Radio signal reveals supernova origin

Artist impression of the double star system with a compact white dwarf star accreting matter from a helium-rich donor companion, surrounded by dense and dusty circumstellar material. It was the interaction of the exploded star and the material left over from this companion that gave rise to the strong radio signal, the conspicuous helium lines in the optical spectra and the infrared emission from SN 2020eyj.
Video Credit: Adam Makarenko/W. M. Keck Observatory

In the latest issue of the journal Nature, an international team including astronomers from University of Turku reveal the origin of a thermonuclear supernova explosion. Strong emission lines of helium and the first detection of such a supernova in radio waves show that the exploding white dwarf star had a helium-rich companion.

Thermonuclear (Type Ia) supernovae are important for astronomers since they are used to measure the expansion of the Universe. However, the origin of these explosions remains an open question. While it is established that the explosion is that of a compact white dwarf star somehow accreting too much matter from a companion star, the exact process and the nature of the progenitor is not known. The new discovery of supernova SN 2020eyj established that the companion star was a so-called helium star that had lost much of its material just prior to the explosion of the white dwarf.

“Once we saw the signatures of strong interaction with the material from the companion, we tried to detect it also in radio emission”, explains Erik Kool, post-doc at the Department of Astronomy at Stockholm University and lead author of the paper. “The detection in radio is actually the first one of a Type Ia supernova – something astronomers have tried to do for decades.”

Are Earth and Venus the only volcanic planets? Not anymore.

LP 791-18 d is an Earth-size world about 90 light-years away. The gravitational tug from a more massive planet in the system, shown as a blue disk in the background, may result in internal heating and volcanic eruptions – as much as Jupiter’s moon Io, the most geologically active body in the solar system.
Illustration Credit: NASA’s Goddard Space Flight Center/Chris Smith/KRBwyle

Imagine an Earth-sized planet that’s not at all Earth-like. Half this world is locked in permanent daytime, the other half in permanent night, and it’s carpeted with active volcanoes. Astronomers have discovered that planet. 

The planet, named LP 791-18d, orbits a small red dwarf star about 90 light years away. Volcanic activity makes the discovery particularly notable for astronomers because volcanism facilitates interaction between a world’s interior and its exterior.

“Why is volcanism important? It is the major source contributing to a planetary atmosphere, and with an atmosphere you could have surface liquid water — a requirement for sustaining life as we know it,” said UC Riverside astrophysicist Stephen Kane. 

Astronomers already knew about two other worlds in this star system, LP 791-18b and c. The outer planet, c, is about 2.5 times Earth’s size, and nearly nine times its mass. 

Latest research provides SwRI scientists close-up views of energetic particle jets ejected from the sun

Southwest Research Institute (SwRI) scientists observed the first close-up views of the source of jets of energetic particles expelled from the Sun. The high-resolution images of the solar event were provided by ESA and NASA Solar Orbiter, a Sun-observing satellite launched in 2020.
Image Credit: Courtesy of SwRI

Southwest Research Institute (SwRI) scientists observed the first close-ups of a source of energetic particles expelled from the Sun, viewing them from just half an astronomical unit (AU), or about 46.5 million miles. The high-resolution images of the solar event were provided by ESA’s Solar Orbiter, a Sun-observing satellite launched in 2020.

“In 2022, the Solar Orbiter detected six recurrent energetic ion injections. Particles emanated along the jets, a signature of magnetic reconnection involving field lines open to interplanetary space,” said SwRI’s Dr. Radoslav Bucik, the lead author of a new study published this month in Astronomy & Astrophysics Letters. “The Solar Orbiter frequently detects this type of activity, but this period showed very unusual elemental compositions.”

Saturn’s rings younger than previously thought — just a few hundred million years

New research reveals that Saturn's rings are much younger than the planet itself.
Photo Credit: NASA/JPL/Space Science Institute.

Saturn’s rings are much younger than scientists once thought, according to new research from Indiana University Professor Emeritus of Astronomy Richard Durisen — and they are not here to stay.

For decades, there has been debate about the origin of Saturn’s icy rings. But according to two new studies from Durisen, published in Icarus, the rings are no more than a few hundred million years old — much younger than the planet itself, which formed 4.5 billion years ago. In fact, Durisen said the rings may well have formed when dinosaurs were still walking on the Earth.

Durisen and co-author Paul Estrada, a research scientist at NASA’s Ames Research Center in California’s Silicon Valley, also concluded that the rings will last only another few hundred million years at most.

“Our inescapable conclusion is that Saturn’s rings must be relatively young by astronomical standards, just a few hundred million years old,” Durisen said. “If you look at Saturn’s satellite system, there are other hints that something dramatic happened there in the last few hundred million years.”

Tidal Shocks Can Light up the Remains of a Star Being Pulled Apart by a Black Hole

In a Tidal Disruption Event, a star moves close enough to a supermassive black hole so that the gravitational pull of the black hole bends the star until it is destroyed (image 1). The stellar matter from the destroyed star forms an elliptical stream around the black hole (image 2). Tidal shocks are formed around the black hole as the gas hits itself on its way back after circling the black hole (image 3). The tidal shocks create bright outbursts of polarized light that can be observed in optical and ultraviolet wavelengths. Over time, the gas from the destroyed star forms an accretion disk around the black hole (image 4) from where it is slowly pulled into the black hole. The scale of the image is not accurate.
Full size image
 Image Credit: Jenni Jormanainen

The Universe is a violent place where even the life of a star can be cut short. This occurs when a star finds itself in a "bad" neighborhood, specifically near a supermassive black hole. 

These black holes weigh millions or even billions of times the mass of the Sun and typically reside in the centers of quiet galaxies. As a star moves closer to the black hole, it experiences the ever-increasing gravitational pull of the supermassive black hole until it becomes more powerful than the forces that keep the star together. This results in the star being disrupted or destroyed, an event known as a Tidal Disruption Event (TDE).

“After the star has been ripped apart, its gas forms an accretion disk around the black hole. The bright outbursts from the disk can be observed in nearly every wavelength, especially with optical telescopes and satellites that detect X-rays,” says Postdoctoral Researcher Yannis Liodakis from the University of Turku and the Finnish Centre for Astronomy with ESO (FINCA).

First-of-its-kind measurement of Universe’s expansion rate weighs in on longstanding astronomy debate

Image Credit: Patrick Kelly, University of Minnesota

Thanks to data from a magnified supernova, a team led by University of Minnesota researchers has successfully used a first-of-its-kind technique to measure the expansion rate of the Universe. Their data provide insight into a longstanding debate in the field of astronomy and could help scientists more accurately determine the Universe’s age and better understand the cosmos.

The work is divided into two papers, published in Science, one of the world’s top peer-reviewed academic journals, and The Astrophysical Journal, a peer-reviewed scientific journal of astrophysics and astronomy.

In astronomy, there are two precise measurements of the expansion of the Universe, also called the “Hubble constant.” One is calculated from nearby observations of supernovae, and the second uses the “cosmic microwave background,” or radiation that began to stream freely through the Universe shortly after the Big Bang. 

However, these two measurements differ by about 10%, which has caused widespread debate among physicists and astronomers. If both measurements are accurate, that means scientists’ current theory about the make-up of the universe is incomplete.

Study could help solve mystery of the disappearing twins

An image of the binary stars Alpha Centauri A (left) and Alpha Centauri B, taken by the Hubble Space Telescope.
Image Credit: NASA

When supermassive stars are born, they’re almost always paired with a twin, and the two stars normally orbit one another.

But astronomers at UCLA’s Galactic Center Group and the Keck Observatory have analyzed over a decade’s worth of data about 16 young supermassive stars orbiting the supermassive black hole at the center of the Milky Way galaxy. Their findings, published today in the Astrophysical Journal, reveal a startling conclusion: All of them are singletons.

But why? Are the stars, which are about 10 times larger than our sun, being formed alone in the hostile environment around the black hole? Have their “twins” been kicked out by the black hole? Or have pairs of stars merged to form single stars?

The findings support a scenario in which the central supermassive black hole drives nearby stellar binaries to merge or be disrupted, with one of the pair being ejected from the system.

Testing a theory of supermassive black holes with 100 newly described 'blazars'

For some supermassive black holes, matter outside the event horizon is propelled at high speed in a jet that can be detected across the universe. When the jet is pointed in the direction of the Earth, it is typically called a blazar. Penn State researchers have characterized more than a hundred relatively dim blazars and used them to test a contentious theory of blazar emissions.
Illustration Credit: NASA/JPL-Caltech/GSFC

More than a hundred blazars — distant and active galaxies with a central supermassive black hole that drives powerful jets — have been newly characterized by Penn State researchers from a catalog of previously unclassified high-energy cosmic emissions. The new blazars, which are dim relative to more typical blazars, have allowed the researchers to test a controversial theory of blazar emissions, informing our understanding of black hole growth and even theories of general relativity and high-energy particle physics.

A paper describing the blazars and the theory has been accepted for publication in the Astrophysical Journal, and the peer-reviewed accepted version appears online on the preprint server arXiv.

Supermassive black holes can be millions or billions of times the mass of our sun. In some cases, matter outside of the black hole’s event horizon is propelled in a jet, accelerating to nearly the speed of light and sending emissions across the universe. When the jet happens to be pointed directly at the Earth, the system is typically called a blazar.

Celestial monsters at the origin of globular clusters

Scientists have found strong evidence that supermassive stars existed within globular clusters when they formed 13 billion years ago. Here, an image of the globular cluster M13, 22 000 light years from Earth, consisting of a million stars squeezed into a space 150 light years across.
Image Credits: HST STScI NASA ESA

A team from the universities of Geneva, Paris and Barcelona has found strong evidence that supermassive stars can explain the anomalies observed in large clusters of stars.

Globular clusters are the most massive and oldest star clusters in the Universe. They can contain up to 1 million of them. The chemical composition of these stars, born at the same time, shows anomalies that are not found in any other population of stars. Explaining this specificity is one of the great challenges of astronomy. After having imagined that supermassive stars could be at the origin, a team from the Universities of Geneva and Barcelona, and the Institut d’Astrophysique de Paris (CNRS and Sorbonne University) believes it has discovered the first chemical trace attesting to their presence in globular proto-clusters, born about 440 million years after the Big Bang. These results, obtained thanks to observations by the James-Webb space telescope, are to be found in Astronomy and Astrophysics.

Globular clusters are very dense groupings of stars distributed in a sphere, with a radius varying from a dozen to a hundred light years. They can contain up to 1 million stars and are found in all types of galaxies. Ours is home to about 180 of them. One of their great mysteries is the composition of their stars: why is it so varied? For instance, the proportion of oxygen, nitrogen, sodium and aluminum varies from one star to another. However, they were all born at the same time, within the same cloud of gas. Astrophysicists speak of "abundance anomalies".

Researchers measure the light emitted by a sub-Neptune planet’s atmosphere for the first time

U-M graduate student Isaac Malsky, a co-author of the study, ran three-dimensional models for the planet, testing models with and without clouds and hazes, to see how these aerosols shape the thermal structure of the planet and help interpret the data.
Illustration Credit: NASA/JPL-Caltech/R. Hurt (IPAC)

For more than a decade, astronomers have been trying to get a closer look at GJ 1214b, an exoplanet 40 light-years away from Earth.

Their biggest obstacle is a thick layer of haze that blankets the planet, shielding it from the probing eyes of space telescopes and stymying efforts to study its atmosphere. But now, NASA’s new JWST has solved that issue. The telescope’s infrared technology allows it to see planetary objects and features that were previously obscured by hazes, clouds or space dust, aiding astronomers in their search for habitable planets and early galaxies.

A team of researchers from the University of Michigan and University of Maryland used JWST to observe GJ 1214b’s atmosphere by measuring the heat it emits while orbiting its host star. Their results, published in the journal Nature, represent the first time anyone has directly detected the light emitted by a sub-Neptune exoplanet—a category of planets that are larger than Earth but smaller than Neptune.

Webb Finds Water Vapor, But from a Rocky Planet or Its Star?

This artist concept represents the rocky exoplanet GJ 486 b, which orbits a red dwarf star that is only 26 light-years away in the constellation Virgo. By observing GJ 486 b transit in front of its star, astronomers sought signs of an atmosphere. They detected hints of water vapor. However, they caution that while this might be a sign of a planetary atmosphere, the water could be on the star itself – specifically, in cool starspots – and not from the planet at all.  GJ 486 b is about 30% larger than the Earth and weighs three times as much. It orbits its star closely in just under 1.5 days.
Illustration Credit: NASA, ESA, CSA, Joseph Olmsted (STScI)

The most common stars in the universe are red dwarf stars, which means that rocky exoplanets are most likely to be found orbiting such a star. Red dwarf stars are cool, so a planet has to hug it in a tight orbit to stay warm enough to potentially host liquid water (meaning it lies in the habitable zone). Such stars are also active, particularly when they are young, releasing ultraviolet and X-ray radiation that could destroy planetary atmospheres. As a result, one important open question in astronomy is whether a rocky planet could maintain, or reestablish, an atmosphere in such a harsh environment.

To help answer that question, astronomers used NASA’s James Webb Space Telescope to study a rocky exoplanet known as GJ 486 b. It is too close to its star to be within the habitable zone, with a surface temperature of about 800 degrees Fahrenheit (430 degrees Celsius). And yet, their observations using Webb’s Near-Infrared Spectrograph (NIRSpec) show hints of water vapor. If the water vapor is associated with the planet, that would indicate that it has an atmosphere despite its scorching temperature and close proximity to its star. Water vapor has been seen on gaseous exoplanets before, but to date no atmosphere has been definitively detected around a rocky exoplanet. However, the team cautions that the water vapor could be on the star itself – specifically, in cool starspots – and not from the planet at all.

New black hole images reveal a glowing, fluffy ring and a high-speed jet

Scientists observing the compact radio core of M87 have discovered new details about the galaxy’s supermassive black hole. In this artist’s conception, the black hole’s massive jet is seen rising up from the center of the black hole. The observations on which this illustration is based represent the first time that the jet and the black hole shadow have been imaged together, giving scientists new insights into how black holes can launch these powerful jets. 
Illustration Credit: S. Dagnello (NRAO/AUI/NSF)

In 2017, astronomers captured the first image of a black hole by coordinating radio dishes around the world to act as a single, planet-sized telescope. The synchronized network, known collectively as the Event Horizon Telescope (EHT), focused in on M87*, the black hole at the center of the nearby Messier 87 galaxy. The telescope’s laser-focused resolution revealed a very thin glowing ring around a dark center, representing the first visual of a black hole’s shadow. 

Astronomers have now refocused their view to capture a new layer of M87*. The team, including scientists at MIT’s Haystack Observatory, has harnessed another global web of observatories — the Global millimeter VLBI Array (GMVA) — to capture a more zoomed-out view of the black hole.

The new images, taken one year after the EHT’s initial observations, reveal a thicker, fluffier ring that is 50 percent larger than the ring that was first reported. This larger ring is a reflection of the telescope array’s resolution, which was tuned to pick up more of the super-hot, glowing plasma surrounding the black hole. 

Scientists discover rare element in exoplanet’s atmosphere

Illustration Credit: Bibiana Prinoth

The rare metal terbium has been found in an exoplanet’s atmosphere for the first time. The researchers at Lund University in Sweden have also developed a new method for analyzing exoplanets, making it possible to study them in more detail.

KELT-9 b is the galaxy’s hottest exoplanet, orbiting its distant star about 670 light years from Earth. The celestial body, with an average temperature of a staggering 4,000 degrees Celsius, has excited the world's astronomers since its discovery in 2016. A new study in Astronomy & Astrophysics reveals discoveries about the scalding-hot oddball's atmosphere.

“We have developed a new method that makes it possible to obtain more detailed information. Using this, we have discovered seven elements, including the rare substance terbium, which has never before been found in any exoplanet's atmosphere”, says Nicholas Borsato, PhD student in astrophysics at Lund University.

Terbium is a rare earth metal that belongs to the so-called lanthanoids. The substance was discovered in 1843 by the Swedish chemist Carl Gustaf Mosander in the Ytterby mine in the Stockholm archipelago. The substance is very rare in nature, and 99 percent of the world's terbium production today takes place in the Bayan Obo mining district in Inner Mongolia.

Dark order in the universe

3D position and shape information for each galaxy helped to measure the magnitude of alignment relative to distant galaxies
Illustration Credit: KyotoU/Jake Tobiyama

Einstein would nod in approval. General relativity may apply even in the farthest reaches of the universe.

Now, scientists from international research institutions, including Kyoto University, have confirmed that the intrinsic alignments of galaxies have characteristics that allow it to be a powerful probe of dark matter and dark energy on a cosmological scale.

By gathering evidence that the distribution of galaxies more than tens of millions of light years away is subject to the gravitational effects of dark matter, the team succeeded in testing general theory of gravity at vast spatial scales. The international team analyzed the positions and orientations of galaxies, acquired from archived data of 1.2 million galaxy observations. With the help of available 3D positional information of each galaxy, the resulting statistical analysis quantitatively characterized the extent to which the orientation of distant galaxies is aligned.

Birth of a very distant cluster of galaxies from the early Universe

This image shows the protocluster around the Spiderweb galaxy (formally known as MRC 1138-262), seen at a time when the Universe was only 3 billion years old. Most of the mass in the protocluster does not reside in the galaxies that can be seen in the centre of the image, but in the gas known as the intracluster medium (ICM). The hot gas in the ICM is shown as an overlaid blue cloud.   The hot gas was detected with the Atacama Large Millimeter/submillimeter Array (ALMA), of which ESO is a partner. As light from the cosmic microwave background –– the relic radiation from the Big Bang –– travels through the ICM, it gains energy when it interacts with the electrons in the hot gas. This is known as the Sunyaev-Zeldovich effect. By studying this effect, astronomers can infer how much hot gas resides in the ICM, and show that the Spiderweb protocluster is in the process of becoming a massive cluster held together by its own gravity. 
Full Size Image
Image Credit: ESO/Di Mascolo et al.; HST: H. Ford

Using the Atacama Large Millimeter/submillimeter Array (ALMA), of which ESO is a partner, astronomers have discovered a large reservoir of hot gas in the still-forming galaxy cluster around the Spiderweb galaxy — the most distant detection of such hot gas yet. Galaxy clusters are some of the largest objects known in the Universe and this result, published today in Nature, further reveals just how early these structures begin to form.

Galaxy clusters, as the name suggests, host a large number of galaxies — sometimes even thousands. They also contain a vast “intracluster medium” (ICM) of gas that permeates the space between the galaxies in the cluster. This gas in fact considerably outweighs the galaxies themselves. Much of the physics of galaxy clusters is well understood; however, observations of the earliest phases of formation of the ICM remain scarce.

Previously, the ICM had only been studied in fully-formed nearby galaxy clusters. Detecting the ICM in distant protoclusters — that is, still-forming galaxy clusters – would allow astronomers to catch these clusters in the early stages of formation. A team led by Luca Di Mascolo, first author of the study and researcher at the University of Trieste, Italy, were keen to detect the ICM in a protocluster from the early stages of the Universe. 

Preserving the stars: light pollution and what you can do about it

Astrophysicist Ms Kirsten Banks explains what we can do to reverse the impact of "light glow".
Photo Credit: UNSW Sydney.

An astrophysicist from UNSW Sydney explains why it’s so important that we can all look up and see the stars. 

Astronomer Carl Sagan famously said that there were more stars in the universe than grains of sand on earth.  

It has been estimated that there are over 100 billion stars in the Milky Way galaxy. While there is a limit to how many stars we can see from earth with the naked eye, that number is dramatically reducing due to light pollution. 

“We should be able to see around 2500 stars with the naked eye on any night, and we can see about 125 of them at best in Sydney,” says astrophysicist, proud Wiradjuri woman and UNSW PhD candidate Ms. Kirsten Banks.

In fact, in a recent study published in Science, data collected by citizen scientists around the world found light pollution is increasing at a rate that is equivalent to the brightness of the sky doubling every eight years.  

This latest research continues to expose the extent to which we’re losing the darkness of our night sky. Not being able to look up and see the stars will have significant cultural impacts, but there are steps we can all be taking to reduce the effect of light pollution.

New Study Reveals Potential Link Between Two of Astronomy’s Most Mysterious Phenomena

Artist's conception of fast radio burst reaching Earth.
Illustration Credit: Jingchuan Yu, Beijing Planetarium

International team of scientists reports a possible correlation between gravitational waves from neutron star mergers and fast radio bursts; results could improve understanding of how some deep-space bursts occur.

The secrets of deep space may be starting to reveal themselves, as rapid advances in technology and stronger research collaborations are making it possible for astronomers to piece together cosmological clues like never before.

  In the March 27 issue of the journal Nature Astronomy, an international team of scientists shows for the first time a possible relationship between neutron star mergers and fast radio bursts (FRBs) – two of the most mysterious cosmological phenomena studied over the past two decades.

  The team, which includes researchers from UNLV, University of Western Australia (UWA), and Curtin University, reports on the observation of a deep space neutron star merger followed just 2 ½ hours later by an observed FRB. If confirmed, the correlation between the two events could unlock part of the mystery of how FRBs are generated.

  Fast radio bursts (FRBs) are millisecond-long pulses of electromagnetic radio waves that occur in deep space and produce the energy equivalent to the sun’s annual output. Most FRBs occur as one-off events, while others present as repeating bursts. Though their origins are still a bit of a mystery, the fraction of FRBs emitted as repeated bursts are likely produced by highly magnetized neutron stars known as magnetars.

Giant planet atmospheres vary widely, JWST confirms

 A ‘hot Jupiter’ called HD 149026b, is about 3 times hotter than the rocky surface of Venus, the hottest planet in our solar system.
Illustration Credit: NASA/JPL-Caltech

Gas giants orbiting our sun show a clear pattern; the more massive the planet, the lower the percentage of “heavy” elements (anything other than hydrogen and helium) in the planet’s atmosphere. But out in the Galaxy, the atmospheric compositions of giant planets do not fit the solar system trend, an international team of astronomers has found.

Using NASA’s James Webb Space Telescope (JWST), the researchers discovered that the atmosphere of exoplanet HD149026b, a ‘hot Jupiter’ orbiting a star comparable to our sun, is super-abundant in the heavier elements carbon and oxygen – far above what scientists would expect for a planet of its mass. In addition, the diagnostic carbon-to-oxygen ratio of HD149026b, also known as “Smertrios,” is elevated relative to our solar system.

A ‘hot Jupiter’ called HD 149026b, is about 3 times hotter than the rocky surface of Venus, the hottest planet in our solar system.

These findings, published in “High atmospheric metal enrichment for a Saturn-mass planet,” in Nature on [March 27] are an important first step toward obtaining similar measurements for a large sample of exoplanets in order to search for statistical trends, the researchers said. They also provide insight into planet formation.

Searching for life with space dust

Space dust. This piece of interplanetary dust is thought to be part of the early solar system and was found in our atmosphere, demonstrating lightweight particles could survive atmospheric entry as they do not generate much heat from friction.
Photo Credit: 2023 NASA CC-0

Following enormous collisions, such as asteroid impacts, some amount of material from an impacted world may be ejected into space. This material can travel vast distances and for extremely long periods of time. In theory this material could contain direct or indirect signs of life from the host world, such as fossils of microorganisms. And this material could be detectable by humans in the near future, or even now.

When you hear the words vacuum and dust in a sentence, you may groan at the thought of having to do the housework. But in astronomy, these words have different connotations. Vacuum of course refers to the void of space. Dust, however, means diffuse solid material floating through space. It can be an annoyance to some astronomers as it may hinder their views of some distant object. Or dust could be a useful tool to help other astronomers learn about something distant without having to leave the safety of our own planet. Professor Tomonori Totani from the University of Tokyo’s Department of Astronomy has an idea for space dust that might sound like science fiction but actually warrants serious consideration.

Surprisingly simple explanation for alien comet ‘Oumuamua’s weird orbit

An artist’s depiction of the interstellar comet ‘Oumuamua, as it warmed up in its approach to the sun and outgassed hydrogen (white mist), which slightly altered its orbit. The comet, which is most likely pancake-shaped, is the first known object other than dust grains to visit our solar system from another star.
Image Credit: NASA, ESA and Joseph Olmsted and Frank Summers of STScI

In 2017, a mysterious comet dubbed ‘Oumuamua fired the imaginations of scientists and the public alike. It was the first known visitor from outside our solar system, it had no bright coma or dust tail, like most comets, and a peculiar shape — something between a cigar and a pancake — and its small size more befitted an asteroid than a comet.

But the fact that it was accelerating away from the sun in a way that astronomers could not explain perplexed scientists, leading some to suggest that it was an alien spaceship.

Now, a University of California, Berkeley, astrochemist and a Cornell University astronomer argue that the comet’s mysterious deviations from a hyperbolic path around the sun can be explained by a simple physical mechanism likely common among many icy comets: outgassing of hydrogen as the comet warmed up in the sunlight.

What made ‘Oumuamua different from every other well-studied comet in our solar system was its size: It was so small that its gravitational deflection around the sun was slightly altered by the tiny push created when hydrogen gas spurted out of the ice.