This page highlights topical astronomy facts and research of interest.

Earendel the Methuselah star

This is the oldest star detected to date by a fluke of location, namely that it was ideally aligned with a huge foreground galaxy cluster known as WHL0137-08, which magnified the star by gravitational lensing. This means the star becomes visible in the general glow of its home galaxy, as a result, its magnitude (brightness) is magnified a thousandfold or more! The star now named Earendel (old English for Dawn star), formed around 800 million years after the Big Bang. These first-generation stars called “Population III” stars formed from primordial hydrogen and helium created during the big bang, and consequently contain virtually no heavy elements, those beyond helium in the periodic table of elements. Computer modelling suggests these first-generation stars would be massive and low in heavy elements, those beyond helium in the periodic table of elements, and Earendel weighs in at 50 solar masses or more. Currently, astronomers are not able to determine if it’s a binary star, though most massive stars have at least one smaller companion star.

The recently launched James Webb telescope when fully commissioned will be able to study this star in greater detail and establish the properties of these first-generation Population III stars!

Comet C/2020 F3 NEOWISE, 11/07/2020 (updated 18/07 & 30/8/2020)

This bright long-period comet with a diameter estimated to be 5 km, is now visible by eye over the UK (binoculars will be helpful) if you look towards the north northwest sky, the comet is currently circumpolar at the latitude of Southport, so it can, in theory, be seen throughout the entire day, although late evening will be the best time provided the sky is clear. This passage will increase the comet’s orbital period from c 4,500 years to c 6,800 years and confirms it originated in the Oort cloud. Click this link to go to the relevant ‘Live sky’ page for details, and this link to show where the comet is currently located in the sky (image credit Jamie Cooper/ SWNS)

When these icy bodies from the outer solar system reach the region around the orbit of Mars, solar heating becomes strong enough to begin sublimating (ice to gas) their ices (mostly methane, carbon monoxide & dioxide, and water) that begin sweeping the entrained smoke sized dust-producing a coma around the icy nucleus. This coma is the head of the comet, and two types of tail develop, the bright dust tail and the fainter ion tail see this photo of Neowise that illustrates its two distinct tails. The micron (10^-6 m) sized dust particles are swept away by solar radiation pressure and begin forming the dust tail. Since dust particles have different sizes, they will each have unique velocities and thus enter a range of different solar orbits that result in their broad dust tail that are typically between 1 and 10 million miles long that reflect sunlight, often giving them a pale-yellow colour. The faint striations (striae) form when dust grains released from the comet’s head break up into smaller particles, creating these faint narrow bands. The ion tail, by comparison, consists of ionised gases that always point along the direction of the solar wind plasma and interact with its associated magnetic field. This faint blue ion tail thus follows the magnetic field lines rather than the orbital trajectory of the dust tail and can extend for up to 100 million km shining faintly due mostly to the de-excitation of carbon monoxide ions (CO⁺). (image credit Юкатан )

Interstellar visitors, 1/5/2020

To date two Interstellar Objects (ISO’s) have been observed travelling through our solar system, the first discovered on 19/10/17 by researchers using the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS 1) telescope in Hawaii, was subsequently named  1I/Oumuamua (Scout in Hawaiian), and the second discovered by the Crimean amateur astronomer Gennady Borisov on 30/8/2019 and named 2I/Borisov. It’s now believed a small sub-set of solar system objects collectively called Centaurs that orbit between Jupiter and Neptune may be captured ISO’s.

1I/Oumuamua was travelling at 57,000 mph in a hyperbolic orbit (V-shaped trajectory) see image 1 (Credit: Tomruen- Own work), and detailed studies reveal it was very unusual, tumbling in a puzzling manner, had a dry surface, and was elongated with an estimated length between 328 & 3,281 ft, and similar breadth and thickness between 115 & 548 ft; see image2. Credit: Meech et al./ESO. Such an unusual shape implies there’s a large population of rocky ISO’s that would need exoplanetary systems to each eject on average about 100 trillion objects like Oumuamua. Recent computer simulations show these elongated ISO’s can be produced prolifically by tidal fragmentation during a close encounter of their parent body with their host star (possibly a Red dwarf that represent 76% of all stars), which then ejects the elongated tidal fragments into interstellar space. Modelling confirms these close encounters with their host star plus heat generated by tidal disruption, will volatilise any ices and melt their surface. Following these close encounters their surface will then re-condense and solidify as they cool, thus preserving their elongated shape.  Modelling also suggests it’s possible some higher melting point volatiles like water could remain in their interior, explain the reddish surface colours detected by spectroscopic analysis, the absence of a visible coma, and the inferred dryness of these ISO’s.

The slight non-gravitational acceleration detected at perihelion implies it may still contain some subsurface water ice that’s sublimating and venting as it approaches the sun, explaining its increasing speed; see RH image. see outgassing in image 3. (Credit: ESA/Hubble, NASA, ESO, M. Kornmesser)

Interestingly, spectral analysis confirms it has a reddish surface colour due to the presence of complex organic chemicals called Tholins, produced by cosmic irradiation of simpler organic chemicals like carbon monoxide (CO), methane (CH4), water (H2O), and hydrogen cyanide (HCN), These chemicals are also found on the moons of planets beyond Jupiter, and on Pluto and smaller objects in the Kuiper belt. This suggests it may be an exocomet from the outer zone of another planetary system that’s lost its more volatile ices like CO and CH4.

2I/Borisov is definitely an exocomet because it displays a coma. It was travelling even faster than Oumuamua at ~75,600 mph when detected, see image 4; Credit NASA/ ESA/ K. Meech (University of Hawaii)/ D. Jewitt (UCLA)/ Hubblesite.

The Atacama Large Millimeter/submillimeter Array (ALMA) telescope in Chile has detected between 9 and 26 times more CO compared with any other comet approaching perihelion. Further studies using the Cosmic Origins Spectrograph (COS) on Hubble, observed 50% more CO than H2O in the coma, and 3 times more CO than any other comet seen near the sun. This confirmed 2I Borisov must have formed from material unusually rich in CO ice, which only occurs below -250^o C, implying it probably formed in the chilly outer region of a carbon-rich planetary disc of icy material around a cooler star than the sun, most likely a Red dwarf star that accounts for ~76% of all stars formed. Like 1I/Oumuamua was probably gravitationally ejected by a large gas giant.

Centaurs originate mainly from the outer edge of the Kuiper belt, but recent studies of the origin of 17 high-inclination Centaurs and 2 trans-Neptunian objects with unusual orbital dynamics, suggests they are captured ISO’s. Due to the inherent instability of Centaurs, because their orbits always cross the orbital plane between Jupiter, Saturn, Uranus, and Neptune, consequently gravitational interactions arising from a close encounter with any one of these planets will result in either their ejection from the Solar System or deflection onto a collision trajectory with another planet or the sun. This means they all have relatively short lifetimes, <100 million years, implying these captured interstellar Centaurs must have been captured during the recent past. This raises the interesting thought that captured ISO’s may be a means of introducing prebiotic chemicals or simple organisms capable of seeding life in our solar system, which originally developed in an exoplanetary system, a process known as panspermia.

A Martian mystery 3/12/19

NASA’s Mars rover Curiosity has been exploring the surface of the 96 miles wide Gale crater in the northern hemisphere of Mars, which was believed to have been a lake in the distant past. In addition, Curiosity has also been studying its atmosphere since landing in August 2012, and one instrument, the Sample Analysis at Mars (SAM) has produced intriguing data about its atmosphere. This module consists of two instruments capable of analysing different gases in the atmosphere (and those released from heated solid samples), and a third that can distinguish between different isotopes of oxygen, and carbon isotopes in carbon dioxide and importantly methane which can be produced by anaerobic microorganisms (methanogens), to try to establish if their origins were geochemical processes (abiotic), or due to living processes (biogenic).

Over the course of three Mars years (1 Mars year = 687 Earth days) SAM inhaled the atmosphere above Gale Crater establishing that it contained 95% carbon dioxide (CO₂), 2.6% nitrogen, 1.6% argon, 0.16% oxygen, 0.06% carbon monoxide (CO), and an infinitesimal 0.4 parts per billion (ppb) methane (CH₄).   In addition, during winter periods CO₂ freezes onto the respective polar surface and sublimes during the spring and summer periods, so this cycling of CO₂ alters the atmospheric mass by tens of percent over the course of the Martian year. As expected, nitrogen and argon wax and wane with the seasons as the atmospheric pressure changes, yet oxygen and methane don’t wax and wane. Oxygen is higher than expected in the late spring and summer but lower in the winter (chart 1), while methane increases and peaks during the summer and then declines during the autumn, coupled with occasional sporadic spikes in concentration (chart 2).

Chart 1

Chart 2

Given the strong evidence for the existence of liquid water on the surface of Mars during its first 1.5 billion years, and it’s postulated Mars may have harboured simple life during this period. Some astrobiologists conjecture that life may still exist today, and some have speculated that life on Earth may have seeded Mars, or alternatively, that early life on Mars may have seeded life on Earth. Thus it’s currently unclear if these observed changes in atmospheric oxygen and methane are abiotic (geochemical) or biogenic (due to bacterial life). That said, it’s interesting to note that Mars and Earth have similar atmospheric carbon and oxygen isotope ratios for ¹²C/¹³C and ¹⁶O/¹⁸O.

Whilst it is unclear how the changes in oxygen concentration arise, but it’s suspected they may be connected with surface soil that contains abundant oxidants (peroxides), given there’s insufficient oxygen in its atmosphere to account for these seasonal changes. On average Mars’s atmosphere contains ~0.4 ppb of methane in its atmosphere, which would be destroyed within about 4 years, yet this level varies from ~0.24 ppb during the northern hemisphere winter to ~0.65 ppb during the summer, whilst at the same time spikes in concentrations have been detected indicating it’s periodically released from discrete nearby regions. Again, it’s not known with any certainty how methane is created nor how its concentration can briefly spike, but Chart 3 below illustrates some potential mechanisms.

The first image of a black hole (14/4/2019), revised and reposted 3/12/19.

On 12^th April 2017, a global array of 8 radio telescopes called the Event Horizon Telescope (EHT) using a technique known as long baseline interferometry, spent 4 days studying the supermassive black hole at the center of the giant elliptical galaxy Messier 87. This galaxy lies ~53 million Light Years (LY) away near the center of our galactic local group, which contains about 2,000 galaxies including our Milky Way, known as the Virgo cluster, which itself is part of the Virgo supercluster.  The EHT created the equivalent of an earth-sized dish capable of achieving the angular resolution needed to create this first image of a black hole. It wasn’t until 10^th April 2019 that this first image 1 (Credit: Event Horizon Telescope collaboration et al.) was published, confirming the predictions of Einstein’s General Theory of Relativity, namely that spinning (Kerr) black hole event horizons were spherical,  which are defined by their Schwarzschild radius, this is the point where the escape velocity exceeds the speed of light.

The actual EHT image is a computer-generated image of the microwave shadow of the photon sphear embedded in the distorted image of the accretion disc and two relativistic polar jets of plasma. The accretion disc that’s external to the photon ring is about 25,000 AU wide (0.39 light-years), and spins at about 1,000 km/second (3.6 million km/hr), and the two polar jets streaming out of the photon sphere along the spin axis of the black hole have been accelerated to 99.9% the speed of light (300,000 km/sec). The inner temperature of these jets is around 10 trillion degrees Celsius, and they can extend to a distance of about 5,000 LY’s. As they propagate to these great distances they cool and lose coherence, forming extensive structures resembling plumes and lobes that eventually disperse into interstellar and/or intergalactic space. The variation in image brightness around the black hole shadow is due to Relativistic beaming, also known as Doppler beaming, which results in radiation from the accretion disc moving towards us appearing brighter and away from us appearing fainter.

This next image (2) is a NASA supercomputer simulation that illustrates what outside the horizon of a Schwarzschild black hole illuminated by a thin accretion disk looks like due to the way its intense gravitational field grossly distorts spacetime in its immediate vicinity.

The photon ring often referred to as the photon sphere is the region where gravity is so strong, electromagnetic radiation from the accretion disc is constrained to travels in unstable. near-circular orbits due to the extreme gravitational distortion of space. In theory, in this region, looking directly ahead you would be able to see the photons emitted from the back of your head! Within the photon ring particles of charged plasma either follow trajectories into the black hole or are accelerated to relativist velocities by twisting magnetic fields and ejected in opposite directions along the spin axis of the black hole. The dark area inside the photon ring is about twice the area of the actual black hole event horizon that has a radius of about 18 billion km (~120 AU), and an estimated mass of ~6.5 billion times larger than the Sun.

Image 3 infographic illustrates the undistorted appearance of the accretion disc and two polar jets. Image 4 is an X-ray image taken by the space-based Chandra X-ray telescope showing the wider region of hot plasma surrounding the region around the black hole at the center of M87, and show the polar jets.

              Image 3                                                                                          Image  4

 

The probe is named after the fossilized hominid named Lucy, who walked in eastern Africa 3.2 million years ago, and the mission will be the first to visit Jupiter’s Trojan asteroids. The asteroids in question occupy the two most stable Lagrange points where Jupiter and the sun’s gravity are the same, and these are located 60 degrees ahead (L4) and 60 degrees behind (L5) Jupiter’s orbit. These Trojan’s orbit with the same period as Jupiter meaning they orbit in a 1:1 resonance with Jupiter. These two co-orbiting clouds are named after heroes from the Trojan War, with L4 members named after the Greek camp and L5 members named after the Trojan camp as the diagram illustrates. Note Hildas are not classed as Trojans because they have orbital periods 2/3 that of Jupiter, i.e. they are in a 3:2 resonance with Jupiter, they also orbit well inside Jupiter’s orbit close to the outer region of the asteroid belt, and have a third concentration at L3 (directly on opposite side of the sun) as well as at their L4 & L5 points.

These Trojans are remnants of the primordial material that formed the outer planets and should be capable of revealing information about the formation and early evolution of the giant planets, the physical environment that existed at the locations where they formed, and the planetary dynamical evolution that resulted in these planetary remnants becoming trapped Trojans. Both clusters of Trojans will provide our first close-up view of all three major types of planetary fossils from the birth of our solar system, the dark-red P and D-type Trojans similar to those found in the Kuiper Belt of icy bodies beyond the orbit of Neptune, and the C-types found mostly in the outer region of the asteroid belt between the orbits of Mars and Jupiter.

The diagram illustrates the complex trajectory of Lucy, due for launch in October 2021. En route to Jupiter, it will visit the small inner asteroid belt c-type asteroid 52246 Donaldjohanson (named after the fossil Lucy’s discoverer) in April 2025.  When the probe reaches Jupiter in August 2027 it will start its flyby of four L4 Trojan asteroids, the c-type (3548) Eurybates, p-type (15094) Polymele, d-type (11351) Leucus, ending with the c/d type (21900) Orus in August 2028. The probe will then dive back past Earth for a gravity boost enabling it to return to the L5 cloud, where it will encounter the large binary Trojan P-type  617 Patroclus/Menoetius in March 2033.

Transit of Mercury 11/11/2019, start 12.35 pm end 18.04 pm (for Mersyside it ends at 16.22 pm with sunset)

In November 2019 Mercury transits (crosses) the face of the Sun in a Southeast to northwest direction compared with the Northeast to Southwest direction during its previous transit in May 2016. The reason is that November and May transits of Mercury are viewed from opposite sides of the Earth’s orbit. In November Mercury will be seen ascending from South to North of the ecliptic [the apparent path of the Sun across the sky (background stars)], and by comparison, May transits are descending from North to South of the ecliptic.

May transits also occur at aphelion when Mercury is furthest from the Sun and traveling slower, meaning transit times take longer, whilst November transits occur at perihelion when its closest to Sun and thus moving faster, so transit times are shorter.

Click view from Merseyside for details, including a 500X animation of the transit.

 

The lost water of Mars (12/5/2019)

NASA recently announced evidence that a massive ancient ocean once covered about 20% of Mars, which filled the low-lying area of its northern hemisphere to a depth of upto 1 mile in places. Credits: Mars Geronimo Villanueva/Nasa.

 

This indicates that shortly after Mars formed 4.5 billion years ago, it was wetter and warmer with an Earth-like water cycle, where rain fell onto upland areas, forming trickling streams that fed winding rivers, which formed deltas as they flowed into this ancient ocean.

It is estimated these conditions lasted for over 1.5 billion years before fading away, a period long enough to have allowed primitive life to develop, given it had already developed on earth during this period. Due to its smaller size, Mars’s gravity was less than the earth’s, meaning its atmosphere leaked away at a much faster rate than earth. As the atmosphere gradually thinned the surface pressure slowly fell, causing this ocean to evaporate at an increasing rate. Due to the highly elliptical orbit of Mars compared with Earth, its southern summer coincided with its closest approach to the sun (perihelion). This results in warmer summer temperatures compared with the northern hemisphere, and at certain times of the day, water vapour could rise locally with warmer air masses into the upper atmosphere, and as it travels to the colder north polar region some of it’s ionised by solar UV forming H⁺, OH^–  and O^-2 radicals. This enables the traces of atomic hydrogen and atomic oxygen to escape into space, with the remaining unionised water falling like rain onto the surface of the colder winter northern hemisphere. Over time this steady loss of water depletes the planet of its inventory of water, leading to today’s desiccated low atmospheric pressure (~0.6% of Earth’s) chilly surface (about -143 °C polar winter to +35 °C equatorial summer) of Mars.

Evidence for this progress of loss of water comes from infra-red studies of the permanent mostly water (~70%) polar ice caps. Since water contains mainly light hydrogen (H₂O) it evaporates faster than heavy water, which contains heavier deuterium (D₂O), so the residual Martian water becomes steadily enriched with heavy water, enabling the estimated 90% loss of water since the formation of Mars. Today subsurface water can still be seen leaking from the sides of cliffs during the summer period, as the dark streaks towards the bottom of this photo shows. (Image: © NASA/JPL-Caltech/Univ. of Arizona)

Saturn’s disappearing rings (23/12/18)

When the Cassini probe made its final dive into Saturn’s atmosphere, it passed between Saturn’s mostly water ice ring system and the top of its atmosphere, where it confirmed the existence of micron (10^-9 meter) sized dusty water ice particles falling into the planets equatorial upper atmosphere (ionosphere). NASA estimated the flow rate of water ice was equivalent to an Olympic swimming pool full of water every 30 minutes and concluded its spectacular ring system must be <100 million years old based on the current appearance of the inner C and D rings. Further, at this rate of water loss, they concluded Saturn’s spectacular rings will only last for about another 300 million years.

Micron-sized particles can become electrically charged by either solar ultraviolet light or by plasma clouds created by micrometeoroid impacts with Saturn’s sand grain to meter-sized boulders of disc ice. When these tiny particles become charged they’re attracted to the planet’s magnetic field that curves inward toward the planet at Saturn’s rings, where gravity can then pull them in along the magnetic field lines into the upper atmosphere.

If planetary rings are temporary features we’re lucky to be able to see Saturn’s current ring system, and we’ve probably missed out seeing similar ring systems around Jupiter, Uranus, and Neptune, which only have faint thin ringlets today.

Total Lunar Eclipse (Blood Moon) (26/7/18)

These occur where the orbit of a full Moon intersects the plane of the Earth’s orbit around the Sun, and the Moon passes behind the Earth into the Earth’s shadow (umbra). As Sunlight passes through the Earth’s atmosphere the shorter blue wavelengths are scattered (the reason the sky is blue during the day), and the remaining longer red wavelengths are refracted onto the Moon giving it a reddish hue. The first image illustrates the alignment of the Sun, Earth, and Moon together with the light paths needed to produce a Total Lunar Eclipse. The second image shows what a Blood Moon should look like.                                                                                   

According to NASA, there will be 230 lunar eclipses during the 21st century, and 85 will be total lunar eclipses. Friday 27th July will be the longest of the century, with a duration of 1 hour 43 minutes and 35 seconds. To see this Blood Moon from the UK, look South-East after 9.00 pm when the Moon rises and will be at mid-phase around 10.30 pm. It will be the 17th total lunar eclipse of the century, and the next will occur on 21 January 2019.

Venus, Earth’s twin, or maybe not. (19/6/18)

It is often stated that Venus is Earth’s twin given its similar size, mass, density, composition, and gravity, but it has several key characteristics that challenge this popular trope.

The Earth rotates prograde (sun rises in the east and sets in the west) taking 23 h 56 m to spin once about its axis, 365.256 days to complete one orbit of the Sun, has an atmosphere consisting of 21% oxygen, 78% nitrogen, and 1% argon with a surface pressure of 1 atmosphere, and has an average surface temperature of 14.9 ^o C, ranging from -89.2 .^o C in the Antarctic to 58 ^o C in North Africa.

The generation of a strong global magnetic field requires fluid core convection, which in turn requires a flow of heat from the core into the overlying mantle that is the driving force of plate tectonics. Only the Earth’s outer core is fluid and convective and able to generate an electric current, which in turn creates a magnetic field. As charged convecting core metals flow through this magnetic field they, in turn, generate an electric current, creating a self-sustaining loop known as a Geodynamo. The spiralling caused by the Coriolis force arising from the Earths rotation, roughly aligns the separate magnetic fields lines in the same direction, forming a global scale magnetic field with a magnitude ranging from 25 to 65 microteslas (0.25 to 0.65 gauss) at the surface.

By comparison, Venus’s rotation is both slow and variable taking on average 243 Earth days, yet despite this slow rotation, its upper atmosphere completes one rotation in only 4 Earth days!  Venus orbits the sun every 225 Earth days, but due to its retrograde rotation (results in the Sun rising in the west and setting in the east), the time from one sunrise to the next is about 117 Earth days. The atmosphere of Venus (by volume) consists of 96.5% carbon dioxide, 3.5% nitrogen with a surface pressure of 92 atmospheres. Due to an atmospheric Greenhouse effect, this results in a uniform surface temperature of 462 ^o C, with the lowest temperature a (balmy?) 330 ^o C on the peak of Maxwell Montes, the 11,000 m high mountain, where it’s still hot enough to melt lead (m.p. 327.46°C). This makes Venus the hottest planet in the solar system. Despite being closer to the Sun the dayside of Mercury reaches an average of 427 ^o C, peaking at up to 450 ^o C in places.

Compared with Earth, Venus has a tiny magnetic field only 0.000015 times that of the Earth’s. The reason is due to a combination of its very slow rotation, the absence of plate tectonics suggests an absence of any significant internal convection, which could mean the core may be solid. If a fluid metallic outer core does exist it would not be rotating fast enough to produce a geodynamo capable of generating a significant global magnetic field.

Venus’s variable rotation has recently been explained by data from the Akatsuki spacecraft of JAXA, the Japanese space agency. In a new study, researchers showed how the interaction between Venus’s fast-moving atmosphere and slower moving surface, with its volcanoes and extensive mountain ranges, alters the speed of the planet’s rotation. This interaction generates huge bow-shaped atmospheric structures that keep disappearing and reappearing, yet remain in the same location above mountains on the planet’s surface. These strange structures are believed to be mountain waves that cause Venus to rotate at varying speeds because of the different directions of the wind flowing upstream and downstream against the mountains. This generates a net force on the mountains, in other words, angular momentum is transferred between the solid body of Venus and its atmosphere.

The image shows the most prominent stationary white bow-shaped mountain wave located in the upper atmosphere of Venus, where clouds in this region generally move at speeds of about 100 meters per second. Credit: Planet-C

Supermoon (and Micromoon) 28/01/18

Since the Moon’s orbit is elliptical and not circular, its distance from Earth varies meaning its apparent size when seen from Earth must change. A Supermoon is a full moon (or a new Moon) that is within 90% of its closest distance to Earth or its perigee. 

A Full Moon at perigee is 362,600 km from Earth and appears approximately 14% larger in diameter than at its greatest distance (apogee) of 405,400 km when it is known as a Micromoon.

Since the Full Moon’s surface luminance and hence perceived visual brightness is constant, at perigee, it appears about 30% brighter than at apogee, a difference that’s due to the inverse square law of light.

On  31st January 2018 the first super Blue Moon and total lunar eclipse by the Earth since 1866 occurs. During this rare event, the Moon will be within 90% of its perigee (Supermoon), be the second full Moon in the month (Blue Moon), and will undergo a full lunar eclipse. Due to the scattering of the blue wavelengths of sunlight, as they pass through the Earths’ atmosphere, it will take on a red/orange hue due to the loss of the shorter wavelengths.

Total solar eclipses (21/8/17)

Total solar eclipses are due to a remarkable coincidence. Despite the fact the Moon is slowly moving further away from the Earth at a rate of 3.78 cm/year, within the current range of varying Earth-Moon distances, it will occasionally be ~400 times less than the distance of the Earth from the Sun. When the Moon’s orbital geometry results in a New Moon passing directly in front of the Sun, because the diameter of the Moon is 400 times less than the diameter of the Sun, their angular diameters will appear to be the same when seen from the Earth, resulting in a total solar eclipse within a region called the umbra. The optical geometry of an eclipse is illustrated in these diagrams.

Under the umbra shadow, a long narrow path of totality (darkness) is traced across the surface of the earth that varies depending on one’s local geographical position but is typically 140 miles wide and up to 6,000 miles long. The duration of totality can last from a few seconds to a few minutes, during which the sun’s outer atmosphere called the solar corona can be seen. Beyond the umbra is a region known as the penumbra where partial eclipses of varying degrees can be seen; at the outer edge of the penumbra, only about 1% of the Sun will be obscured.

 These total solar eclipses occur when a New Moon crosses the Earth’s orbital plane (ecliptic) and can thus obscure the Sun, an event that happens on average about every 18 months.

Are there life-bearing planets like Earth  (0.2 to 2.0 M_e) nearby? (Updated 11/4/17)

NASA recently announced the discovery of 7 earth-sized planets orbiting a tiny ultra-cool M dwarf star with a mass 8% that of the sun called TRAPPIST 1 A. This star is only just big enough to be called a star, having a mass a little over 90 times that of Jupiter, and a surface temperature of 2286°C, making it appear about 2000 times less luminous than the sun, yet it will remain a mainstream star for about 10 trillion years compared with 10 billion for the sun. M dwarf stars constitute 76% of all stars in the universe, and ultra-cool dwarf stars plus brown dwarfs (L, T & Y class objects) with masses between 13 & 90 Jupiter masses* represent 15% of all dwarf stars. The accretion model of planet formation predicts earth-sized planets should readily form around these types of objects, implying earth-sized planets may be far more abundant than previously thought. It is thus likely many of these planets would be orbiting within the “Goldilocks” habitability zone where liquid water can exist.

What makes TRAPPIST 1A interesting, is that it’s only 39 light-years away in the constellation of Aquarius, making its 7 orbiting Earth-sized planets close enough to study. They all orbit close to this M dwarf star with orbital periods between 1·51 to over 20 days, which would place them all well within the Sun-Mercury distance – see graphic. Despite its feeble energy output, three planets have surface temperatures that could enable liquid water to exist, provided they had retained an atmosphere containing greenhouse gases that raised surface temperatures high enough to enable liquid water to exist, a key requirement for organisms like simple bacteria or even protozoa to evolve. These planets are designated 1 e, 1 f, and 1 g, with equilibrium surface temperatures of -22°C, -54°C and -74°C respectively. If any possess an atmosphere, the James Webb telescope due for launch in 2018 would be able to identify whether or not any contained oxygen and greenhouse gases like carbon dioxide, water vapour, or methane. The presence of oxygen and/or methane would both be indicators of the possible existence of simple lifeforms.

There are, however, two factors reducing the likelihood of simple lifeforms developing. Orbiting so close to their parent star means planets will be tidally-locked signifying they would have a permanent hot day and frigid night hemispheres. How planetary climates would be affected remains largely unknown, although strong circulating winds could be expected, making twilight zones the most promising locations for life to develop. More importantly, M dwarf stars emit violent flares in their youth that would strip an atmosphere away and emit intense X-ray and extreme UV radiation, neither of which are conducive to life developing.

More recent studies of Trappist-1 over several weeks confirm it is subject to violent flares that occur on average every 28 hours. These storms are also many thousands of times more violent than the Carrington Event, which was the strongest recorded solar geomagnetic storm that hit Earth in 1859. Storms of this magnitude would be very destructive to any planetary atmosphere, reducing the chances of any life developing, especially given their closeness to the star. Computer simulations suggest it may take up to 30,000 years for a planetary atmosphere to recover from events that are generated about every 28 hrs. This situation is not conducive to the development of life, but that said, life would have trillions rather than billions of years to evolve, and nobody really knows how these dwarf stars behave during their mid-life period, since there’s been insufficient time since the big bang for them to reach a (stable?) mid-life phase.

 

Image credit: NASA/JPL-Caltech/R. Hurt, T. Pyle (IPAC)

*IAU defines Brown dwarfs as objects with a mass between 90 and 13 Jupiter masses since their cores are not hot enough to initiate core hydrogen fusion, but still hot enough to initiate core deuterium (heavy hydrogen) fusion. Below 13 Jupiter masses, their cores are too cool for any fusion to occur, and they are defined as planets.

What are sunspots?

The surface of the sun (photosphere) glows at a steady temperature of 5,500 º C, by comparison, sunspots are temporary areas with a lower temperature of 2,700 to 4,200°C that typically last between a few days to a few weeks.  At this lower temperature they appear darker, but on their own would be brighter than a full moon. The reason for this visible difference is because the Sun’s brightness approximates to luminescence that varies to the fourth power of temperature (L ~ T ⁴), so sunspot brightness appears far lower (darker) than the rest of the photosphere.

Sunspots are caused by the sun’s differential rotation steadily winding up its magnetic field below the surface, forming magnetic flux tubes that eventually burst through the surface. When this happens heat conveyed by convection towards the surface slows, reducing the surface temperature. Close up, sunspots display two zones, a black central area (umbra) where the magnetic field lines emerge more or less vertically (maximum suppression of heat flow), and a surrounding area (penumbra) that appears lighter because the emerging magnetic field lines are inclined wrt the surface (reduced suppression of heat flow). This confirms there’s an increasing temperature gradient between the center and the edge.

Yes, we are made of stardust.

The late astronomer Carl Sagan, when he opened the first episode of his TV series about space, “Cosmos: A Personal Voyage”, included these lines in his opening remarks.

“The surface of the Earth is the shore of the cosmic ocean. On this shore we’ve learned most of what we know. Some part of our being knows this is where we came from. We long to return, because the cosmos is also within us. We’re made of star-stuff. We are a way for the cosmos to know itself.”

This version of the Periodic table uniquely identifies the principle cosmic processes responsible for creating the elements in our solar system, which were also made from, confirming his prescient observations that the cosmos is truly within us because we know we are made of star-stuff.

 Image credit: Jennifer A. Johnson/The Ohio State University; NASA; ESA

Note: Promethium (Pm) and Technetium (Tc), (both in grey) do not occur naturally on earth, and are only found in nuclear reactor fission products and fission weapon fallout. In the case of Tc 99 (half-life 4.2 million years), virtually immeasurable traces have been detected in Uranium ore due to the spontaneous fission of U 235, and traces have also been detected in a subclass of Red giant stars called Technetium stars.

Pluto – Is it a dwarf or binary planet?

When a celestial body orbits another celestial body, in reality, they’re both orbiting around a common center of mass called the barycentre. The barycentre usually lies inside the larger body, such as planets orbiting the sun, or the moon orbiting the Earth, in the latter case it lies 1,710 km (1,062 miles) below the surface, causing the earth to wobble as the moon orbits it.

Pluto and its moon Charon are only separated by a distance of 12,000 miles and have an unusually high mass ratio, which means the barycentre lies between them, so they both orbit around this common centre of mass. This situation is typical for many binary asteroids, binary stars, and for Jupiter and the Sun.

Because Pluto and Charon are orbiting this common point, Charon would appear to be suspended at a fixed point in the sky, so try to imagine if our moon was three times closer to Earth and the size of Mars, that’s how Charon would look from the surface of Pluto.

Planet nine, fact or fiction? – The evidence.

Caltech astronomer Michael Brown and theoretical astrophysicist Konstantin Batygin have found evidence for a possible 10 Earth-mass planet that may be tilting the orbits of long-period orbiting dwarf planets with perihelia >36 AU into high inclination eccentric orbits, and shepherding them into clusters well beyond the 30 AU orbit of Neptune.

When planetary systems are born, the planets form within very flat discs around the equatorial plane of their Protostar, and the planetary orbits in our system are consistent with this principle, being aligned within about 1° of each other. The Sun’s rotation was first measured in 1850 and it was immediately realised its spin axis appeared to be tilted by 6°  with respect to the planets. Whilst a 6° angle is relatively small, it’s very significant in terms of what’s seen in the majority of exoplanetary systems.

Over a period of 4 billion years, the presence of Planet Nine would ensure the apparent obliquity of the sun was 6°. However, the direction of the suns axial rotation has not changed since its formation, rather it is the planetary orbits that have all been tilted by this proposed Planet Nine. In other words, while it appears to us it’s the Sun that’s tilted, it’s actually the other way around because the earth lies in the tilted plane of the suns planetary disc.

The reason Planet nine is postulated to be about 10 Earth masses, compared with say, Jupiter’s 300 Earth masses, is because its orbital distance is very large compared with that of Jupiter. This large orbital distance can thus exert a significant torque on the inner planets without needing to apply any significant force, giving it almost the same amount of angular momentum as the rest of the planets combined!

If Planet nine exists, it’s hypothesised it would have a diameter of ~40,000 km, an orbital period of ~15,000 years, be tilted ~30° to the plane of the solar system in a highly eccentric orbit ranging from a perihelion of ~200 AU to an aphelion of ~1200 AU. As to its origin, there are two possibilities. It may have been a ninth planet that was thrown into a distant highly eccentric orbit rather than being expelled during the early evolution of the solar system when Jupiter and Saturn were migrating inwards towards the protosun, and then reversed direction due to a unique gravitational interaction between them. Alternatively, it may have been a rogue planet that was expelled from its own planetary system and captured by the sun as it passed close by around 4 billion years ago.