Right, let's kick off with a study reported recently in the journal Nature by B.L. de Vries et al. In this study, olivine crystals were detected in the Beta Pictoris system.
Background
Beta Pic is a relatively young system (~12 million years old), just over 60 light years away and is a solar system in the making. The star at the centre of the system has just entered the stable main sequence phase but beyond lies swathes of gas, dust, cometary debris and planetesimal belts. Astronomers detect debris disks by measuring strong infra-red (IR) radiation emission. As the light from the host star travels through the gas and dust it's scattered to longer wavelengths. Mapping this radiation flux can then tell you about the geometry of the system. Are we viewing the debris desk edge on? Or is the debris disk inclined to our line of sight? The asymmetry of the disk was found in the 1990s with IR observations and later the Hubble Space Telescope revealed disk bulges close to the host star:
Early imaging in the 1990s with telescopes in Hawaii revealed the asymmetry of the disk in Beta Pictoris. Above is Hubble Space Telescope (HST) imaging of Beta Pic. The high resolution of HST and lack of atmosphere in orbit around the Earth allowed vertical bulging of the disk to be observed close to the star at 50AU (1AU is the distance from the Earth to the Sun). The above image is false colour to accentuate the features of the disk.
Disk asymmetry and bulging was thought to be down to a planet in the system. Remarkably this planet was eventually imaged by the Very Large Telescope (VLT) in 2003 and 2009. In order to replicate the physical properties of the system the planet was found to be about 9 Jupiter masses, orbiting at a distance of 8-15AU. This is a very interesting result as before no disk had been observed hosting a planet! We know that disk lifetimes are around 10 million years, as radiation pressure from the star disperses gas and dust required for planet formation. So actually seeing a planet form with a disk that was still intact was good news for planet formation modellers. Planets can and are forming, in our own backyard.
VLT imaging of the gas giant in orbit around Beta Pic, seen as the bright white spot. The image on the left was taken in 2003 and the one on the right was taken in 2009. Using the projected separation and how the planet orbits in time the orbital period was found to be 17-35 years.
The planet observed in Beta Pic was observed to lie in the plane of the disk. However, we have and are finding planets that aren't orbiting in an orderly fashion. Unlike the solar system, planet hunters are finding planets that orbit in directions opposite to the rotation of their star (as if Earth orbited the opposite way) and in highly oblique orbits. Is this evidence that planets gravitationally interact? Or is the star simply tilted due to its interaction with the disk? I mention this because I'm working on it at the moment. I might talk about it at a later date and possibly bore you with it. Hopefully not.
The Study
Just as was mentioned near the start of this blog post, we can use scattered radiation to reveal a great deal of information. As the Beta Pic debris disk is very much intact planetesimals such as comets and asteroids collide. When these collisions occur dusty material and debris is ejected then bathed in host star radiation. As the radiation scatters from the material we can begin to say something about its chemical composition using spectroscopy. The authors report a detection of the 69 micron band of olivine in the spectrum of Beta Pic. The crystals are then associated with a proto-Kuiper belt (the belt that planet Pluto is a part), the magnesium composition is determined and the fraction of the total dust mass is reported.
Why olivine anyway?
To understand terrestrial planets we need to understand olivine. Olivine makes up around 50% of the Earth's upper-mantle and is one of the Earth's most common minerals. The other terrestrial planets are thought to have high quantities of olivine (the Mars Curiosity rover detected olivine in its first soil sample) but is this true of other solar systems? Do all terrestrial planets form in the same way?
Results
Olivine, chemical formula (Mg,Fe)2SiO4 comes in two types: iron rich and magnesium rich. Asteroids that form close in to the Sun in our solar system are Fe rich and Mg rich further out from the Sun. This is because Fe is refractory and Mg remains gaseous at very high temperatures so only can begin to condense at cooler temperatures and larger distances, where stellar irradiation isn't as intense.
The authors report a detection of the 69 micron emission feature of olivine using the Herschel PACS (Photodetecting Array Camera and Spectrometer) instrument. Measuring the emission feature at 69 micron is especially interesting as the width and position of the emission feature determines the temperature and chemical composition of the olivine crystals:
Modelling the 69 micron feature, determining the width and wavelength position determines the temperature and composition of olivine.
The exact wavelength position of the 69 micron feature indicates Fe/Mg=0.01-magnesium rich olivine. Also the measured temperature from modelling resulted in a temperature of 85 +/- 6K placing the olivine at a distance of between 15 and 45 AU from the central star. This position, beyond the so-called snow line, is the realm of icy cometary bodies i.e. the Kuiper belt in our own Solar System. The Mg rich content is also of a similar composition to that of Solar System cometary bodies suggesting a familiarity in the way olivine was transported through the debris disk.
Final Words
- Study suggests that olivine processing may not be Solar System specific
- Implications for astrobiology suggesting terrestrial planets perhaps form in a similar way
In case you're not super excited about olivine crystals by now check out the Fukang pallasite, found in the Gobi desert, China:
Fukang Pallasite: a staggering ~1000kg was recovered in 2000 in Fukang, Xinjiang Province, China. A Pallasite is a class of meteorite with an iron-nickel matrix encasing beautifully transparent green olivine crystals. Pallasites are thought to have formed in the early Solar System when planetesimals differentiated to form an iron-nickel core and liquid mantle. Pallasites were formed at the core-mantle boundary and were liberated from impacts.
Just take a look at the scale for a moment. Yes, you are reading that correctly-91cm! If you think that's impressive just look at...
THIS!
'World renowned as the most spectacular example of natural cosmic splendor, the Fukang pallasite will undoubtedly become one of the greatest meteorite discoveries of the 21st century. This awe-inspiring main mass weighs over 925 pounds (over 420 kilograms). The Fukang pallasite displays celestial yellow-green olivine crystals in an illustrious nickel-iron matrix. Backlit slices from the Fukang mass are reminiscent of stained glass windows crafted in the ancient solar system'-Southwest Meteorite Lab
