New Zealand astronomers and astrophysicists have helped discover a collection of Jupiter-sized free-floating planets that do not orbit stars, but instead drift through space.
The discovery of the exoplanets differs from those observed in the last 15 years, as these lonely planets are not gravitationally bound to host stars – as the planets in our solar system are. The research was published in the journal Nature today.
The planets were discovered using gravitational microlensing, which enables scientists to study objects that emit little or no light. The scientists, with support from researchers at the University of Auckland, Massey University, Victoria University, the University of Canterbury and Mt. John Observatory, estimate that there are nearly twice as many lonely planets as main-sequence stars.
The lonely planets are at least 10 astronomical units from a star (one astronomical unit is the distance from the Sun to the Earth).
The scientists suggest the planets may have formed in dense gas clouds around newly formed stars and were then scattered into unbound or very distant orbits.
Dr Ian Bond, Senior Lecturer, Institute of Information and Mathematical Sciences Massey University and co-author of the Nature paper comments:
“Microlensing observations in astrophysics uses the technique of gravitational microlensing to study stars and planets in the Galaxy. We use this to detect distant stars and sometime any orbiting planets by utilizing them as naturally occurring lens. These amplify the light of background stars that are physically unrelated to the foreground star but are along the line of sight.
“The probability of microlensing is extremely low, and in order to detect them, one needs the capability of monitoring many simultaneously. Our Japan/NZ MOA (Microlensing Observations in Astrophysics) collaboration uses a 1.8 m 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. MOA surveys an area of the sky towards the Galactic Bulge that contains a backdrop of some 50-100 million potential microlensing source stars. Image processing software is used at the telescope to detect these microlensing events as they occur in real-time. We typically find 500-600 microlensing events each year. The only other facility with the capability of finding microlensing events is the Polish OGLE telescope that operates from Las Campanas in Chile.
“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, eleven 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 my accrete large quantities of ice to form icy/ocean planets.
“In this latest result, we have taken the idea of cold planets 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 analyzing 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 recognized that detecting these could be within our capabilities given our exceptional ability to monitor large numbers of stars simultaneously.
“Theories of planet formation predict planet-planet scattering and stellar encounters could move planets into wide orbits possibly ejecting them from their systems altogether. Our results point to a population of free floating planets of around Jupiter mass. However, 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.”
Denis Sullivan, Professor of Physics, School of Chemical and Physical Sciences, Victoria University and co-author of the Nature paper comments:
“The research just published in a Nature Letter provides convincing evidence of the existence of so-called free-floating planets with Jupiter masses. Ten such astronomical objects have been discovered in our Galaxy by the gravitational microlensing technique.
“This technique uses the amplified light from a background star to ‘measure’ the mass of a foreground lensing object. As predicted by Einstein about 80 years ago, there is an increase in the star’s brightness when the star, lens and observer are in close alignment.
“The research was carried out by a NZ/Japanese collaboration of astronomers known as MOA – ‘Microlensing Observations in Astrophysics’ – together with another collaboration based in Chile called OGLE. The MOA group, established in the mid nineties, uses NZ’s largest telescope based in the South Island to regularly monitor the brightness of a large number of stars using a special large format electronic camera system.
“This work provides unique insights into our understanding of the formation of stars and planets. A key property that astronomers call ‘the initial mass function’ measures the relative values of the resulting masses when interstellar gas and dust condense under the attraction of gravity to form astronomical objects.
“The objects we can readily see are the stars. Little was known about the numbers of isolated planetary mass objects, since in stellar terms they are quite cold and therefore emit little light and energy. Microlensing circumvents this difficulty by using the light from other stars to detect the cold low mass objects.
“When the masses of all the detected objects in the research are taken into account, an interesting inference can be made. The detected Jupiter-mass objects may have actually been formed in protoplanetary disks formed around new stars and were subsequently scattered out of the multi-planet system by collisions with other planets.”
Associate Professor Philip Yock, Department of Physics, University of Auckland and co-author of the Nature paper comments:
“This discovery is the culmination of a 24 year collaboration between Japan and New Zealand in astronomy. The collaboration started when a supernova occurred in the southern sky in 1987 – the first supernova to be visible to the naked eye in 400 years.
“In 1994 the efforts of the collaboration were re-focussed on to the then-new technique of using gravitational fields of stars as naturally occurring astronomical lenses, following an old idea of Einstein. Thus was born the NZ/Japan project now known as MOA – ‘Microlensing Observations in Astrophysics’.
“As with its predecessor, 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.
“The telescope that is used by MOA is located at the Mt John Observatory of the University of Canterbury. It is New Zealand’s largest telescope. It was funded mainly by a grant to Professor Yasushi Muraki of Japan.
“Since 1994 a number of observations of stars and planets have been made using the gravitational lensing effect. Detailed measurements of the shapes of distant stars have been made, and also a number of planets have been discovered orbiting stars beyond their snowlines, where water freezes. These discoveries are described at http://www.phys.canterbury.ac.nz/moa/ .They could not have been made using conventional astronomical techniques.
“Several staff and students from both Japan and New Zealand contributed to the discoveries. In addition, many astronomers in other countries supplied supporting data. Recent papers have included more than 100 authors from more than 20 observatories.
“The most recent discovery is the finding of “free-floating” planets roaming interstellar space. These planets are unattached to stars. It is possible that they were born as normal planets orbiting stars, but were subsequently “kicked out” of their planetary systems, never to return. They are sometimes referred to as “rogue” or “orphan” planets.
“The discovery was made primarily by the MOA group, but important supporting data were supplied by a Polish group known as OGLE.
“The free-floating planets were discovered as gravitational lenses of weaker power than normal stellar lenses. It is estimated that there are twice as many free-floating planets in the galaxy as stars.
“An artist’s conception of a free-floating planet (credit NASA/JPL-Caltech/R. Hurt) is attached. This is a Jupiter-sized planet. Future studies will aim to find smaller free-floating planets, closer to the size of Earth.”