Space Sunday: a ring of fire, 6 billion Earths and an FRB

The “ring of fire” of the June 21st annular eclipse as seen from Taiwan. Credit: unknown, distributed via Twitter.

For parts of East Africa, the Middle East and Asia, the 2020 summer solstice of June 21st was marked by an annular eclipse of the Sun.

Solar eclipses – when the Moon passes between the Earth and the Sun – take a number of forms, of which the most spectacular is, of course, a total eclipse. These occur when the distance between the Earth and the Moon is such that entire disk of the Sun is covered by the Moon, and the Moon’s shadow – called the umbra – falls directly onto the Earth’s surface, reducing the landscape directly below it to a state of dusk-like darkness called Totality. And just before that period of Totality, that can last several minutes, the solar corona is displayed as a beautiful halo of pearly white light.

A combination of pictures showing the June 21st eclipse as seen from (top l to r) Kurukshetra, Allahabad, Bangalore; (bottom l to r) Kolkata, New Delhi, Bangalore. Credits: Jewel Samad, Manjunath Kiran, Sanjay Kanojia, Dibyangshu Sarkar, Sajjad Hussain/AFP via Getty Images

However, as the Moon’s orbit around the Earth is elliptical rather than circular, for a total eclipse to occur, the Moon needs to be around 379,100 km from Earth. At this distance, the conical shadow of the Moon (the umbra) is sufficient for us to witness Totality. When the Moon is further away from Earth – say at the 381,500 km of the June 21st, 2020 event – , we have an annular eclipse, in which the Moon’s umbra “falls short” of reaching the Earth’s surface. This means that only around 99-99.5% of the Sun’s disk is covered by the Moon when observed along the path of the umbra, leaving the Sun and Moon appearing as a “ring of fire” hanging in the sky. It is this “ring of fire” that makes an annular eclipse the second most spectacular type of solar eclipse.

The needle of the Burj Khalifa, Dubai, magnificently set against the backdrop of the June 21st 2020 eclipse. Credit: unknown, distributed via Twitter

This particular event began at 03:45 UTC on June 21st, 2020, with the Moon “cutting in” to the disk of the Sun, and ended at 10:34 UTC as the Moon moved clear of the Sun. However, the period of maximum eclipse – the time at which the “ring of fire” might be seen – occurred at 06:54 UTC and was visible along a narrow track of the eclipse path just 21 km wide for around 35-60 seconds. Even so, it was still spectacular for those who witnessed it.

For people north and south of this narrow band of passage, the eclipse varied in nature from a partial ring of fire (where the disk of the Moon is jut off-centre enough relative to the Sun for the ring not to be completed) to a partial eclipse (where the disk of the Moon partially sits between the Earth and the Sun, but leaves a fair amount of the latter visible.

As direct viewing of the Sun is dangerous, ahead of the event, Astronomers Without Borders – a global group based out of the United States – worked with regional governments and astronomical groups and societies in Africa to get 16,000 pairs of solar glasses distributed to help people view the eclipse safely. For those well outside the path of the event who wished to witness it, the eclipse was streamed through You Tube and other platforms by a number of organisations such as SLOOH.

The track of the June 21st 2020 eclipse. The central orange band marks the track  along with the “ring of fire” could be seen. Credit: timeanddate.com

Eclipses are seasonal in nature, and generally occur in pairs: one lunar – when the Earth is between the Sun and the Moon, so that the later moves within the Earth’s shadow. This annular solar eclipse was preceded by a penumbral lunar eclipse on June 5th. However, and somewhat unusually, it will be followed by a further penumbral lunar eclipse on July 4th / 5th. A penumbral eclipse is one where the Moon is only within the outermost extent of the cone of Earth’s shadow, dimming it as it reflects the Sun’s light, rather than blocking sunlight falling on it entirely.

The next pair of eclipses will take place in November / December 2020, with a penumbral lunar eclipse on November 30th and a total solar eclipse visible from Chile and Argentina occurring on December 14th. For now, here’s a video of the June 21st event.

Six Billion Earths?

A new study from the University of British Columbia estimates that there could be as many as six billion Earth-type planets in the Milky Way galaxy orbiting within the habitable zone of stars with the same G_Type spectral class as our own Sun.

This may seem a surprisingly high number, but it requires context. In this case, it is estimated our galaxy has 400 billion stars of which some seven percent are G-Type. This means that if the study’s findings are correct, Earth-type planets orbiting in the habitable zone of G-Type stars averages out as just 0.18 per star.

Could Earth have as many as 6 billion “cousins” orbiting G-Type stars? Credit: NASA

The study findings are based on extrapolations from the data on 200,000 stars in the Kepler Space Telescope catalogue, with some adjustments to calculations.

The adjustments were required because Kepler used the transit method of exoplanet detection: watching for regular dips in a star’s brightness. However, given that a large planet will cause a correspondingly greater dip in a star’s brightness than one the size of Earth, the Kepler data is naturally biased towards finding larger planets. Further, it is possible that the dips caused by Earth-sized worlds could be mistaken for transient data rather than actual planets. So to handle things, Michelle Kunimoto, one of the researchers in the study used a technique called forward modelling.

I started by simulating the full population of exoplanets around the stars Kepler searched. I marked each planet as ‘detected’ or ‘missed’ depending on how likely it was my planet search algorithm would have found them. Then, I compared the detected planets to my actual catalogue of planets. If the simulation produced a close match, then the initial population was likely a good representation of the actual population of planets orbiting those stars.

– Michelle Kunimoto, University of British Columbia

The focus on the potential number of Earth-like planets orbiting stars of the same spectral type as the Sun was intentional: while all stars have a habitable zone – where planets orbiting them might support both an atmosphere and liquid water on their surface – G-Type stars are more naturally considered to be more likely to nature the formation of life on any planet within its habitable zone, in part because they are of a similar spectral nature to our own Sun, which may be a key factor in helping the formation of life on an Earth-like planet in their orbit.

Habitable zones – the distance where things are neither too hot nor too cold to allow planets orbiting within it to potentially both develop an atmosphere and liquid water, and thus be amenable to life – may exist around multiple types of stars, but G-Type stars are still looked on as being the most likely to nurture life-bearing planets – or at least, life as we know it. Credit: NASA

Further, while it is true that Earth-like planets have been found around stars of other spectral types, their habitability is more questionable. For example, M-Type red dwarf stars are the most numerous in the galaxy, and many have been found to be home to Earth-type planets. However, given their small size, these stars have their habitable zones much closer to them, which tends to mean that planets in that zone tend to be tidally locked to their parent, always keeping the same face towards it. This means they are extremely hot on one side and freezing cold on the other, with potentially violent weather between the two – assuming they have an atmosphere. Even this is questionable, as M-Type stars are generally subject to violent outbursts that could well violently irradiate the surface of an Earth-type planet in orbit and even strip away its entire atmosphere.

But coming up with that number of potential Earth-type planets around G-Type stars was only part of the study; it also helped cast new light on the Fulton gap, first noted in 2017. This shows that exoplanets orbiting their parent star in 100 terrestrial days or less tend to be either roughly the size of Earth or slightly larger or smaller – or are around twice Earth’s size or larger. It is very rare for such planets to be between 1.5 and 2 times that of Earth; the so-called “radius valley”.

The Fulton Gap has shown that for some reason, Earth-type exoplanets that have been discovered are either around the size of Earth or either slightly smaller (as with Kepler 20e, roughly the size of Venus or 0.88 that of Earth), or very slightly larger (as with Kepler 20f, at 1.03 the size of Earth) or substantially larger still. Few have been found to be between 1.5 and 2 times the size of Earth. Credit: NASA

Why this should be is not precisely known. However, the study by Kunimoto and her colleagues suggests that the radius valley occurs over a smaller range of orbital periods than previously believed, and this may further help refine astronomer’s understanding of the constraints at play within planetary systems that define the formation of potentially life-bearing worlds, and where we should be looking for them.

Regular Radio Waves from Unknown Source

A team of astronomers has picked up on a curious, repeating rhythm of fast radio bursts emanating from an unknown source outside our galaxy, 500 million light years away.

As I recent reported, fast radio bursts, or FRBs, transient radio pulses lasting no more than a few milliseconds some of which appear to be linked to magnetic neutron stars (magnetars) either within or beyond our galaxy (see: rockets, landers, FRBs and the Moon). Since their first discovery in 2007, around 100 FRBs have been catalogued, the vast majority appearing to originate beyond our galaxy. Most have been one-off events, flashing briefly before disappearing entirely, although some have repeated, if with no set pattern.

The latest discovery – FRB 180916.J0158+65 – is unusual as it is the first extra-galactic FRB source to produce a regular pattern of bursts. These comprise 4 days of radio bursts followed by 12 days of silence before repeating. This 16-day cycle has been recorded continuously for a period of 500 days thus far.

The team behind the discovery numbers more than 50 scientists operating the Canadian Hydrogen Intensity Mapping Experiment (CHIME), a radio telescope in British Columbia, the telescope responsible for the FRB I wrote about in my May Space Sunday article linked to above.

Canadian Hydrogen Intensity Mapping Experiment (CHIME) – responsible for making recent FRB discoveries. Credit: the Dominion Radio Astrophysical Observatory

This FRB … is like clockwork. It’s the most definitive pattern we’ve seen from one of these sources. And it’s a big clue that we can use to start hunting down the physics of what’s causing these bright flashes, which nobody really understands.

– Kiyoshi Masui, Kavli Institute for Astrophysics and Space Research, MIT

CHIME came on-line in 2017, and comprises four 100 x 20 metre cylindrical parabolic reflectors that can observe the entire sky overhead as the world turns, giving it the ability to continually record the signals it detects. In September 2018, CHIME first detected FRB 180916.J0158+65, tracing it to a star-churning region on the outskirts of a massive spiral galaxy 500 million light years from Earth.

As the researchers plotted each of the first recorded 38 bursts over time, the 16-day pattern was recognised. Such regularity has never previously been encountered. However, what might be behind it is unknown. Four possible scenarios have thus far been put forward.

The first is that the source is indeed a magnetar. If so, it would be the first time that such a magnetar with a periodic “beat” to its outbursts has been detected. The second is that the bursts may be coming from a single, compact neutron star that is both spinning and wobbling in an astrophysical phenomenon known as precession, such that it is only pointing towards Earth for 4 days out of every 16.

A view of one of the CHIME antennae. the Dominion Radio Astrophysical Observatory

The third possibility is that the bursts originate within a binary system comprising a pair of neutron stars, one less dense than the other, or a neutron star orbiting a black hole. In both cases, the orbital period of the neutron star is 16 terrestrial days, four of which are in close proximity to the companion star / black hole. As these times, the gravitational tides between the two could be strong enough to cause the neutron star to deform, generating the 4-day FRB outburst.

For the fourth possibility the researchers suggest the bursts might be the result of a radio-emitting source orbiting a central star. If the star emits a wind, or cloud of gas, then every time the source passes through the stellar wind / cloud, it might periodically magnify the source’s radio emissions, like a lens can magnify a beam of light – and Earth just happens to be in a position to observe the magnified signal.

One thing the team responsible for the discovery have pointed out is that the signals are unlikely to be of artificial origin, although investigations into their nature will continue,as will the search for FRB sources in general.

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