galaxy pic Small Rocky Planet Discovered In Our Galaxy

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Local Tasmanian contacts from the University of Tasmania, School of Mathematics and Physics.

Dr John Greenhill :    phone: (03) 6226 2429 (w)
Dr Kym Hill           :            phone: (03) 6226 2437 (w)
Dr Stefan Dieters   :    phone: (03) 6226 2438 (w)

The University of Tasmania Press Office can be contacted:
Phone: (03) 6226 2124 Mobile: 0417 517 291

Other Australian contacts:
Dr Andrew Williams, Perth Observatory :
            phone (08) 9293 8255
Professor Penny Sackett, Research School  of Astronomy and Astrophysics ANU :
            phone (02) 6125 0266

The Milky Way runs vertically, with Scorpio in the centre, with Sagittarius just below center. The large glow (just below centre) marks the location where OGLE-2005-BLG-390L and it's planet are located, which is toward the center of our galaxy.
The cloud of stars seen in this image is near the centre of our galaxy, about 30,000 light years away. These are generally stars much brighter than our Sun. Between us and them there are many stars, most too faint to be seen. These foreground stars orbit the centre of the galaxy. If one of these foreground star becomes nearly perfectly aligned with a background star, it's gravity acts like a giant lens, creating miniture multiple micro-images of the background star. These images combine, and make the background star brighten. For a single foreground (lens) star and a single background star the brightness of the background star increases and decreases in a characteristic manner. These microlensing events are very rare because the alignment has to be nearly exact and stars are very small compared to the distances between them. In order to find a resonable number of microlensing events the brightness of millions of stars needs to be monitored on a nightly basis.  This is done very sucessfully by the OGLE and MOA groups.
Note that the images taken from Mt Canopus are rotated 45 degrees with respect to images taken by the PLANET group.
The position of the planet is not marked but is between the small yellowish star image from the planet and larger star image formed by the foregroud star (star of planet). At the middle of the planetary anomaly the images combine to give a donut shaped image of the background star. The planet is located in the middle of the donut. Changes in these combined images cause the small brightening of the background star as shown in the lightcurve.
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Artist Impression of planetThe parent star is only one fifth the mass of the Sun. It is a much cooler, and so shines with a red colour. These stars are called red dwarfs and are the most common type of star in our galaxy. Even though the planet obirts only 3 times further out from its star than the Earth about the Sun, it is much colder; about the temperature of Neptune or Pluto in our solar system. The mass of the planet (only 5 Earth Masses) is too small to be a gas planet like Jupiter (317 Earth masses). So it is probably made largly of rock and ice; a giant Pluto or Europa (large moon of Jupiter).


Just below the centre of the frame are two round star images. The left hand image is that of OGLE-2005-BLG-390Lb. As the microlensing event progresses the star first brightens then fades with a brief resurgence in brightness. As the unseen foreground star moves closer into line with the background which we see here, the gravity of the foreground star acts like a giant lens. The star brightens as the alignment between foreground and background star becomes closer and then fades as they move appart. Briefly the star brightens again as the planet's gravity creates another image.
This is a simulation of the images a hypothetical super high resolution telescope would see. We observe the combined image and measure the the brightness of the background star as these images change. The position of the foreground star is shown as a small green circle. The viewpoint is centred on the foreground star and so the position of the background star (a small red circle) seems to move. The blue circle is the Einstein ring. If the alignment was perfect a circular image of the background star would be seen here. Initially the foreground star creates two images (yellowish) images of the background star. As the foreground star and background star move relative to each other the images move and grow larger corresponding to a brightening of the background. The whole microlensing event lasts 40 days. Toward the end, the animation slows down and zooms in upon one of the primary images created by the foreground lens star. A third very small image created by the planet appears. The postion of the planet is marked by a blue dot. As the planet moves it's image grows larger and merges with the primary image from the star (circle with hole). This coresponds to the 12 hour long brightening of the background star causing the bump in the lightcurve. Later the images separate and the planetary image rapidly fades away.

Adaptive view (whole event with zoom for planentary anomaly)
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Whole event view
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Zoomed view of the planetary anomaly only
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This is a plot of brightness versus time for the background star of OGLE-2005-BLG-390Lb. As the unseen foreground star aligns with the background star it's gravity acts like a giant lens. As the alignment changes the brightness of the background star varies in a characteristic manner; a smooth rise and fall.  If the foreground star has a planet, the planet's gravity distorts the gravitational lens adding extra micro-images, causing an extra brightening of the background stars overall image.  The length of this extra bump (anomaly) depends mostly on the planets mass. The larger the planet the longer the deviation in brightness. For OGLE-2005-BLG-390Lb the deviation lasted about 12 hours. (See the insert of the lightcurve plot).  In this case the Perth Observatory got most of the brightness measurements during the anomaly caused by the planet.  Each colour on the plot represents a separate telescope. The changing bands of colour shows how as daylight approaches at one telescope the next telescope takes over, giving roughly continous coverage.
As the Earth rotates at least one of the telescopes is in darkness, so the between them the PLANET telescopes have 24hours of darkness for the 3-4 months of the observing campaign.
Mt Canopus with a 1 m diameter telescope is near Hobart Tasmania and is run by the School of Mathematics and Physics of the University of Tasmania.
The Perth Observatory has a 0.6m diameter telescope and is run by (CALM = Conservation and Land Management) department of the West Australian state government.

Planet Infomation Page

Optical and X-ray Astronomy home page
Jan 2006