Astro briefings

This page highlights topical astronomy facts and research of interest.

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

It is often stated that Venus is Earths 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 day side of Mercury reaches an average of 427 o C, peaking at upto 450 o C in places.

Further, 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 volcano’s 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 causes 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.

venus mountain wave

Credit: Planet-C

Supermoon (and Micromoon) 28/01/18

Since the moons 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.

super-moon-real-deal-comparison

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 (a 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 moons 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.

Under the umbra shadow a long narrow path of totality (darkness) is traced across the earths surface that varies depending on one’s local geographical position, but is typically 140 miles wide and upto 6,0000 miles long. The duration of totality can last from a few seconds to a few minutes, during which the suns 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 earths orbital plane (ecliptic) and can thus obscure the sun, an event that happens on average about every 18 months. The optical geometry of an eclipse is illustrated in the diagrams below.

Imaging black holes (18/4/17).

On 12th April 2017 six radio telescopes collectively called the Event Horizon Telescope (EHT) located around the world, made the first sub-millimeter microwave high angular resolution observations of the closest super-massive black hole to earth over a period of 6 days. This black hole is called Sagittarius A and is located 25,000 L Y away at the centre of the Milky Way; observations of a second black hole located at the centre of the giant elliptical galaxy M 87 about 53.5 million L Y away near the centre of the Virgo cluster are also planned. Sagittarius A has a mass of 4.1 million solar masses, which is tiny compared with black hole at the centre of M 87 with an estimated mass of between 3 & 7 billion solar masses. This black hole is emitting collimated jets of relativistic plasma with an energy of 5.1 × 1049 joules. By comparison the total energy output of the Milky Way is estimated to be 5 × 1036 joules per second.

These individual radio telescopes when combined, create the equivalent of a single earth sized dish capable of achieving the angular resolution comparable to the size of the event horizons of black holes, using a technique called very long baseline interferometry. It will be several months before all the data sets have been merged and analysed, but if successful the resultant image should be able to confirm or disprove many of the predictions of Einstein’s General Theory of Relativity that describes the behaviour of matter close to the immensely strong gravitational fields adjacent to black hole event horizons.

Are there life bearing planets like earth  (0.2 to 2.0 Me) 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 type 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 1 A 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 places 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 mean planets will be tidally-locked signifying they would have 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 atmospheres 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 that was the strongest recorded solar geomagnetic storm that hit earth in 1859. Storms of this magnitude would be very destructive to any planetary atmospheres, reducing the chances of any life developing, especially given their closeness to the star. Computer simulations suggest it may take upto 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.

trappist-1-system

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

*IAU define 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 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 suns 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 emerges 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 to the surface (reduced suppression of heat flow). This confirms there’s an increasing temperature gradient between the centre 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 making the elements in our solar system, which we’re also made from, confirming his prescient observations that the cosmos is truly within us, because we know we are made of star-stuff.

origin-of-elements 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 sub class 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 centre 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 plane of 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 plant that was expelled from its own planetary system, and captured by the sun as it passed close by around 4 billion years ago.

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