Tag: Kuiper Belt

Why you should care about the New Horizons probe nearing Pluto

The Wire
May 29, 2015

Alex Parker is a planetary astronomer at the Southwest Research Institute, Texas, and he posted his tweet just as I started writing this piece. And not just for Parker – it’s an exciting time for everyone, an exhilarating period in the history of space exploration. In just under 48 days – on July 14, 2015 – the NASA New Horizons space-probe will make its first fly-by of our favourite dwarf planet Pluto. Until then, it will be relaying less and less grainy pictures to Earth, each of more interest than the last, of a cold and distant world discovered by Clyde Tombaugh in 1930. One batch of images taken from May 8 to May 12 has already added to old evidence that Pluto hosts icy polar caps, and variations in surface brightness suggest a more uneven composition. On May 28, New Horizons restarted another phase of imaging – and as each day takes the probe 1.2 million km closer to its target, this is Pluto finally emerging out of the blur.

What more could we stand to find out? Quite a lot, as it turns out, from three points of view:

1. Toward the outer limits

The engineers operating the Voyager 1 space-probe (currently the farthest human-made object from Earth) had exciting news in September 2013: they claimed that about a year earlier, the probe had entered the interstellar medium – the space between stars, where the Sun’s influence was no longer the dominant one but had to contend with particulate emissaries from other stars in the galaxy. At the time, V1 was running on what little remained of its battery, a feeble ingot of blinking lights 19 billion km from Earth, and the occasion was replete with symbolism: humankind (or a representative) had set foot into the universe.

Actually, that moment could’ve transpired earlier. The engineers said that, in February 2012, the readings to indicate if V1 had entered the interstellar medium were spotted by the probe. However, they couldn’t be verified because the instrument that could do that had run out of juice. Luckily for them, a solar flare that erupted in March 2012 set the region of space around the probe thrumming with energy, which V1’s weak were able to pick up on and settle the matter.

Pluto, now, is much closer to the Sun than the threshold of the interstellar medium – in fact, the distance between Pluto and the Sun is 3.7 times smaller than the distance between Pluto and the medium. However, it is still quite far, and any space-probe sent to study it will either have to use up as little of its battery as it can until the rendezvous or be able to make only perfunctory observations of the dwarf planet. New Horizons is of the former kind – its primary mission is the farthest till date, and unlike the Voyager and Pioneer probes, will be able to respond to its environment agilely and be less susceptible to the vagaries of a dying battery.

2. Within the outer limits

Even if Pluto is among the outermost significant, planet-like bodies to orbit the Sun, it’s equally significant as being the largest body in the Kuiper Belt, a ring of asteroids like the one between the orbits of Mars and Jupiter. The belt starts from around the orbit of Neptune and extends to six AUs beyond the orbit of Pluto (AU is the astronomical unit, the distance between Earth and the Sun: 149.5 million km). It is also 200 times heavier than the Mars-Jupiter belt. Overall, both belts are important for two reasons in the context of New Horizons.

Before the Solar System took the form we now know – with a star at the centre, eight planets orbiting it, and two rings of asteroids – it comprised a young Sun at the centre of a massive disk of gases, dust and other materials called the protoplanetary disk. It is so named because it is out of this disk that the Solar System’s planets condensed, born as clumps of matter whose gravity accrued more matter, growing in size. And even as a planet formed, its gravitational pulls would ‘clean’ out a space in the protoplanetary disk, forming gaps. This phenomenon is visible among Saturn’s rings as well, with the space between rings having been cleaned out by the formation of small moons. The gaps in the disk survived to this day as the space between planets’ orbits. On the other hand, parts of the disk that didn’t get fully cleaned out formed the asteroid belts. So, they’re residues of the matter that the first planets were formed of, and studying them throws a lot of light on the history of the Solar System’s formation.

The second reason is that the asteroid belt between the orbits of Mars and Jupiter and the Kuiper Belt are separated by 4.2 billion km – even on the cosmological scale, that’s a non-trivial gap. However, many objects in the two belts share chemical and physical properties as if they were once part of a common larger body. One logical explanation is that the belts were ‘mixed’ after they were formed. And to explain such mixing, astronomers have an awe-inspiring yet plausible explanation. According to them, as Jupiter was forming, its orbits moved closer to the Sun and then farther away, before shrinking down to place it between the inner asteroid belt and Saturn. The increase and decrease in the orbit’s size could’ve been due to the formation of other planets in the system, which would’ve disrupted the gravitational equilibrium. And while Jupiter moved, its prodigious gravity could’ve tugged a part of the inner asteroids out and vice versa, resulting in a mixed composition of asteroids in both belts. Since Pluto is the largest among Kuiper Belt objects, New Horizons studying it in detail could provide more clues about if such mixing could’ve happened.

3. Beyond the outer limits

Pluto is all of 2,300-km across – the distance between Kanyakumari in south Tamil Nadu and New Delhi – and it has five moons all to itself: Nix, Styx, Hydra, Charon and Kerberos. All of them are Kuiper Belt objects, too, and astronomers are curious to know if Pluto has a ring system as well, populated by smaller asteroids. The dwarf planet will also likely have smaller rocks orbiting it, and dust particles kicked up as a result of collisions between them. Such dust will be dangerous for New Horizons because they could impact the probe at some 50,000 km/hr and damage on-board systems. In January 2014, Simon Porter, one of the probe’s mission scientists, had told Wired that to protect against such collisions, his team had a contingency plan in mind: to turn the probe’s 2.1-metre-wide dish antenna into a shield.

If the probe does make it through the danger zone and get to within 12,500 km of the surface of Pluto, its observations of any rings as well as the dwarf planet’s surface, atmosphere and any craters/seismic activity will reveal more about the composition of Kuiper Belt objects, how they interact with each other, whether they sport any signs of violence from the past, and if at all they have atmospheres, what they’re composed of – information important to understand how and where the Solar System’s other planets could’ve formed. Astronomers also already know that Pluto’s surface has frozen methane and carbon monoxide.

This and other data gleaned from Pluto and its surroundings will take until late-2016 to be transmitted to Earth but the probe’s journey will continue – rather, has to continue because a probe that’s gone so far might as well just go farther because of the considerable time taken to travel such distances. Because the primary mission will almost exhaust its battery, the probe will subsequently become less manoeuvrable – like the Voyager and Pioneer probes did, yet still boast of a sophisticated suite of instruments. To take advantage, astronomers from the Southwest Research Institute, including Alex Parker, had spotted three other Kuiper Belt objects in New Horizons‘ path in late 2014 that would be interesting to study. All three objects are about 30-55 km across and located about 44 AU from the Sun, meaning the probe will reach them around 2020. This timeline is very interesting because NASA plans to launch the James Webb Space Telescope – successor to the Hubble and Spitzer space telescopes – in 2018. The JWST will be better equipped to study the Kuiper Belt objects than Hubble is, and its observations could be complemented by New Horizons‘.

It is probably from all these expectations that the probe draws its promising name. There are parallels to be drawn between its (impending) exploration of Pluto and the Kuiper Belt, and the space beyond, and how astronomers look into the older universe. The speed of light in vacuum is the highest possible speed in the universe, so when astronomers train their telescopes to look billions of lightyears in one direction, they’re simply looking billions of years into our past. The farther a part of the cosmos is from us, the older the light from it is – and the older the information it is carrying is. A parallel of this ingrained association between space and time can be drawn with the distance New Horizons is travelling and the more than four billions years into our past it will be able to see. Here’s waiting with bated breath…

For planets, one thing leads to another

One of the biggest benefits of being a journalist is that you become aware of interesting things from various fields. As a science journalist, the ambit is narrowed but the interestingness, not at all. And one of the most interesting things I’ve come across is a relationship between the rate at which planets rotate – the equatorial rotation velocity – and their mass. It seems the lighter the planet, the lower its equatorial rotation velocity. This holds true for all the planets in the Solar System, as well as large asteroids and Kuiper belt objects such as Pluto. The plot below shows logarithm of planet mass (kg) on the x-axis and logarithm of equatorial rotation velocity (km/h) on the y-axis.

The blue line running through the points is a local regression fit and represents a statistical connection. However, there are four prominent outliers. They represent Mercury, Venus, Earth and Mars, the System’s rocky, inner planets. Their spin-mass correlation deviates from the normal because they are close to the Sun, whose gravitational pull exerts a tidal force on the planets that slows them down. Earth and Mars are also influenced by the gravitational effects of their moons. Anyway, I’ve already written at length about this fascinating connection. What I want to highlight here are more such connections.

And before that, a note: it’s probably obvious that they exist because the connection between mass and equatorial velocity could just as well be a connection between mass and a string of other properties that eventually influence the equatorial velocity. This thought was what led me to explore more connections.

Mass and rotation period

The rotation period of a planet describes the time taken for the planet to rotate once around its axis. If mass and equatorial velocity are related, then mass and rotation period can be related, too, if there is a connect between mass and planetary radius, which in turn implies there is a connect between radius and density, which in turn implies that planets that can get only so big and so dense before they become implausible, presumably – a conclusion borne out by a study released on May 26.

Image: A log-log plot between planetary mass and rotation period.

Density and rotation period

If a planet can only get so big before it becomes puffy, and its mass is related to its rotation period, then its density and rotation period must be connected, too. The chart below shows that that hypothesis is indeed borne out (it’s not my hypothesis, FYI): log(density) increases as log(rotation period) does, so denser planets rotate faster. However, the plot shows significant variation. How do you explain that?

Image: A log-log plot between density and rotation period.

Turn to the Solar System’s early days. The Sun has formed and there is a huge disk of gas, rocks, dust and other debris floating around it. It exerts a gravitational pull that draws heavier things in the disk toward itself. As it becomes more energetic, however, it exerts a radiation pressure that pushes lighter things in the disk away. Thus, the outermost areas of the disk are mostly dust while the inner areas are heavier and denser. It’s possible that this natural stratification could have created different bands of material in the disk of different densities. These different kinds of materials could have formed different kinds of planets, each now falling on a specific part of the log(density) v. log(rotation period) plot.

Mass and density

Of course mass and density are related: the relation is called volume. But what’s interesting is that the volume isn’t arbitrary among the Solar System’s planets. It rises and then dips, which means heavier planets are less dense and, thus, more voluminous. For example, Jupiter is the heaviest planet in the Solar System – it weighs as much as 317 Earths. Its volume is more than 1,300-times Earth’s. However, its density – 1.33 g/cm3 versus Earth’s 5.52 g/cm3 – shouldn’t be surprising because it’s made of mostly hydrogen and helium. Which is just what we deduced based on the mass-period connection.

Image: A log-log plot between mass and density.

One way to find out if there’s any merit in this exercise is to conduct a detailed study. Another way is to perform a regression analysis. I’m not smart enough to do that, but I’ll blog in detail about that when I am.

Studying our primal horizons at the Kuiper belt

In August this year, the New Horizons spacecraft will cross into the region of space beyond Neptune’s orbit. It won’t be the first human object to go this far: the two Voyager space probes have already done that, and then Pioneer 10 with them. What will be special about New Horizons is that it’ll be the only one with enough power to receive commands from Earth, perform observations, and relay its findings back. Unlike the Voyager and Pioneer probes, New Horizons will not be a symbolic, space-born artifact but the first fully functional scientific experiment to travel that far. To be fair, Voyager 1 at the cusp of the interstellar medium still has its ears open for out-of-the-ordinary stuff but it doesn’t have enough juice to turn its head.

From its new perch, New Horizons will be privy to the lives of a belt of bodies named for the astronomer Gerard Kuiper, who speculated on them in the 1950s. The Kuiper belt, like the asteroid belt between Mars and Jupiter, bears signatures of the formative days of the Solar System, which were quite tumultuous. Various studies of asteroids, Kuiper belt objects (KBOs) and satellite systems of the gas giants Jupiter and Saturn have shown that after the planets formed, they moved around quite a bit before settling in their current orbits. One interesting way we know this is because of some similar properties between the asteroid belt and the KBOs. Even though they’re so far apart (~4.2 billion km between them), how could they have had a shared history?

Look to Jupiter. According to one of the models of planetary formation, called the Grand Tack Model, Jupiter once came as close to the Sun as Mars is today, adulterating the asteroid belt with objects from the Kuiper belt its prodigious gravitational pull would’ve tugged along, before moving back. Then, according to the Nice Model, Jupiter pulled in more KBOs into orbit around itself – explaining why many moons of the ice- and gas-giants in that part of the Solar System look and feel like large KBOs. However, as compelling as these models seem, they’re far from being known to be absolute true. Astronomers need to make more observations.

That’s why it’s exciting that New Horizons is entering the vicinity of the Kuiper belt. Its findings would be both seminal and extremely important in understanding how the Solar System was born, why it has an anomalous constitution of planets, and how the ice giants Uranus and Neptune came to be.

 Featured wallpaper: hqdesktop.net

The secrets of how planets form

Astronomers who were measuring the length of one day on an exoplanet for the first time were in for a surprise: it was shorter than any planet’s in the Solar System. Beta Pictoris b, orbiting the star Beta Pictoris, has a sunrise every eight hours. On Jupiter, there’s one once every 10 hours; on Earth, every 24 hours.

This exoplanet is located 63.4 light-years from the Solar System. It is a gas giant, a planet made mostly of gases of light elements like hydrogen and helium, and more than 10 times heavier than Earth. In fact, Beta Pictoris b is about eight times as heavy as Jupiter. It was first discovered by the Very Large Telescope and the European Southern Observatory in 2003. Six years and more observations later, it was confirmed that it was orbiting the star Beta Pictoris instead of the star just happening to be there.

On April 30, a team of scientists from The Netherlands published a paper in Nature saying Beta Pictoris b was rotating at a rate faster than any planet in the Solar System does. At the equator, its equatorial rotation velocity is 25 km/s. Jupiter’s equatorial rotation velocity is almost only half of that, 13.3 km/s.

The scientists used the Doppler effect to measure this value. “When a planet rotates, part of the planet surface is coming towards us, and a part is moving away from us. This means that due to the Doppler effect, part of the spectrum is a little bit blueshifted, and part of it a little redshifted,” said Ignas Snellen, the lead author on the Nature paper and an astronomy professor at the University of Leiden.

So a very high-precision color spectrum of the planet will reveal the blue- and redshifting as a broadening of the spectral lines: instead of seeing thin lines, the scientists will have seen something like a smear. The extent of smearing will correspond to the rate at which the planet is rotating.

Bigger is faster

So much is news. What is more interesting is what the Leiden team’s detailed analysis tells us, or doesn’t, about planet formation. For starters, check out the chart below.

Spin_rate_chart
Image: Macclesfield Astronomical Society

This chart shows us the relationship between a planet’s mass (X-axis) and its spin angular momentum (Y-axis), the momentum with which it spins on an axis. Clearly, the heavier a planet is, the faster it spins. Pluto and Charon, its moon, are the lightest of the lot and their spin rate is therefore the lowest. Jupiter, the heaviest planet in the Solar System, is the heaviest and its spin rate is also the highest. (Why are Mercury and Venus not on the line, and why have Pluto and Earth been clubbed with their moons? I’ll come to that later.)

“Apparently the more massive the planet, the more angular momentum it acquires,” Prof. Snellen said. This would put Beta Pictoris b farther along the line, possibly slightly beyond the boundaries of this chart – as this screenshot from the Leiden team’s pre-print paper shows.

planet_spin_rate1

Unfortunately, science doesn’t yet know why heavier planets spin faster, although there are some possible explanations. A planet forms from grains of dust floating around a star into a large, discernible mass (with many steps in between). This mass is rotating in order to conserve angular momentum. As it accrues more matter over time, it has to conserve the kinetic and potential energy of that matter as well, so its angular momentum increases.

There have been a few exceptions to this definition. Mercury and Venus, the planets closest to the Sun, will have been affected by the star’s gravitational pull and experienced a kind of dragging force on their rotation. This is why their spin-mass correlations don’t sit on the line plotted in the chart above.

However, this hypothesis hasn’t been verified yet. There is no fixed formula which, when plotted, would result in that line. This is why the plots shown above are considered empirical – experimental in nature. As astronomers measure the spin rates of more planets, heavy and light, they will be able to add more points on either side of the line and see how its shape changes.

At the same time, Beta Pictoris b is a young planet – about 20 million years old. Prof. Snellen used this bit of information to explain why it doesn’t sit so precisely on the line:

planet_spin_rate2

Sitting precisely on the line would be an equatorial velocity of around 50 km/s. But because of its youth, Prof. Snellen explained, this exoplanet is still giving off a lot of heat (“this is why we can observe it”) and cooling down. In the next hundreds of millions of years, it will become the size of Jupiter. If it conserves its angular momentum during this process, it will go about its life pirouetting at 50 km/s. This would mean a sunrise every 3 hours.

I think we can stop complaining about our days being too long.

Spin velocity v. Escape velocity

Should the empirical relationship hold true, it will mean that the heaviest planets – or the lightest stars – will be spinning at incredible rates. In fact, the correlation isn’t even linear: even the line in the first chart is straight, the axes are both logarithmic. It is a log-log plot where, like shown in the chart below, even though the lines are straight, equal lengths of the axis demarcate exponentially increasing values.

log-log
Image: Wikipedia

If the axes were not logarithmic, the line f(x) = x3 (red line) between 0.1 and 1 would look like this:

plot3
Image: Fooplot.com

The equation of a line in a log-log plot is called a monomial, and goes like this: y = axk. In other words, y varies non-linearly with x, i.e. a planet’s spin-rate varies non-linearly with its mass. Say, if k = 5 and a (a scaling constant) = 1, then if x increases from 2 to 4, y will increase from 32 to 1,024!

Of course, a common, and often joked-about, assumption among physicists has been made: that the planet is a spherical object. In reality, the planet may not be perfectly spherical (have you known a perfectly spherical ball of gas?), but that’s okay. What’s important is that the monomial equation can be applied to a rotating planet.

Would this mean there might be planets out there rotating at hundreds of kilometres per second? Yes, if all that we’ve discussed until now holds.

… but no, if you discuss some more. Watch this video, then read the two points below it.

  1. The motorcyclists are driving their bikes around an apparent centre. What keeps them from falling down to the bottom of the sphere is the centrifugal force, a rotating force that, the faster they go, pushes them harder against the sphere’s surface. In general, any rotating body experiences this force: something in the body’s inside will be fleeing its centre of rotation and toward the surface. And such a rotating body can be a planet, too.
  2. Any planet – big or small – exerts some gravitational pull. If you jumped on Earth’s surface, you don’t shoot off into orbit. You return to land because Earth’s gravitational pull doesn’t let you go that easy. To escape once and for all, like rockets sometimes do, you need to jump up on the surface at a speed equal to the planet’s escape velocity. On Earth, that speed is 11.2 km/s. Anything moving up from Earth’s surface at this speed is destined for orbit.

Points 1 and 2 together, you realize that if a planet’s equatorial velocity is greater than its escape velocity, it’s going to break apart. This inequality puts a ceiling on how fast a planet can spin. But then, does it also place a ceiling on how big a planet can be? Prof. Snellen to the rescue:

Yes, and this is probably bringing us to the origin of this spin-mass relation. Planets cannot spin much faster than this relation predicts, otherwise they would spin faster than the escape velocity, and that would indeed break the planet apart. Apparently a proto-planet accretes near the maximum amount of gas such that it obtains a near-maximum spin-rate. If it accretes more, the growth in mass becomes very inefficient.

(Emphasis mine.)

Acting forces

The answer will also depend on the forces acting on the planet’s interior. To illustrate, consider the neutron star. These are the collapsed cores of stars that were once massive but are now dead. They are almost completely composed of neutrons (yes, the subatomic particles), are usually 10 km wide, and weigh 1.5-4 times the mass of our Sun. That implies an extremely high density – 1,000 litres of water will weigh 1 million trillion kg, while on Earth it weighs 1,000 kg.

Neutron stars spin extremely fast, more than 600 times per second. If we assume the diameter is 10 km, the circumference would be 10π = ~31 km. To get the equatorial velocity,

Vspin = circumference × frequency = 31 × 600/1 km/s = 18,600 km/s.

Is its escape velocity higher? Let’s find out.

Ve = (2GM/r)0.5

G = 6.67×10-11 m3 kg-1 s-2

M = density × volume = 1018 × (4/3 × π × 125) = 5.2×1020 kg

r = 5 km

∴ Ve = (2 × 6.67×10-11 × 5.2×1020/5)0.5 =  ~37,400 km/s

So, if you wanted to launch a rocket from the surface of a neutron star and wanted it to escape the body’s gravitational pull, it has to take off at more than 30 times the speed of sound. However, you wouldn’t get this far. Water’s density should have given it away: any object would be crushed and ground up under the influence of the neutron star’s phenomenal gravity. Moreover, at the surface of a neutron star, the strong nuclear force is also at play, the force that keeps neutrons from disintegrating into smaller particles. This force is 1032 times stronger than gravity, and the equation for escape velocity does not account for it.

However, neutron stars are a unique class of objects – somewhere between a white dwarf and a black hole. Even their formation has nothing in common with a planet’s. On a ‘conventional’ planet, the dominant force will be the gravitational force. As a result, there could be a limit on how big planets can get before we’re talking about some other kinds of bodies.

This is actually the case in the screenshot from the Leiden team’s pre-print paper, which I’ll paste here once again.

planet_spin_rate1

See those circles toward the top-right corner? They represent brown dwarfs, which are gas giants that weigh 13-75 times as much as Jupiter. They are considered too light to sustain the fusion of hydrogen into helium, casting them into a limbo between stars and planets. As Prof. Snellen calls them, they are “failed stars”. In the chart, they occupy a smattering of space beyond Beta Pictoris b. Because of their size, the connection between them and other planets will be interesting, since they may have formed in a different way.

Disruption during formation is actually why Pluto-Charon and Earth-Moon were clubbed in the first chart as well. Some theories of the Moon’s formation suggest that a large body crashed into Earth while it was forming, knocking off chunks of rock that condensed into our satellite. For Pluto and Charon, the Kuiper Belt might’ve been involved. So these influences would have altered the planets’ spin dynamics, but for as long as we don’t know how these moons formed, we can’t be sure how or how much.

The answers to all these questions, then, is to keep extending the line. At the moment, the only planets for which the spin-rate can be measured are very massive gas giants. If this mass-spin relation is really universal, than one would expect them all to have high spin-rates. “That is something to investigate now, to see whether Beta Pictoris b is the first of a general trend or whether it is an outlier.”

Reference:

Fast spin of the young extrasolar planet β Pictoris b. Nature. doi:10.1038/nature13253