[ Update: What’s Up: March 2021 Skywatching Tips from NASA – NASA JPL
What are some skywatching highlights in April 2021? Look for the rosy arch known as the Belt of Venus at sunset, then find the constellation Leo overhead on April evenings. Also, check out Jupiter and Saturn with the Moon on April 6. Additional information about topics covered in this episode of What’s Up, along with still images from the video, and the video transcript, are available at https://solarsystem.nasa.gov/whats-up….
Clear April nights are filled with starry creatures. Near the Big Dipper, you will find several interesting binary stars. You can also spot galaxies like the Pinwheel Galaxy, M82, and M96—the last of which is an asymmetric galaxy that may have been gravitationally disrupted by encounters with its neighbors. Keep watching for space-based views of these celestial objects.
What can you see in the night sky tonight? Astronomers Pete Lawrence and Paul Abel guide us through April’s night sky highlights and reveal the stars, constellations and planets worth looking out for over the coming weeks.
New observations with the European Southern Observatory’s Very Large Telescope (ESO’s VLT) indicate that the rogue comet 2I/Borisov, which is only the second and most recently detected interstellar visitor to our Solar System, is one of the most pristine ever observed. Astronomers suspect that the comet most likely never passed close to a star, making it an undisturbed relic of the cloud of gas and dust it formed from.
2I/Borisov was discovered by amateur astronomer Gennady Borisov in August 2019 and was confirmed to have come from beyond the Solar System a few weeks later.
“2I/Borisov could represent the first truly pristine comet ever observed,”
says Stefano Bagnulo of the Armagh Observatory and Planetarium, Northern Ireland, UK, who led the new study published today in Nature Communications. The team believes that the comet had never passed close to any star before it flew by the Sun in 2019.
Bagnulo and his colleagues used the FORS2 instrument on ESO’s VLT, located in northern Chile, to study 2I/Borisov in detail using a technique called polarimetry . Since this technique is regularly used to study comets and other small bodies of our Solar System, this allowed the team to compare the interstellar visitor with our local comets.
The team found that 2I/Borisov has polarimetric properties distinct from those of Solar System comets, with the exception of Hale–Bopp. Comet Hale–Bopp received much public interest in the late 1990s as a result of being easily visible to the naked eye, and also because it was one of the most pristine comets astronomers had ever seen. Prior to its most recent passage, Hale–Bopp is thought to have passed by our Sun only once and had therefore barely been affected by solar wind and radiation. This means it was pristine, having a composition very similar to that of the cloud of gas and dust it — and the rest of the Solar System — formed from some 4.5 billion years ago.
By analysing the polarisation together with the colour of the comet to gather clues on its composition, the team concluded that 2I/Borisov is in fact even more pristine than Hale–Bopp. This means it carries untarnished signatures of the cloud of gas and dust it formed from.
“The fact that the two comets are remarkably similar suggests that the environment in which 2I/Borisov originated is not so different in composition from the environment in the early Solar System,”
says Alberto Cellino, a co-author of the study, from the Astrophysical Observatory of Torino, National Institute for Astrophysics (INAF), Italy.
Olivier Hainaut, an astronomer at ESO in Germany who studies comets and other near-Earth objects but was not involved in this new study, agrees.
“The main result — that 2I/Borisov is not like any other comet except Hale–Bopp — is very strong,” he says, adding that “it is very plausible they formed in very similar conditions.”
[..] Ludmilla Kolokolova, of the University of Maryland in the US, who was involved in the Nature Communications research [explains,]
“The arrival of 2I/Borisov from interstellar space represented the first opportunity to study the composition of a comet from another planetary system and check if the material that comes from this comet is somehow different from our native variety,”
Bagnulo hopes astronomers will have another, even better, opportunity to study a rogue comet in detail before the end of the decade.
“ESA is planning to launch Comet Interceptor in 2029, which will have the capability of reaching another visiting interstellar object, if one on a suitable trajectory is discovered,”
he says, referring to an upcoming mission by the European Space Agency.
An origin story hidden in the dust
Even without a space mission, astronomers can use Earth’s many telescopes to gain insight into the different properties of rogue comets like 2I/Borisov.
“Imagine how lucky we were that a comet from a system light-years away simply took a trip to our doorstep by chance,”
says Bin Yang, an astronomer at ESO in Chile, who also took advantage of 2I/Borisov’s passage through our Solar System to study this mysterious comet. Her team’s results are published in Nature Astronomy.
Yang and her team used data from the Atacama Large Millimeter/submillimeter Array (ALMA), in which ESO is a partner, as well as from ESO’s VLT, to study 2I/Borisov’s dust grains to gather clues about the comet’s birth and conditions in its home system.
They discovered that 2I/Borisov’s coma — an envelope of dust surrounding the main body of the comet — contains compact pebbles, grains about one millimetre in size or larger. In addition, they found that the relative amounts of carbon monoxide and water in the comet changed drastically as it neared the Sun. The team, which also includes Olivier Hainaut, says this indicates that the comet is made up of materials that formed in different places in its planetary system.
The observations by Yang and her team suggest that matter in 2I/Borisov’s planetary home was mixed from near its star to further out, perhaps because of the existence of giant planets, whose strong gravity stirs material in the system. Astronomers believe that a similar process occurred early in the life of our Solar System.
While 2I/Borisov was the first rogue comet to pass by the Sun, it was not the first interstellar visitor. The first interstellar object to have been observed passing by our Solar System was ʻOumuamua, another object studied with ESO’s VLT back in 2017. Originally classified as a comet, ʻOumuamua was later reclassified as an asteroid as it lacked a coma.
 Polarimetry is a technique to measure the polarisation of light. Light becomes polarised, for example, when it goes through certain filters, like the lenses of polarised sunglasses or cometary material. By studying the properties of sunlight polarised by a comet’s dust, researchers can gain insights into the physics and chemistry of comets.
The Event Horizon Telescope (EHT) collaboration, who produced the first ever image of a black hole, has today revealed a new view of the massive object at the centre of the Messier 87 (M87) galaxy: how it looks in polarised light. This is the first time astronomers have been able to measure polarisation, a signature of magnetic fields, this close to the edge of a black hole. The observations are key to explaining how the M87 galaxy, located 55 million light-years away, is able to launch energetic jets from its core.
“We are now seeing the next crucial piece of evidence to understand how magnetic fields behave around black holes, and how activity in this very compact region of space can drive powerful jets that extend far beyond the galaxy,”
says Monika Mościbrodzka, Coordinator of the EHT Polarimetry Working Group and Assistant Professor at Radboud University in the Netherlands.
On 10 April 2019, scientists released the first ever image of a black hole, revealing a bright ring-like structure with a dark central region — the black hole’s shadow. Since then, the EHT collaboration has delved deeper into the data on the supermassive object at the heart of the M87 galaxy collected in 2017. They have discovered that a significant fraction of the light around the M87 black hole is polarised.
“This work is a major milestone: the polarisation of light carries information that allows us to better understand the physics behind the image we saw in April 2019, which was not possible before,”
explains Iván Martí-Vidal, also Coordinator of the EHT Polarimetry Working Group and GenT Distinguished Researcher at the University of Valencia, Spain. He adds that
“unveiling this new polarised-light image required years of work due to the complex techniques involved in obtaining and analysing the data.”
Light becomes polarised when it goes through certain filters, like the lenses of polarised sunglasses, or when it is emitted in hot regions of space where magnetic fields are present. In the same way that polarised sunglasses help us see better by reducing reflections and glare from bright surfaces, astronomers can sharpen their view of the region around the black hole by looking at how the light originating from it is polarised. Specifically, polarisation allows astronomers to map the magnetic field lines present at the inner edge of the black hole.
“The newly published polarised images are key to understanding how the magnetic field allows the black hole to ‘eat’ matter and launch powerful jets,”
says EHT collaboration member Andrew Chael, a NASA Hubble Fellow at the Princeton Center for Theoretical Science and the Princeton Gravity Initiative in the US.
The bright jets of energy and matter that emerge from M87’s core and extend at least 5000 light-years from its centre are one of the galaxy’s most mysterious and energetic features. Most matter lying close to the edge of a black hole falls in. However, some of the surrounding particles escape moments before capture and are blown far out into space in the form of jets.
Astronomers have relied on different models of how matter behaves near the black hole to better understand this process. But they still don’t know exactly how jets larger than the galaxy are launched from its central region, which is comparable in size to the Solar System, nor how exactly matter falls into the black hole. With the new EHT image of the black hole and its shadow in polarised light, astronomers managed for the first time to look into the region just outside the black hole where this interplay between matter flowing in and being ejected out is happening.
The observations provide new information about the structure of the magnetic fields just outside the black hole. The team found that only theoretical models featuring strongly magnetised gas can explain what they are seeing at the event horizon.
“The observations suggest that the magnetic fields at the black hole’s edge are strong enough to push back on the hot gas and help it resist gravity’s pull. Only the gas that slips through the field can spiral inwards to the event horizon,”
explains Jason Dexter, Assistant Professor at the University of Colorado Boulder, US, and Coordinator of the EHT Theory Working Group.
To observe the heart of the M87 galaxy, the collaboration linked eight telescopes around the world — including the northern Chile-based Atacama Large Millimeter/submillimeter Array (ALMA) and the Atacama Pathfinder EXperiment (APEX), in which the European Southern Observatory (ESO) is a partner — to create a virtual Earth-sized telescope, the EHT. The impressive resolution obtained with the EHT is equivalent to that needed to measure the length of a credit card on the surface of the Moon.
“With ALMA and APEX, which through their southern location enhance the image quality by adding geographical spread to the EHT network, European scientists were able to play a central role in the research,” … “With its 66 antennas, ALMA dominates the overall signal collection in polarised light, while APEX has been essential for the calibration of the image.”
[says Ciska Kemper, European ALMA Programme Scientist at ESO.]
“ALMA data were also crucial to calibrate, image and interpret the EHT observations, providing tight constraints on the theoretical models that explain how matter behaves near the black hole event horizon,”
adds Ciriaco Goddi, a scientist at Radboud University and Leiden Observatory, the Netherlands, who led an accompanying study that relied only on ALMA observations.
The EHT setup allowed the team to directly observe the black hole shadow and the ring of light around it, with the new polarised-light image clearly showing that the ring is magnetised. The results are published today in two separate papers in The Astrophysical Journal Letters by the EHT collaboration. The research involved over 300 researchers from multiple organisations and universities worldwide.
“The EHT is making rapid advancements, with technological upgrades being done to the network and new observatories being added. We expect future EHT observations to reveal more accurately the magnetic field structure around the black hole and to tell us more about the physics of the hot gas in this region,”
concludes EHT collaboration member Jongho Park, an East Asian Core Observatories Association Fellow at the Academia Sinica Institute of Astronomy and Astrophysics in Taipei.
Using the Atacama Large Millimeter/submillimeter Array (ALMA), in which the European Southern Observatory (ESO) is a partner, a team of astronomers have directly measured winds in Jupiter’s middle atmosphere for the first time. By analysing the aftermath of a comet collision from the 1990s, the researchers have revealed incredibly powerful winds, with speeds of up to 1450 kilometres an hour, near Jupiter’s poles. They could represent what the team have described as a “unique meteorological beast in our Solar System”.
Jupiter is famous for its distinctive red and white bands: swirling clouds of moving gas that astronomers traditionally use to track winds in Jupiter’s lower atmosphere. Astronomers have also seen, near Jupiter’s poles, the vivid glows known as aurorae, which appear to be associated with strong winds in the planet’s upper atmosphere. But until now, researchers had never been able to directly measure wind patterns in between these two atmospheric layers, in the stratosphere.
Measuring wind speeds in Jupiter’s stratosphere using cloud-tracking techniques is impossible because of the absence of clouds in this part of the atmosphere. However, astronomers were provided with an alternative measuring aid in the form of comet Shoemaker–Levy 9, which collided with the gas giant in spectacular fashion in 1994. This impact produced new molecules in Jupiter’s stratosphere, where they have been moving with the winds ever since.
A team of astronomers, led by Thibault Cavalié of the Laboratoire d’Astrophysique de Bordeaux in France, have now tracked one of these molecules — hydrogen cyanide — to directly measure stratospheric “jets” on Jupiter. Scientists use the word “jets” to refer to narrow bands of wind in the atmosphere, like Earth’s jet streams.
“The most spectacular result is the presence of strong jets, with speeds of up to 400 metres per second, which are located under the aurorae near the poles,” says Cavalié.
These wind speeds, equivalent to about 1450 kilometres an hour, are more than twice the maximum storm speeds reached in Jupiter’s Great Red Spot and over three times the wind speed measured on Earth’s strongest tornadoes.
“Our detection indicates that these jets could behave like a giant vortex with a diameter of up to four times that of Earth, and some 900 kilometres in height,” explains co-author Bilal Benmahi, also of the Laboratoire d’Astrophysique de Bordeaux. “A vortex of this size would be a unique meteorological beast in our Solar System,” Cavalié adds.
Astronomers were aware of strong winds near Jupiter’s poles, but much higher up in the atmosphere, hundreds of kilometres above the focus area of the new study, which is published today in Astronomy & Astrophysics. Previous studies predicted that these upper-atmosphere winds would decrease in velocity and disappear well before reaching as deep as the stratosphere.
“The new ALMA data tell us the contrary,“
says Cavalié, adding that finding these strong stratospheric winds near Jupiter’s poles was a “real surprise“.
The team used 42 of ALMA’s 66 high-precision antennas, located in the Atacama Desert in northern Chile, to analyse the hydrogen cyanide molecules that have been moving around in Jupiter’s stratosphere since the impact of Shoemaker–Levy 9. The ALMA data allowed them to measure the Doppler shift — tiny changes in the frequency of the radiation emitted by the molecules — caused by the winds in this region of the planet.
“By measuring this shift, we were able to deduce the speed of the winds much like one could deduce the speed of a passing train by the change in the frequency of the train whistle,”
explains study co-author Vincent Hue, a planetary scientist at the Southwest Research Institute in the US.
In addition to the surprising polar winds, the team also used ALMA to confirm the existence of strong stratospheric winds around the planet’s equator, by directly measuring their speed, also for the first time. The jets spotted in this part of the planet have average speeds of about 600 kilometres an hour.
The ALMA observations required to track stratospheric winds in both the poles and equator of Jupiter took less than 30 minutes of telescope time.
“The high levels of detail we achieved in this short time really demonstrate the power of the ALMA observations,” says Thomas Greathouse, a scientist at the Southwest Research Institute in the US and co-author of the study. “It is astounding to me to see the first direct measurement of these winds.”
[ Cavalié says, ]
“These ALMA results open a new window for the study of Jupiter’s auroral regions, which was really unexpected just a few months back,” […]
[ Greathouse adds, ]
“They also set the stage for similar yet more extensive measurements to be made by the JUICE mission and its Submillimetre Wave Instrument,” […]
referring to the European Space Agency’s JUpiter ICy moons Explorer, which is expected to launch into space next year.
ESO’s ground-based Extremely Large Telescope (ELT), set to see first light later this decade, will also explore Jupiter. The telescope will be capable of making highly detailed observations of the planet’s aurorae, giving us further insight into Jupiter’s atmosphere.
With the help of the European Southern Observatory’s Very Large Telescope (ESO’s VLT), astronomers have discovered and studied in detail the most distant source of radio emission known to date. The source is a “radio-loud” quasar — a bright object with powerful jets emitting at radio wavelengths — that is so far away its light has taken 13 billion years to reach us. The discovery could provide important clues to help astronomers understand the early Universe.
Quasars are very bright objects that lie at the centre of some galaxies and are powered by supermassive black holes. As the black hole consumes the surrounding gas, energy is released, allowing astronomers to spot them even when they are very far away.
The newly discovered quasar, nicknamed P172+18, is so distant that light from it has travelled for about 13 billion years to reach us: we see it as it was when the Universe was just around 780 million years old. While more distant quasars have been discovered, this is the first time astronomers have been able to identify the telltale signatures of radio jets in a quasar this early on in the history of the Universe. Only about 10% of quasars — which astronomers classify as “radio-loud” — have jets, which shine brightly at radio frequencies .
P172+18 is powered by a black hole about 300 million times more massive than our Sun that is consuming gas at a stunning rate.
“The black hole is eating up matter very rapidly, growing in mass at one of the highest rates ever observed,”
explains astronomer Chiara Mazzucchelli, Fellow at ESO in Chile, who led the discovery together with Eduardo Bañados of the Max Planck Institute for Astronomy in Germany.
The astronomers think that there’s a link between the rapid growth of supermassive black holes and the powerful radio jets spotted in quasars like P172+18. The jets are thought to be capable of disturbing the gas around the black hole, increasing the rate at which gas falls in. Therefore, studying radio-loud quasars can provide important insights into how black holes in the early Universe grew to their supermassive sizes so quickly after the Big Bang.
“I find it very exciting to discover ‘new’ black holes for the first time, and to provide one more building block to understand the primordial Universe, where we come from, and ultimately ourselves,” says Mazzucchelli.
P172+18 was first recognised as a far-away quasar, after having been previously identified as a radio source, at the Magellan Telescope at Las Campanas Observatory in Chile by Bañados and Mazzucchelli.
“As soon as we got the data, we inspected it by eye, and we knew immediately that we had discovered the most distant radio-loud quasar known so far,” says Bañados.
However, owing to a short observation time, the team did not have enough data to study the object in detail. A flurry of observations with other telescopes followed, including with the X-shooter instrument on ESO’s VLT, which allowed them to dig deeper into the characteristics of this quasar, including determining key properties such as the mass of the black hole and how fast it’s eating up matter from its surroundings. Other telescopes that contributed to the study include the National Radio Astronomy Observatory’s Very Large Array and the Keck Telescope in the US.
While the team are excited about their discovery, to appear in The Astrophysical Journal, they believe this radio-loud quasar could be the first of many to be found, perhaps at even larger cosmological distances.
“This discovery makes me optimistic and I believe — and hope — that the distance record will be broken soon,” says Bañados.
Observations with facilities such as ALMA, in which ESO is a partner, and with ESO’s upcoming Extremely Large Telescope (ELT) could help uncover and study more of these early-Universe objects in detail.