it.

The problem with poor Pluto is that it is indistinguishable from all its other Trans-Neptunian brethren. Sentimentality does not make it special, unfortunately. Nor would it in being the first discovered, which it also isn't. Too bad Clyde Tombaugh didn't keep looking, but he was simply lucky to have spotted Pluto in the first place...

In short, Pluto is not a planet because it never was. A case of mistaken identity. (Hello, Alan Stern?) Still, paying it a visit is very cool, and this issue doesn't detract from that.

PLUTO PLANET – WHY PLUTO IS NOT PLANET ?

The first image of a black hole - Image of the black hole at the center of the Messier 87 galaxy shows the effect of the accretion disc as well as the black hole's "shadow" in the center. (Courtesy: Akiyama et al and ApJL)

The first direct visual evidence of a black hole and its “shadow” has been revealed today by astronomers working on the Event Horizon Telescope (EHT). The image is of the supermassive black hole that lies at the center of the huge Messier 87 galaxy, in the Virgo galaxy cluster. Located 55 million light-years from Earth, the black hole has been determined to have a mass 6.5-billion times that of the Sun, with an uncertainty of 0.7 billion solar masses. Although black holes are inherently invisible because of their extreme density and gravitational field, the researchers have managed to obtain images near the point where matter and energy can no longer escape – the so-called event horizon.

“We are giving humanity its first view of a black hole — a one-way door out of our universe,” says Sheperd Doeleman of the Haystack Observatory at the Massachusetts Institute of Technology (MIT) who is the EHT’s lead astronomer. “This is a landmark in astronomy, an unprecedented scientific feat accomplished by a team of more than 200 researchers.” Doeleman says that the result would have “presumed to be impossible just a generation ago”, adding that breakthroughs in technology and the completion of new radio telescopes over the past decade have allowed researchers to now “see the unseeable”. The results, announced today at multiple press conferences around the world, have been published in six papers in a special issue of Astrophysical Journal Letters, which is published by the Institute of Physics on behalf of the American Astronomical Society.

Flashing Disk

Supermassive black holes are thought to lie at the centers of most galaxies in the universe, and astronomers are keen to decipher their key properties – such as how their extreme gravity affects the space-time around them, and how some of them fuel the massive jets of material that spew out from the galaxies that host them. A key feature of a black hole is its event horizon – the boundary at which even light cannot escape its gravitational pull, as the velocity required to do so would be greater than the speed of light, which is forbidden by Einstein’s general theory of relativity. And while that theory has passed many tests, researchers want to see how well it holds up at the “ultimate proving ground” – a black hole’s edge.

Despite their name, black holes are not, however, all dark. The gas and dust trapped around them in an accretion disc are so compact that it is often heated to billions of degrees even before the matter eventually succumbs to the black hole, making them glow brightly. Indeed, general relativity also predicts that a black hole will have a “shadow” around it, measuring around three times larger than the event horizon. The shadow is of great interest as its size and shape depend mainly on the mass and – to a lesser extent – on any possible spin of the black hole, thereby revealing its inherent properties.

“If immersed in a bright region, like a disc of glowing gas, we expect a black hole to create a dark region similar to a shadow — something predicted by Einstein’s general relativity that we’ve never seen before,” says Heino Falcke from Radboud University in the Netherlands, who chair the EHT’s science council. “This shadow, caused by the gravitational bending and capture of light by the event horizon, reveals a lot about the nature of these fascinating objects.”

An orange on the black hole Colour

To directly observe the black hole at the center of Messier 87 – dubbed M87* – astronomers require a telescope with an angular resolution comparable to the event horizon, which is on the order of tens of micro-arcseconds across. But to achieve that resolution with an ordinary telescope – which is like spotting an orange on the surface of the Moon – would require a dish the size of our planet, which is clearly impractical.

EHT astronomers instead use the radio-astronomy technique of very-long-baseline interferometry (VLBI). It involves picking up radio signals from an astronomical source by a network of individual radio telescopes and telescopic arrays scattered across the globe. The EHT, which first turned on in 2007, consists of eight radio dishes in six different locations across the globe all operating at a wavelength of 1.3 mm. These telescopes include the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, the South Pole Telescope (SPT) in Antarctica, and the IRAM 30-meter telescope in Spain (see image left). The distance between individual EHT telescopes – known as the “baseline” – ranges from 160 m to 10 700 km.

Schematic of the Event Horizon Telescope

Global Networking: Event Horizon Telescope incorporates signals from eight radio telescope observatories, including the Atacama Large Millimeter / Submilimeter Array (ALMA) in Chile and the South Pole Telescope (SPT) in Antarctica. Solid lines represent the telescope involved with an overview of M87 *, while the dashed lines are used for calibration run. (Courtesy: Akima et al and APJL)

The signals received at each individual telescope dish in the network are precisely tagged with a very accurate time stamp, normally using an atomic clock at each location. Each telescope produces roughly 350 terabytes per day, which is stored on high-performance helium-filled hard drives. The data is then later correlated and used to build up a complete image by supercomputers that are located at the Max Planck Institute for Radio Astronomy in Bonn, Germany, and the MIT Haystack Observatory in the US. This process makes the EHT the highest-resolution instrument on Earth, capable of taking images up to 2000 times better resolution than the Hubble Space Telescope and able to resolve features as small as 20 micro-arcseconds.

An astronomy "milestone"

As a black hole’s size is proportional to its mass, the more massive a black hole, the larger its shadow. Thanks to its enormous mass and relative proximity, M87* was predicted to be one of the largest viewable from Earth — making it a perfect target for the EHT. Astronomers observed M87* on 5, 6, 10 and 11 April 2017 with the telescope taking a series of scans of three to seven minutes in duration each day.

These multiple independent EHT observations have now resulted in the first image of a black hole including its shadow, revealing a ring-like structure with a dark central region. The diameter of the ring is 42 micro-arcseconds with a width less than 20 micro-arcseconds. By comparing the image with theoretical models such as general relativistic magnetohydrodynamic (GRMHD) simulations, the observed image is consistent with expectations for the shadow of a Kerr black hole – one that is uncharged and rotates about a central axis – as predicted by the general relativity.

The researchers were able to deduce the mass of the M87* at 6.5-billion times that of the Sun. Previous estimates — based on models as well as spectroscopic observations of the galaxy by the Hubble Space Telescope — range between 3.5-7.7 billion solar masses. EHT scientists also deduced the radius of the event horizon as 3.8 micro-arcseconds. They also found that the rotation of the black hole is in a clockwise direction and that it is spin points away from us. The brightness in the lower part of the image is due to the relativistic movement of material in a clockwise direction as seen by us so that it is moving towards us.

Image of black hole and simulations Shadowlands of black hole : From left to right, EHT observations of M87* taken on 6 April 2017; a simulation of M87*; simulation convoluted to the resolution of the Event Horizon Telescope. (Courtesy: Akiyama et al and ApJL)

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