Wednesday, November 28, 2012

Not like the other one

When we look to other solar systems is it really so surprising that there is a huge diversity of planets and systems out there. We only need look at our own to see how wacky some of these places are. So I thought I would compile a feature on the weird and wacky things setting our own solar system planets apart from each other.

The messenger is the smallest planet in our solar system; land of the double sunrise; and home of the Caloris Basin. Mercury orbits the Sun in just 87 days while experiencing only one and a half of its own. After Pluto was excluded from the ‘Planet Club’ in 2006, it became the most eccentric planet in the solar system experiencing non-Newtonian motion as it approached perihelion each orbit.  This motion also causes a unique phenomenon, the double sunrise, when the speed of the planet as it orbits the Sun overtakes that of its rotation; casing the sun to set and then rise again when the planet passes the point of closest approach.

The goddess of love and beauty that will crush you down and melt your skin. Although not the closest to the Sun it is the hottest planet in the solar system, temperatures can reach up to nearly 500oC. This is due to high levels of volcanic activity on the surface and a surface pressure 92 times greater than that we experience on Earth; that is the equivalent of going 920 meters underwater nearly 3 times deeper than any scuba diver has ever gone. Labeled as Earth’s sister planet due to its similar size, its climate is anything but. With an atmosphere of over 95% Carbon dioxide, rain clouds of sulfuric acid, and little to no temperature variation between day and night it is a hellish place. 


Terra Firma. What more do I need to say about this weird and wacky world other than it has managed to produce a civilization of walking talking primates, who not only have an Internet of knowledge available at the click of a button, but who have ventured into space, beyond the realms of their heavenly home.  

Named after the God of War due to its vibrant rust red surface of iron oxide that covers a silica-rich dust bed. Mars is the most explored planet in the solar system, other than the Earth, with over 30 probes and satellites bound for the ‘Red planet’ since the 1960’s. Most of these missions, however, met their end before they had even begun. Those that have made it into Martian orbit, or successfully landed on its surface, have sent us staggering information about this not so distant world. And with three active satellites in orbit (Mars express, Mars odyssey, and Mars Reconnaissance Orbiter) and two live rovers on the ground (Curiosity and Opportunity), we are set to learn even more in the years to come. 

The largest planet in our solar system you could line up ten earths next to each other and it would still not stretch all the way across the face of this mammoth world. With an atmosphere of hydrogen and helium that stretches over 5,000 km in altitude it is truly the god of the Gas giants. Jupiters atmosphere is host to the largest storm in the solar system that was first recorded in 1665, The Great Red Spot. The storm rotates anticlockwise roughly every 6 Earth days and is big enough to swallow the Earth whole three times over. The center of the storm is hotter and redder than the surrounding area, although it is still not known what exactly causes the brick-red to pale salmon color of the gas as it swirls and processes around the planet. 

Apart from having a mean density less than water, meaning that with a bath tub big enough you would find yourself with a pretty impressive bath toy, Saturn is host to one of the most impressive sites in the solar system. Its rings. Saturn’s main ring system spans a distance almost as wide as the planet itself extending from around 6,500 km from the surface out to over 120,000 km. The latest ring was discovered in 2009 using the Spitzer space telescope and is the largest ring studied in the solar system with a thickness of 2.5 million km and reaching 50 times further out into space than the central rings.

The first and only planet in the solar system named after a Greek god, Uranus is the largest of the ‘ice giants’ with an atmosphere of frozen; water, ammonia and methane crystals. While the axial tilt of the Earth set at 23.5°, affords us a beautiful mix of four seasons a year. Uranus is tilted at a staggering 97.77° making it barrel roll around the Sun like a drunken uncle at a wedding. This means that it has the most extreme seasons in the solar system with each pole experiencing 42 years of continuous sunlight in the summer, followed by 42 years of darkness in the winter.

Aptly named after the God of the sea, Neptune gets its rich blue color from the methane in the atmosphere. Voyager 2 made the one and only fly-by of Neptune in August 1999 observing immense storm systems and winds wrapping round the planet with speeds approaching 600m/s, over 1,300 miles per hour. If you could stand on Neptune, without getting blown over or sinking into the atmosphere, you would weigh almost the same as you do on Earth with a surface gravity of 11.5m/s compared to the 9.81m/s here on Earth.

Each of the planets in our solar system are wonderfully unique, so I can’t begin to imagine what the hundreds of exoplanets out there have to offer us. 

Saturday, November 24, 2012

Turning out the lights

What is stellar occultation and how does it tell us about the atmosphere of the solar system objects?

Just this week Nature published a report on the stellar occultation of a star by Makemake a dwarf planet in the outer solar system, showing that unlike Pluto Makemake has no atmosphere to speak of. But what is stellar occultation and how can it tell us about the atmosphere around solar system objects?

Littered across the sky are our very own atmospheric probes in the form of bright shining stars. On the occasion that these stars are obscured by an intervening body its absence can often tell us more than its previously visible twinkling light. As an object passes in front of the star it obscures the light, if that object has and atmosphere then the light will not be blinked out instantaneously but its light will refract through the atmosphere causing the light to slowly drop off. 

The most famous stellar occultation study resulted in the discovery of Uranus’ ring system in 1977. In the picture you can see the time series of observed light as a star is obscured by a ringed planet. As the star passes behind the rings the light is attenuated by the material, this can tell us about the structure of the rings, whether there is more than one or if some parts of the ring system are thicker than others obscuring more light from the star. The star then re-emerges the distance between the ring and the first obscuration of the planet can tell us about the orbit and symmetry of the ring system, given a known motion of the planetary body. As the star passes behind the planet the light is refracted through the atmosphere imposing its molecular signatures on the light observed from the star. In 1952 observations of σ-Arietis as it was occulted by Jupiter revealed that the upper atmosphere of Jupiter is rich in Hydrogen and Helium, confirming previous predictions. When the star lies directly behind the planet a flash of light is often observed. This central flash is caused by the light from the star being refracted round the atmosphere from either side of the planet creating a halo-like effect that is observed as a general brightening. This can tell us information about the lower atmosphere of the planet. As the star then continues past the center of the planet the light is refracted from only one side of the atmosphere and a symmetric increase in light is observed until the star fully re-emerges. 

Stellar occultation by a ringed planet
Using observatories from both the ground and in space many occultation studies have been carried out on our closes neighbours. Orbiting spacecraft sent to these planets have not only been able to corroborate the results obtained from this method they have also used it in-situ to get a closer look at what the planets atmospheres have to offer. 

In the case of the dwarf worlds in the outer solar system like Pluto and Makemake stellar occultation’s are an important tool in the understanding the diversity of our solar system. In the case of Makemake no atmosphere was detected meaning that there was a sharp change in light as it passed in front of the star, unlike the situation is previously described. But detecting no atmosphere is still a result and helps with formation models and an understanding of the extremes of our solar system.


If you want to know more about the recent observations of Makemake check out some of the links below.

For more on Stellar occultations see
ESA's site for some good descriptions and images
or the Harvard website for previous observations

Top - Animation of Venus Express performing stellar occultation at Venus. CREDIT: ESA (Animation by AOES Medialab)
Bottom - James L. Elliot. Stellar occultation studies of the solar system. Astronomy and Astrophysics, (17):445–475, 1979.

Monday, November 5, 2012

Size is Everything

There are not enough adjectives in the human language to describe how immensely, inconceivably, indescribably vast the universe is. So how can we expect to convey the beauty of the universe to those without the time to dedicate their life to such pursuits if it requires us to warp our brains to succeed?

Why do I bring up this mind-warping task? Because that is exactly what I had to do when I started working on my PhD. How could I expect myself to study the universe without a true sense of the scale of it all, and most importantly how could I do this without giving myself a planet sized headache.

I was recently at the Rose Center for Earth and Space in Manhattan, just a week before Sandy hit. In the center of the Rose Center is the Hayden planetarium, an 87 foot diameter (23 meter) sphere suspended in the center of a giant glass cube that makes up the fascade of the building looking in from West 81st street. With the sphere as the central point the exhibits are displayed around and below its rotund shell. One of these exhibits is The Scales of the Universe, a 400 foot walkway around the glass edge of the cube surrounding the Hayden Sphere.  The exhibit uses the Hayden sphere to take you on a journey from the smallest atomic particle to the expanse of the universe giving you a physical scale to wrap your mind around. Even then, with it placed in front of you the ability to visualize the 87 foot sphere as a raindrop or an entire galaxy requires some imagination.

Over the past year I have been studying exoplanets and their atmospheres and desperately attempting to bring them down to an imaginable scale. I am currently looking at the hot Jupiter HAT-P-1b using the Hubble Space Telescope. The term hot Jupiter means that it is a very large planet orbiting very close to its parent star which when given the numbers floats solidly in the realm of the unimaginable. So where do you begin? Well I start with the Earth. How many times the size of the Earth is a hot Jupiter sized planet? Jupiter is in-fact just over 11 times the radius of the Earth, but HAT-P-1b is 1.225 times the radius of Jupiter; that is a further 2 ½ Earths. So having dealt with the size we now have to take a look at the orbital distance from its star. This is where the mind starts to boggle and we have to make our way in towards the center of our solar system to the closest planet orbiting our Sun, Mercury. Mercury’s orbit is almost 3 times closer to the Sun as the Earth, giving it a maximum surface temperature of 700 degrees, but we are not quite there yet. To get to the orbit of HAT-P-1b we have to go 8 times closer still at an orbit 95% closer to the Sun than the Earth. To draw it on a landscape A4 piece of paper the Earth would be a the size of a full stop at one end with HAT-P-1b a 1.5cm circle just 14mm away from the other end of the page that marks the surface of the Star; and you wouldn’t even be able to see the dot that would represent Mercury 11cm safely away from the star. If you look at the image below there is a scaled picture showing the size of HAT-P-1b in relation to the Earth and its orbit in relation to that of the Earth and Mercury with a zoom in needed on the orbit of HAT-P-1b. This shows just how close hot Jupiters are to their host star and helps explain the extremely hot environments detected for these exoplanets ~1500 K. 

Total Solar Eclipse (c) NASA 
Nature offers us a unique opportunity to witness how scale can really put things in perspective via a remarkable coincidence, the solar eclipse. This occurs when the moon passes directly between us and the sun blocking out its light. The Sun is 400 times the diameter of the moon and is 400 times as far away meaning that during a total eclipse the moon fits perfectly over the disc of the sun revealing the shining corona of its extended atmosphere. It is the equivalent of a pea orbiting a grape blocking out the view of a human sized sphere over a block away.

So with a pinch of imagination and a tiny bit of patience the universe can be broken down to the scale of the world around us, where recognition and reasoning reside. 


There are so many different 'The Scale of the Universe' websites out there so if you are interested I have listed a few below.

Also if you are ever in the area make sure you check out The Rose Center for Earth and Space on W 81st, Manhattan, NYC or the Smithsonian Air and Space museum on the National Mall in Washington D.C