Double rubble

Our Solar System contains a vast belt of rocky asteroids sitting in an orbit roughly between the planets Mars and Jupiter, but astronomers have found a reason for us to be envious of other star systems. They have discovered that a single belt is not the only possible arrangement of the primordial debris that remains following planet formation around a star.

Dana Backman of the SETI Institute and colleagues have found that the nearby star Epsilon Eridani has two rocky asteroid belts and an outer icy ring, making it a triple-ring system. The inner asteroid belt is a virtual twin of the belt in our solar system, while the outer asteroid belt is much more massive, holding twenty times as much material. The astronomers explain that the existence of these three rings of material suggests that undiscovered planets in the Epsilon Eridani system confine and shape the rings.

Massimo Marengo

Massimo Marengo

Epsilon Eridani is a smaller, younger, cooler star than the Sun, nestling in the southern constellation of Eridanus, the river, about 10.5 light-years from Earth. It is a mere 850 million years old and astronomers consider the Epsilon Eridani planetary system to have some remarkable similarities with our solar system at a comparable age. Studying Epsilon Eridani is like having a time machine to look at our solar system when it was young, according to team member Massimo Marengo of the Harvard-Smithsonian Center for Astrophysics. Backman agree pointing out that, This system probably looks a lot like ours did when life first took root on Earth.

Using NASA’s Spitzer Space Telescope, the team identified an asteroid belt orbiting Epsilon Eridani at an equivalent distance as our own asteroid belt – 3 astronomical units. However, they also spotted a second asteroid belt at 20 astronomical units from Epsilon Eridani, the approximate distance Uranus is located in our solar system. The total mass of this second asteroid belt is about the same as Earth’s Moon.

Dr Dana Backman

Dr Dana Backman

The third, ring of icy material sits at about 35 to 100 astronomical units from Epsilon Eridani and is the equivalent of the Kuiper Belt in our solar system, but this previously discovered third ring has a mass one hundred times that of the Kuiper Belt.

Some star systems are more equal than others (Credit: NASA/JPL-Caltech)

Some star systems are more equal than others (Credit: NASA/JPL-Caltech)

Theorists have suggested that when our Sun was 850 million years old, the Kuiper Belt would have resembled more that seen in Epsilon Eridani, but in the intervening billions of years, much of its material has been swept away, hurled out of the solar system, or plunged into the inner planetary region in the Late Heavy Bombardment. The enormous craters on our Moon are evidence of this bombardment.

Running rings around a star system (Credit: NASA/JPL-Caltech)

Running rings around a star system (Credit: NASA/JPL-Caltech)

It is possible that Epsilon Eridani will undergo a similar dramatic clearing in the future, the astronomers say. Epsilon Eridani looks a lot like the young solar system, so it’s conceivable that it will evolve similarly, said Marengo. The team points out that the pattern of rings and their total masses suggest that three giant planets with masses between those of Neptune and Jupiter are also orbiting Epsilon Eridani. Further observations will soon reveal more about the character of our distant stellar cousin.

D. Backman, M. Marengo, K. Stapelfeldt, K. Su, D. Wilner, C. D. Dowell, D. Watson, J. Stansberry, G. Rieke, T. Megeath, G. Fazio, M. Werner (2009). Epsilon Eridani’s Planetary Debris Disk: Structure and Dynamics based on Spitzer and CSO Observations Astrophysical Journal

Further reading

Dr Dana Backman homepage
http://www.seti.org/Page.aspx?pid=390

Massimo Marengo homepage
http://www.cfa.harvard.edu/~mmarengo/me/home.html

Suggested searches

asteroids
Kuiper belt

The Uber-Pluto

New measurements published in the February 2 issue of Nature support claims last year that the planetary membership of our Solar System should be extended to include a tenth that is bigger than Pluto. 2003 UB313 was first spotted in January 2005 by Mike Brown’s team at Caltech, but recent thermal emission measurements have recently allowed German scientists to estimate its diameter at approximately 3100 km, some 700 km larger than Pluto. This makes it the biggest object to be discovered in the Solar System since the discovery of Neptune in 1846. For comparison, Earth’s Moon has a diameter of about 3500 km.

Like Pluto, UB313 is an icy body circling the Sun in the vast expanse beyond the orbit of Neptune known as the Kuiper belt. UB’s orbit takes it almost 100 times farther away from the Sun as the Earth and about twice as far as the most distant point of Pluto’s orbit. When astronomers first sighted it, there were hints that it was at least as big as Pluto, but a more accurate estimate would only be possible once astronomers had determined its albedo, or surface reflectance.

Planets

Planets

Now, Frank Bertoldi of the University of Bonn and the Max Planck Institute for Radio Astronomy (MPIfR) and MPIfR’s Wilhelm Altenhoff have resolved this albedo problem using measurements of the heat radiated by UB313 and comparing this with optical observations. The researchers detected the very faint millimetre wavelength emissions from UB313 using the IRAM 30-m telescope on Pico Veleta in southern Spain, and the very sensitive heat sensor MAMBO-2, which was developed and built at the MPIfR in Bonn by Ernst Kreysa’s group.

Astronomers have discovered numerous small planetary objects beyond the orbits of Neptune and Pluto since the early 1990s and so confirmed the prediction of Kenneth Edgeworth (1880-1972) and Gerard Kuiper (1905-1973) of a belt of such objects. These objects are thought to be the leftovers from the formation of the Solar System some 4.5 billion years ago. They evaded the gravitational clean-up that took place closer to the Sun but occasionally venture into this region as short-period comets.

Planetary detector

Planetary detector

As evidence of small planetary objects accrues, the notion of what it takes to be a planet has changed. Since UB313 is decidedly larger than Pluto, Bertoldi explain, it is now increasingly hard to justify calling Pluto a planet if UB313 is not also given this status. One cynical journalist has already dubbed Pluto’s big brother Ooby, while others have mistakenly given it the name Xena. Astronomers are more likely to take a name from Roman mythology but the International Astronomical Union will have the final say.

Planetary telescope

Planetary telescope

Frank Bertoldi

Frank Bertoldi

Further reading

Nature 2006, 439, 563-564
http://dx.doi.org/10.1038/nature04494

Prof Dr Frank Bertoldi
http://www.astro.uni-bonn.de/~bertoldi/

New planet press release
http://www.astro.uni-bonn.de/~bertoldi/ub313/

Kuiper Belt
http://www.ifa.hawaii.edu/faculty/jewitt/kb.html

MAMBO
http://www.astro.uni-bonn.de/~bertoldi/projects/mambo/

IRAM
http://www.iram.fr/

Suggested searches

Kuiper Belt
Solar System
planets
thermal emission spectroscopy

Interplanetary tales

In a galaxy not so far away … the presence of a planet orbiting a distant star in the Milky Way galaxy was observed during a week-long window of opportunity during observations by a team of 32 researchers. The existence of the planet was deduced, not by the standard method of observing a slight change in the star’s spectrum when tugged by the planet, but rather by the way in which the planet and the star about which it orbits some 17,000 light years away towards the central part of the galactic disc, in the constellation of Sagittarius, distorted the image of a more distant, background, star, some 24,000 light years away.

Several detector groups are set up to monitor the passage of stars in the Milky Way passing behind or near foreground objects with the aim of spotting dark matter, brown dwarves and other stars. The detectors hope to make sense of the changes in the way the light curves for the background objects. Ian Bond of the Institute for Astronomy in Edinburgh and his colleagues at two detector groups, the Microlensing Observations in Astrophysics (MOA) and Optical Gravitational Lensing Experiment (OGLE) have now reported their analysis of a week’s observations last summer that show that for one distant star the characteristic brightening light curve, just before a gravitational lensing event, had several unexpected extra spikes. The team interpreted these spikes as being due to two neighbouring massive objects.

Schematic diagram showing how microlensing helps astronomers find new planets (Credit: NASA)

Schematic diagram showing how microlensing helps astronomers find new planets (Credit: NASA)

Further analysis showed that one of the pair was less than half a percent as massive as the other. This suggests a star-planet pairing with the putative planet having a mass 1.5 times that of the biggest planet in the Solar system, Jupiter. The discovery represents the most distant giant planet to be detected orbiting an ordinary star in the Milky Way and the first planet outside the Solar system found using the gravitational microlensing technique. Astronomers are pinning their hopes on this technique revealing the presence of much smaller planets, about the size of the Earth, around other stars.

Gravitational microlensing was first proposed by Bohdan Paczynski of Princeton University and his research student, Shude Mao (now at Jodrell Bank Observatory), in 1991. Two years later, the first reported microlensing events were observed due to stellar mass objects by three different groups: the OGLE, MACHO, and EROS collaborations.

Data used to discover another planet (Credit: NASA)

Data used to discover another planet (Credit: NASA)

More than a decade later, OGLE, MACHO, EROS have drawn together to report their discovery of the first extra-Solar planet observed using the technique. Paczynski is apparently thrilled as a theoretician to see the prediction come true.

Artist’s impression of the planet, believed to be 1.5 times larger than Jupiter, orbiting its tiny parent star, a red dwarf. The distance between the star and planet is three times the distance between Earth and the Sun. (Credit: NASA)

Artist’s impression of the planet, believed to be 1.5 times larger than Jupiter, orbiting its tiny parent star, a red dwarf. The distance between the star and planet is three times the distance between Earth and the Sun. (Credit: NASA)

However, the real strength of the microlensing technique will be its ability to detect low mass planets, Bond, a member of the MOA collaboration explains. MOA and OGLE expect to spot Neptune-mass planets in Jupiter-like orbits in the next few years using the new large field-of-view OGLE-III camera, the MOA-II 1.8 m telescope which is now being built, and with the improved cooperation between the different groups that observe microlensing events. Such events require a telescope that produces images 1000 times sharper than the Hubble Space Telescope.

Bohdan Paczynski

Bohdan Paczynski

Finding a planet outside the Solar system that is of a similar size is not only of interest to astronomers studying the evolution of other star systems, but could one day provide the first view of a place where extraterrestrial life may have evolved.

Further reading

Astrophys J. Lett, 10 May 2004
http://arxiv.org/abs/astro-ph/0404309

Planetary Microlensing
http://bulge.astro.princeton.edu/~ogle/ogle3/blg235-53.html

Bohdan Paczynski
http://www.astro.princeton.edu/~bp/

Dr. Shude Mao
http://www.jodrellbank.manchester.ac.uk/~smao/

OGLE III Early Warning System
http://ogle.astrouw.edu.pl/ogle3/ews/ews.html

MOA Transient Alert Page
https://it019909.massey.ac.nz/moa

Microlensing movie – from NASA (2 MB Quicktime .mov file)
http://www.jpl.nasa.gov/videos/microlensing/movingmag.mov

Suggested searches

gravitational lensing