Lonely planets wander galaxy

Less than 20 years ago, whether planets existed in other solar systems was pure speculation. Since 1995, over 500 planets orbiting other stars have been observed and mapped using various methods. Now a team of scientists including New Zealand astronomers and astrophysicists has gone a step further and discovered a collection of 10 Jupiter-sized free-floating planets. These ‘rogue’, ‘orphan’ or ‘lonely’ planets are either so far from any potential host star it is doubtful whether they are orbiting them or are wandering free through the Milky Way with no host star at all.

The research involved scientists from Massey University, Victoria University, The University of Auckland, Canterbury University and their Mt John University Observatory at Lake Tekapo. It was published in the 19 May 2011 edition of the prestigious international journal Nature. The Kiwi scientists and their collaborating Japanese scientists are calling themselves the MOA collaboration, an appropriate acronym for Microlensing Observations in Astrophysics – the technique that was used to discover the wandering planets.

Gravitational microlensing

Gravitational microlensing enables scientists to study objects that emit little or no light. In the past, astronomers could only observe celestial bodies that emitted lots of light, such as stars, or large objects that blocked background light, such as clouds of gas and dust. However, these easily observable objects in space make up only a tiny fraction of the mass of a galaxy.

Gravitational microlensing detects mass rather than light. It works when an object (such as a planet, a brown dwarf star or a black hole) is between an observer on Earth and a bright star or quasar as a background object. The foreground object is called a gravitational lens. The light from the background object is bent by the gravitational field of the foreground object. To the observer on Earth, this leads to 2 distorted unresolved images, which astrophysicists can use to calculate the mass of the foreground object and the relative motions of the background object and the foreground gravitational lens object.

The MOA collaboration has been gathering microlensing survey data from the Galactic Bulge where they monitored 50 million Milky Way stars for microlensing events for the last 2 years. To qualify as lonely or wandering planets, the planets had to be at least 10 astronomical units from a star (one astronomical unit is the distance from the Sun to the Earth). The team recorded 474 individual microlensing events, 10 of which lasted for less than 2 days, indicating that they are the planetary masses the researchers were hunting for. The researchers then used independent data obtained by the OGLE (Optical Gravitational Lensing Experiment) team of astronomers and astrophysicists from Poland, who are working in Chile, to substantiate their findings. They found that the Polish team had also recorded 7 of the 10 events the MOA collaboration had recorded.

The scientists estimate that there are nearly twice as many lonely planets as main-sequence stars – there are potentially almost 2 Jupiter-mass wandering planets for every dwarf star in the Milky Way. Dr Ian Bond from the Institute of Information and Mathematical Sciences at Massey University and co-author of the Nature paper says, “Free-floating planets of ‘terrestrial’ mass (less than 15 Earth masses) could be more common. Our next aim is to make the first detections of these types of planets.”

The scientists believe the planets may have formed in dense gas clouds around newly formed stars in the usual way but then were scattered into unbound or very distant orbits. They theorise that systems with too many large planets can be dynamically unstable – some of the big planets are knocked out of the system by planet/planet collisions.

Dr Bond explains that the MOA collaboration uses a 1.8m telescope at the Mt John University Observatory near Lake Tekapo. “Attached to this telescope is a scientific grade 80 megapixel CCD camera that is capable of imaging a wide field-of-view on the sky.

Microlensing planet discoveries

“We have used the microlensing technique to good effect in discovering planets. MOA led the discovery of the first microlensing planet in 2004, based on observations take in 2003. Since then, 11 more microlensing planet discoveries have been published, with several more undergoing analysis. It turns out that microlensing is most sensitive to planets that orbit stars beyond their ‘snowlines’ – the distance from the star where it is cold enough for water to condense into solid ice grains. The planets found by microlensing are expected to be cold. It has been speculated that low mass planets that form beyond the snowline may accrete large quantities of ice to form icy/ocean planets.

“In this latest result, we have taken the idea of cold planets or planets in wide orbits to extremes. Here, we have found planets that are at such large distances from any host stars that they are effectively ‘free-floating’ planets. We detected these by analysing the measure microlensing timescales for a sample of events. For stellar mass lenses, these timescales are typically 10–20 days, but for planetary mass lenses, these timescales are less than 2 days. This makes them hard to detect, but early on in our project, we recognised that detecting these could be within our capabilities given our exceptional ability to monitor large numbers of stars simultaneously.”

Associate Professor Philip Yock from the University of Auckland’s Physics Department and co-author of the Nature paper, observes that the MOA project capitalises on New Zealand’s southern location. “The centre of the galaxy is in the southern sky, and the dense stellar fields there provide frequent examples of the gravitational lensing effect.”

This is also why the Polish OGLE team operate from Chile.