Discovery of coolest known exo-planet
Two astronomers at Perth Observatory have played a crucial part in the discovery of a new planet around a distant star, significantly more Earth-like than any other planet discovered so far. The planet, which is only about 5 times as massive as the Earth, orbits its parent star about every 10 years, with an orbital radius of about 3 times the Earth-Sun distance in our solar system. The star it orbits (its 'sun') is much like our Sun, except it is only 1/5th as massive, and much closer to the Galactic center.
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| CAPTION: The OGLE-2005-BULGE-390 field, showing the microlensing event. Over the course of a few weeks in July, 2005, the indicated star brightened and dimmed due to gravitational lensing by an invisible foreground star. |
The planet was discovered using a technique known as gravitational microlensing, and was reported in the prestigious scientific journal Nature as the joint effort of three independent microlensing campaigns: PLANET/RoboNet, OGLE, and MOA, involving a total of 73 collaborators affiliated with 32 institutions in 12 countries (France, United Kingdom, Poland, Denmark, Germany, Austria, Chile, Australia, New Zealand, United States of America, South Africa, Japan). Andrew Williams and Ralph Martin at Perth Observatory were founding members of the PLANET (Probing Lensing Anomalies NETwork) in 1995, and have been involved with the group ever since.
Planet Properties
Using the brightness of the parent star, and the orbital distance, this planet has a calculated surface temperature of only 50°Kelvin or so (220°C below zero). Due to its low mass and low temperature it's probably solid: An icy or rocky planet, similar to (but larger than) Pluto in our solar system, rather than gaseous like Jupiter, Saturn, Uranus and Neptune.
This new planet (with the unglamorous identifier of "OGLE-2005-BLG-390Lb") is probably the smallest detected so far. Only one other known extra-solar planet comes close: Gliese 876d, at 7.3 ± 1 Earth masses, is within the statistical errors on mass but in a very different orbit - orbiting its sun every 2 days, at 1/50th of the Earth-Sun distance.
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| CAPTION: The sky above Perth Observatory at 1am on 2005/8/10, at the peak of the planetary anomaly. The main research telescope is in the tower at the far right. |
Extra-solar Planet Detection
More than 170 planets outside the solar system (known as extra-solar planets or exo-planets) have been discovered to date. Almost all were found using the 'Radial Velocity' technique, which detects planets by measuring the tiny back-and-forth movements of the parent star as the planet orbits. This technique is heavily biased towards finding large planets which are very close to their parent star. If a distant star had a set of planets with masses and orbits identical to the ones in our solar system, they would be almost impossible to detect using the radial velocity technique with current technology.
Microlensing uses a different strategy, and can find planets more like the ones we are familiar with. In this technique the gravity of a dim nearby star acts as a giant natural telescope, magnifying a distant, bright star. A small 'defect' in this gravitational lens revealed the existence of the planet near its "parent" nearby star. Note that we cannot measure the brightness or image the planet, or even the star that it's orbiting, we just see the effect of their gravity.
The existence of a planet around a lens star often causes a distortion that lasts only a few hours. In order to be able to catch and characterize these, nearly-continuous round-the-clock high-precision monitoring is required. This is achieved by the PLANET network.
Microlensing Observations
Microlensing observations determine the light curve (a graph of brightness versus time, as above) of the lensing event. Each point on the graph represents the brightness in a single image and the colour of the point shows which telescope took that image. The regular cycle of colors shows how observing is taken over by the next telescope in turn as the night ends at each site.
The inset in the top right of the graph is an enlargement of the deviation from a 'perfect' lensing curve caused by the gravity of the planet. The dark blue points indicate images from Perth Observatory. Note that the blue (Perth Observatory) points are crucial, ruling out the best non-planet model for the event - the grey dashed curve representing a binary star as a lens, instead of star and planet.
Microlensing - Close Up
An observer on Earth would see (given an imaginary telescope in space, with perfect optics) what is illustrated in the following computer simulations. They show:
- The image/s of the background star (in yellow) that you would actually see
- The real position of the background star as a red circle (where you would see if without the bending of light caused by the lens star)
- The position of the lens star as a green circle (very much dimmer than the bright background star)
- The Einstein Radius around the lens as a blue circle, defining the region where the lensing effect is strongest
Whole event (1280x720 pixel
MPEG4, 487kB) - the distortion
caused by the planet is the short-lived 'glitch' in the larger yellow image
just as the source (red circle) is leaving the Einstein Radius (blue circle).
Close up of the planetary anomaly
(1200x900 pixel MPEG4, 5.6MB), then it slows down so you can see
more detail. The planet itself is shown as a small blue dot.
(Both movies are in MPEG-4 format, so you'll need an MPEG4 viewer, eg a recent version of Quicktime.)
Here's a mosaic (jpg, 190kB)of the computer simulated images of the microlensing event.
All above videos and frames from videos created by Andrew Williams and David Bennett
More Details of the Planet Discovery
The OGLE search team discovered the event OGLE-2005-BLG-390 on 11 July 2005, allowing PLANET to start taking data. A light curve consistent with a single lens star peaking at an amplification of about 3 on 31 July 2005 was observed, until PLANET member Dr. Pascal Fouque, observing at the Danish 1.54m at ESO LaSilla, noticed a planetary deviation seen in the data taken on 10 August. An OGLE point from the same night showed the same trend, while Dr. Andrew Williams reported that the last half of the planetary deviation, lasting about a day, had been covered by images from Perth Observatory. The MOA collaboration was later able to identify the source star on its frames and confirmed the deviation with two data points.
The observed light curve - the graph of the brightness of the background star over time - doesn't give the mass of the planet directly. Instead, it gives the ratio between the mass of the planet, and the mass of the star it orbits. Since this lensing star, like the planet, is too faint to resolve, its mass can't be measured directly. Instead, the mass of the lensing star was derived from its location, plus other clues from the light curve, along with a statistical model of the distribution of stars within our galaxy. This statistical model is responsible for most of the uncertainty in the planet properties. Improved technology over the next decade or so should allow the lens star to be resolved directly, and pin down the parameters more precisely.
OGLE-2005-BLG-390Lb is only the third extra-solar planet resulting so far from microlensing searches. This is due to the fact that planets of Jupiter-mass or above, which are much easier to detect, appear to be rare around M-dwarfs, the stellar type most commonly found as the 'lens' stars in microlensing. This rarity has been indicated independently by microlensing and radial-velocity searches. While the other two microlensing planets have masses of a few times that of Jupiter, the discovery of a sub-Neptune mass object so soon is a strong hint that these smaller objects, in contrast, are quite common, but much harder to detect.
Computer simulations of planet formation using core-accretion models predict that giant planets like Jupiter and Saturn begin as lower-mass planetary cores of rock and ice, which then accrete large amounts of Hydrogen and Helium gas from the proto-planetary disk. However, the host star for this new planet is an M-dwarf star, like most stars in our Galaxy. These stars have less than half the mass of the Sun, and the core accretion theory predicts that planets in these systems will usually form too slowly to grow to the mass of Jupiter. For most stars, the simulations predict a large fraction of planets with masses below 10 Earth-asses, at orbits between 0.1 AU and 10 AU. By coincidence, these orbital separations match well the range preferred by microlensing, making it an ideal technique for studying this population down to Earth mass.
The discovery of a sub-Neptune mass planet should encourage the intensification of microlensing planet searches, using current and additional facilities from the ground - or even with a space-based campaign in the near feature - by providing an observational hint that further low-mass planets will be detected. It shows that microlensing is a straight path to the discovery of a twin Earth.
