A false-colour image of the plumes erupting from Enceladus. Image Credit: NASA/ESA
The Search for Life in our Solar System leads seekers to strange places. From our Earthbound viewpoint, an ice-covered moon orbiting a gas giant far from the Sun can seem like a strange place to search for life. But underneath all that ice sits a vast ocean. Despite the huge distance between the moon and the Sun and despite the thick ice cap, the water is warm.
Of course, we’re talking about Enceladus, and its warm, salty ocean—so similar to Earth’s in some respects—takes some of the strangeness away.
Artist’s illustration of HD 104067 b, which is the outermost exoplanet in the HD 104067 system, and responsible for potentially causing massive tidal energy on the innermost exoplanet candidate, TOI-6713.01. (Credit: NASA/JPL-Caltech)
Can tidal forces cause an exoplanet’s surface to radiate heat? This is what a recent study accepted to The Astronomical Journal hopes to address as a team of international researchers used data collected from ground-based instruments to confirm the existence of a second exoplanet residing within the exoplanetary system, HD 104067, along with using NASA’s Transiting Exoplanet Survey Satellite (TESS) mission to identify an additional exoplanet candidate, as well. What’s unique about this exoplanet candidate, which orbits innermost compared to the other two, is that the tidal forces exhibited from the outer two exoplanets are potentially causing the candidates’ surface to radiate with its surface temperature reaching as high as 2,300 degrees Celsius (4,200 degrees Fahrenheit), which the researchers refer to as a “perfect tidal storm”.
Among the thousands of known exoplanets, there are some that are very odd. Take, for example, the exoplanet known as WASP-103b. It’s a large planet with a mass about 1.5 times that of Jupiter, but 103b is so close to its star it makes a complete orbit every 22 hours. At this proximity, many astronomers wonder if the world is on the edge of being ripped apart by tidal forces. But a new study shows us that something much more interesting is going on.
The asteroid 99942 Apophis. Image Credit: NASA/Caltech/JPL
If NASA and other space agencies don’t want us to freak out about asteroids colliding with Earth, why do they give them names like Apophis? It sounds apocalyptic.
Apophis was the ancient Egyptian god of Chaos. He was an evil serpent that dwelled in endless darkness, the enemy of light and truth. So when they informed us that an asteroid named Apophis was due for a close encounter with Earth in 2029, people were understandably anxious. After all, Earth’s previous dominant inhabitants were evicted by an asteroid.
An asteroid torn apart by the strong gravity of a white dwarf has formed a ring of dust particles and debris orbiting the Earth-sized burnt out stellar core. Image Credit: University of Warwick/Mark Garlick
Sad fact of the Universe is that all stars will die, eventually. And when they do, what happens to their babies? Usually, the prognosis for the planets around a dying star is not good, but a new study says some might in fact survive.
A group of astronomers have taken a closer look at what happens when stars, like our Sun for instance, become white dwarfs late in their lives. As it turns out, denser planets like Earth might survive the event. But, only if they’re the right distance away.
Close-up of star near a supermassive black hole (artist’s impression). Credit: ESA/Hubble, ESO, M. Kornmesser
In 2015, the All-Sky Automated Survey for Supernovae (aka. ASAS-SN, or Assassin) detected something rather brilliant in a distant galaxy. At the time, it was thought that the event (named ASASSN-15lh) was a superluminous supernova – an extremely bright explosion caused by a massive star reaching the end of its lifepsan. This event was thought to be brightest supernova ever witnessed, being twice as bright as the previous record-holder.
But new observations provided by an international team of astronomers have provided an alternative explanation that is even more exciting. Relying on data from several observatories – including the NASA/ESA Hubble Space Telescope – they have proposed that the source was a star being ripped apart by a rapidly spinning black hole, an event which is even more rare than a superluminous supernova.
According to the ASAS-SN’s findings – which were published in January of 2016 in Science – the superluminous light source appeared in a galaxy roughly 4 billion light-years from Earth. The luminous source was twice as bright as the brightest superluminous supernova observed to date, and its peak luminosity was 20 times brighter than the total light output of the entire Milky Way.
This artist’s impression depicts a rapidly spinning supermassive black hole surrounded by an accretion disc. Credit: ESA/Hubble, ESO, M. Kornmesse
What seemed odd about it was the fact that the superluminous event appeared within a massive, red (i.e. “quiescent”) galaxy, where star formation has largely ceased. This was in contrast to most super-luminous supernovae that have been observed in the past, which are typically located in blue, star-forming dwarf galaxies. In addition, the star (which is Sun-like in size) is not nearly massive enough to become an extreme supernova.
As such, the international team of astronomers – led by Giorgos Leloudas of the Weizmann Institute of Science in Israel and the Dark Cosmology Center in Denmark – conducted follow-up observations using space-based and Earth-based observatories. These included the Hubble Space Telescope, the Very Large Telescope (VLT) at the ESO’s Paranal Observatory and the New Technology Telescope (NTT) at the La Silla Observatory.
With information from these facilities, they arrived at a much different conclusion. As Dr. Leloudas explained in a Hubble press release:
“We observed the source for 10 months following the event and have concluded that the explanation is unlikely to lie with an extraordinary bright supernova. Our results indicate that the event was probably caused by a rapidly spinning supermassive black hole as it destroyed a low-mass star.”
The process is colloquially known as “spaghettification”, where an object is ripped apart by the extreme tidal forces of a black hole. In this case, the team postulated that the star drifted too close to the supermassive black hole (SMBH) at the center of the distant galaxy. The resulting heat and the shocks created by colliding debris led to a massive burst of light – which was mistakenly believed to be a very bright supernova.
Multiple lines of evidence support this theory. As they explain in their paper, this included the fact that over the ten-months that they observed it, the star went through three distinct spectroscopic phases. This included a period of substanial re-brightening, where the star emitted a burst of UV light that accorded with a sudden increase in its temperature.
Combined with the unlikely location and the mass of the star, this all pointed towards tidal disruption rather than a massive supernova event. But as Dr. Leloudas admits, they cannot be certain of this just yet. “Even with all the collected data we cannot say with 100% certainty that the ASASSN-15lh event was a tidal disruption event.” he said. “But it is by far the most likely explanation.”
As always, additional observations are necessary before anyone can say for sure what caused this record-breaking luminous event. But in the meantime, the mere fact that something so rare was witnessed should be enough to cause some serious excitement! Speaking of which, be sure to check out the simulation videos (above and below) to see what such an event would look like:
When a star encounters a black hole, tidal forces stretch the star into an elongated blob before tearing it apart, as seen in these images from a computer simulation by James Guillochon of Harvard University.
A telescope peers into the blackness of deep space. Suddenly – a brilliant flash of light appears that wasn’t there before. What could it be? A supernova? Two massively dense stars fusing together? Perhaps a gamma ray burst?
Five years ago, researchers using the ROTSE IIIb telescope at McDonald Observatory noticed just such an event. But far from being your run-of-the-mill stellar explosion or neutron star merger, the astronomers believe that this tiny flare was, in fact, evidence of a supermassive black hole at the center of a distant galaxy, tearing a star to shreds.
Astronomers at McDonald had been using the telescope to scan the skies for such nascent flashes for years, as part of the ROTSE Supernova Verification Project (SNVP). And at first blush, the event seen in early 2009, which the researches nicknamed “Dougie,” looked just like many of the other supernovae they had discovered over the course of the project. With a blazing – 22.5-magnitude absolute brightness, the event fit squarely within the class of superluminous supernovae that the researchers were already familiar with.
But as time went on and more data on Dougie rolled in, the astronomers began to change their minds. X-ray observations made by the orbiting Swift satellite and optical spectra taken by McDonald’s Hobby-Eberly Telescope revealed an evolving light curve and chemical makeup that didn’t fit with computer simulations of superluminous supernovae. Likewise, Dougie didn’t appear to be a neutron star merger, which would have reached peak luminosity far more quickly than was observed, or a gamma ray burst, which, even at an angle, would have appeared far brighter in x-ray light.
That left only one option: a so-called “tidal disruption event,” or the carnage and spaghettification that occurs when a star wanders too close to a black hole’s horizon. J. Craig Wheeler, head of the supernova group at The University of Texas at Austin and a member of the team that discovered Dougie, explained that at short distances, a black hole’s gravity exerts a much stronger pull on the side of the star nearest to it than it does on the star’s opposite side. He explained, “These especially large tides can be strong enough that you pull the star out into a noodle.”
The team refined their models of the event and came to a surprising conclusion: having drawn in Dougie’s stellar material a bit faster than it could handle, the black hole was now “choking” on its latest meal. This is due to an astrophysical principle called the Eddington Limit, which states that a black hole of a given size can only handle so much infalling material. After this limit has been reached, any additional intake of matter exerts more outward pressure than the black hole’s gravity can compensate for. This pressure increase has a kind of rebound effect, throwing off material from the black hole’s accretion disk along with heat and light. Such a burst of energy accounts for at least part of Dougie’s brightness, but also indicates that the original dying star – a star not unlike our own Sun – wasn’t going down without a fight.
Combining these observations with the mathematics of the Eddington Limit, the researchers estimated the black hole’s size to be about 1 million solar masses – a rather small black hole, at the center of a rather small galaxy, three billion light years away. Discoveries like these not only allow astronomers to better understand the physics of black holes, but also properties of their often unassuming home galaxies. After all, mused Wheeler, “Who knew this little guy had a black hole?”
To get a simulated glimpse of Dougie for yourself, check out the amazing animation below, courtesy of team member James Guillochon:
The research is published in this month’s issue of The Astrophysical Journal. A pre-print of the paper is available here.
Using a precision formation-flying technique, the twin GRAIL spacecraft will map the moon's gravity field, as depicted in this artist's rendering. Radio signals traveling between the two spacecraft provide scientists the exact measurements required as well as flow of information not interrupted when the spacecraft are at the lunar farside, not seen from Earth. The result should be the most accurate gravity map of the moon ever made.
The mission also will answer longstanding questions about Earth's moon, including the size of a possible inner core, and it should provide scientists with a better understanding of how Earth and other rocky planets in the solar system formed. GRAIL is a part of NASA's Discovery Program.
Image credit: NASA/JPL-Caltech
The shape of the moon deviates from a simple sphere in a way that scientists have struggled to explain. But new research shows that tidal forces during the moon’s early history can explain most of its large-scale topography. As the moon cooled and solidified more than four billion years ago, the sculpting effects of tidal and rotational forces became frozen in place.
Astronomers think the moon formed when a rogue planet, larger than Mars, struck the Earth in a great, glancing blow. A cloud rose 13,700 miles (22,000 kilometers) above the Earth, where it condensed into innumerable solid particles that orbited the Earth. Over time these moonlets combined to form the moon.
So the moon was sculpted by Earth’s gravity from the get-go. Although scientists have long postulated that tidal forces helped shape the molten moon, the new study provides a much more detailed understanding of the additional forces at play.
Not long after the moon’s formation, the crust was decoupled from the mantle below by an intervening ocean of magma. This caused immense tidal forces. At the poles, where the flexing and heating was greatest, the crust became thinner, while the thickest crust formed at the equators. Garrick-Bethel likened this to a lemon shape with the long axis of the lemon pointing at the Earth.
But this process does not explain why the bulge is now only found on the far side of the moon. You would expect to see it on both sides, because tides have a symmetrical effect.
“In 2010, we found one area that fits the tidal heating effect, but that study left open the rest of the moon and didn’t include the tidal-rotational deformation. In this paper we tried to bring all those considerations together,” said Garrick-Bethell in a press release.
Any rotational forces would cause the spinning moon to flatten slightly at the poles and bulge out near the equator. It would have had a similar effect on the moon’s shape as the tidal heating did — both of which left distinct signatures in the moon’s gravity field. Because the crust is lighter than the underlying mantle, gravity signals reveal variations in the moon’s internal structure, many of which may be due to previous forces.
Interestingly, Garrick-Bethell and colleagues discovered that the moon’s overall gravity field is no longer aligned with the topography. The long axis of the moon doesn’t point directly toward Earth as it likely did when the moon first formed; instead, it’s offset by about 30 degrees.
“The moon that faced us a long time ago has shifted, so we’re no longer looking at the primordial face of the moon,” said Garrick-Bethell. “Changes in the mass distribution shifted the orientation of the moon. The craters removed some mass, and there were also internal changes, probably related to when the moon became volcanically active.”
The details and timing of these processes are still uncertain, but the new analysis should help shed light on the tidal and rotational forces abundant throughout the Solar System and the Galaxy. These simple forces, after all, have helped shape our nearest neighbor and the most distant exoplanet.
Comet Lovejoy re-emerging from behind the Sun on Dec. 15, 2011. (NASA/SDO)
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It was just about three months ago that the astronomy world watched in awe as the recently-discovered comet Lovejoy plummeted toward the Sun on what was expected to be its final voyage, only to reappear on the other side seemingly unscathed! Surviving its solar visit, Lovejoy headed back out into the solar system, displaying a brand-new tail for skywatchers in southern parts of the world (and for a few select viewers above the world as well.)
How did a loosely-packed ball of ice and rock manage to withstand such a close pass through the Sun’s blazing corona, when all expectations were that it would disintegrate and fizzle away? A few researchers from Germany have an idea.
Scientists from the Max Planck Institute for Extraterrestrial Physics and the Braunschweig University of Technology have hypothesized that Comet Lovejoy managed to hold itself together through the very process that, to most people, defines a comet: the outgassing of sublimated icy material.
As a comet near the Sun, the increased heating from solar radiation causes the frozen materials within the nucleus to sublimate — go directly and suddenly from solid to gas, skipping the liquid middle stage — and, in doing so, burst through the surface of the comet and create the long, hazy reflective tail that is so often associated with them.
Overview of the forces acting on sungrazing comets. (Illustration from paper.)
In the case of Lovejoy, which was on a direct path toward the Sun, the sublimation itself may have provided enough outward force across its surface to literally keep it together, according to the team’s research.
“The reaction force caused by the strong outgassing (sublimation) of the nucleus near the Sun acts to keep the nucleus together and to overcome the tidal disruption,” the paper claims.
In addition, the team states that the size of the comet’s nucleus can be derived using an equation that takes into consideration the combined forces of outgassing, the material composition of the comet’s nucleus, the comet’s own gravity and the tidal forces exerted by the comet’s close proximity to the Sun (i.e., the Roche limit).
Using that equation, the team concluded that the diameter of Comet Lovejoy’s nucleus is anywhere between 0.2 km and 11 km (.125 miles and 6.8 miles). Any smaller and it would have lost too much material during its pass (and had too little gravity); any larger and it would have been too thick for outgassing to provide enough counterbalancing force.
If this hypothesis is correct, taking a trip around the Sun may not mean the end for all comets… at least not those of a certain size!
Watch the video of Lovejoy’s Dec. 15 solar swing below:
The paper was submitted to the journal Icarus on March 8, 2012 by Bastian Gundlach. See the full text here.
Illustration of WASP-12b in orbit about its host star (Credit: ESA/C Carreau)
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WASP-12b, discovered in 2008, is a real outlier among the 400 or so exoplanets discovered to date. Not that it’s particularly massive (it’s a gas giant, not unlike Jupiter), nor that its homesun (host star) is particularly unusual (it’s rather similar to our own Sun), but it orbits very close to its homesun, and is considerably larger than any other gas giant discovered to date.
Results from recent research explain why WASP-12b is so unusual; we’re watching it die a painful death at the hands of its homesun, which is snacking on it.
“This is the first time that astronomers are witnessing the ongoing disruption and death march of a planet,” says UC Santa Cruz professor Douglas N.C. Lin. Lin is a co-author of the new study and the founding director of the Kavli Institute for Astronomy and Astrophysics (KIAA) at Peking University, which was deeply involved with the research.
The research was led by Shu-lin Li of the National Astronomical Observatories of China. A graduate of KIAA, Li and a research team analyzed observational data on the planet to show how the gravity of its parent star is both inflating its size and spurring its rapid dissolution.
WASP-12b, like most known exoplanets discovered to date, is large and gaseous, resembling Jupiter and Saturn; however, unlike Jupiter, Saturn, or most other exoplanets, it orbits its homesun at extremely close range – 75 times closer than the Earth is to the Sun, or just over 1.5 million km. It is also larger than astrophysical models predict. Its mass is estimated to be almost 50% larger than Jupiter’s and it is 80% larger, giving it six times Jupiter’s volume. It is also unusually toasty, with a daytime temperature of more than 2500° C.
Some mechanism must be responsible for expanding this planet to such an unexpected size, say the researchers. They have focused their analysis on tidal forces, which they say are strong enough to produce the effects observed on WASP-12b.
On Earth, tidal forces between the Earth and the Moon cause local sea levels rise and fall, modestly, twice a day. WASP-12b, however, is so close to its homesun that the gravitational forces are enormous. The tremendous tidal forces acting on the planet completely change the shape of the planet into something similar to that of a rugby or American football.
These tides not only distort the shape of WASP-12b. By continuously deforming the planet, they also create friction in its interior. The friction produces heat, which causes the planet to expand. “This is the first time that there is direct evidence that internal heating (or ‘tidal heating’) is responsible for puffing up the planet to its current size,” says Lin.
Huge as it is, WASP-12b faces an early demise, say the researchers. In fact, its size is part of its problem. It has ballooned to such a point that it cannot retain its mass against the pull of its homesun’s gravity. As the study’s lead author Li explains, “WASP-12b is losing its mass to the host star at a tremendous rate of six billion metric tons each second. At this rate, the planet will be completely destroyed by its host star in about ten million years. This may sound like a long time, but for astronomers it’s nothing. This planet will live less than 500 times less than the current age of the Earth.” The WASP-12 system (Courtesy: KIAA/Graphic: Neil Miller) About this image: The massive gas giant WASP-12b is shown in purple with the transparent region representing its atmosphere. The gas giant planet’s orbit is somewhat non-circular. This indicates that there is probably an unseen lower mass planet in the system, shown in brown, that is perturbing the larger planet’s orbit. Mass from the gas giant’s atmosphere is pulled off and forms a disk around the star, shown in red.
The material that is stripped off WASP-12b does not fall directly onto the parent star; instead it forms a disk around the star and slowly spirals inwards. A careful analysis of the orbital motion of WASP-12b suggests circumstantial evidence of the gravitational force of a second, lower-mass planet in the disk. This planet is most likely a massive version of the Earth – a so-called “super-Earth.”
The disk of planetary material and the embedded super-Earth should be detectable with currently available telescope facilities. Their properties can be used to further constrain the history and fate of the mysterious planet WASP-12b.
In addition to KIAA, support for the WASP-12b research came from NASA, the Jet Propulsion Laboratory, and the National Science Foundation. Along with Li and Lin, co-authors include UC Santa Cruz professor Jonathan Fortney and Neil Miller, a graduate student at the university.
