A new statistical study of planets found by a technique called gravitational microlensing suggests that Neptune-mass worlds are likely the most common type of planet to form in the icy outer realms of planetary systems. The study provides the first indication of the types of planets waiting to be found far from a host star, where scientists suspect planets form most efficiently.
Neptune-mass worlds are likely the most common type in the outer realms of planetary system
Credits: NASA's Goddard Space Flight Center.
This video can be downloaded from the Scientific Visualization Studio
This video can be downloaded from the Scientific Visualization Studio
This graph plots 4,769 exoplanets and planet
candidates according to their masses and relative distances from the
snow line, the point where water and other materials freeze solid
(vertical cyan line). Gravitational microlensing is particularly
sensitive to planets in this region. Planets are shaded according to the
discovery technique listed at right. Masses for unconfirmed planetary
candidates from NASA's Kepler mission are calculated based on their
sizes. For comparison, the graph also includes the planets of our solar
system. Credits: NASA's Goddard Space Flight Center. Hi-res image
"We've found the apparent sweet spot in the sizes of cold planets.
Contrary to some theoretical predictions, we infer from current
detections that the most numerous have masses similar to Neptune, and
there doesn't seem to be the expected increase in number at lower
masses," said lead scientist Daisuke Suzuki, a post-doctoral researcher
at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and the
University of Maryland Baltimore County. "We conclude that Neptune-mass
planets in these outer orbits are about 10 times more common than
Jupiter-mass planets in Jupiter-like orbits."
Gravitational microlensing takes advantage of the light-bending
effects of massive objects predicted by Einstein's general theory of
relativity. It occurs when a foreground star, the lens, randomly aligns
with a distant background star, the source, as seen from Earth. As the
lensing star drifts along in its orbit around the galaxy, the alignment
shifts over days to weeks, changing the apparent brightness of the
source. The precise pattern of these changes provides astronomers with
clues about the nature of the lensing star, including any planets it may
host.
"We mainly determine the mass ratio of the planet to the host star
and their separation," said team member David Bennett, an astrophysicist
at Goddard. "For about 40 percent of microlensing planets, we can
determine the mass of the host star and therefore the mass of the
planet."
More than 50 exoplanets have been discovered using microlensing compared to thousands detected by other techniques, such as detecting the motion or dimming of a host star
caused by the presence of planets. Because the necessary alignments
between stars are rare and occur randomly, astronomers must monitor
millions of stars for the tell-tale brightness changes that signal a
microlensing event.
However, microlensing holds great potential. It can detect planets
hundreds of times more distant than most other methods, allowing
astronomers to investigate a broad swath of our Milky Way galaxy. The
technique can locate exoplanets at smaller masses and greater distances
from their host stars, and it's sensitive enough to find planets
floating through the galaxy on their own, unbound to stars.
NASA's Kepler and K2 missions
have been extraordinarily successful in finding planets that dim their
host stars, with more than 2,500 confirmed discoveries to date. This
technique is sensitive to close-in planets but not more distant ones.
Microlensing surveys are complementary, best probing the outer parts of
planetary systems with less sensitivity to planets closer to their
stars.
"Combining microlensing with other techniques provides us with a
clearer overall picture of the planetary content of our galaxy," said
team member Takahiro Sumi at Osaka University in Japan.
From 2007 to 2012, the Microlensing Observations in Astrophysics (MOA)
group, a collaboration between researchers in Japan and New Zealand,
issued 3,300 alerts informing the astronomical community about ongoing
microlensing events. Suzuki's team identified 1,474 well-observed
microlensing events, with 22 displaying clear planetary signals. This
includes four planets that were never previously reported.
To study these events in greater detail, the team included data from
the other major microlensing project operating over the same period, the
Optical Gravitational Lensing Experiment (OGLE), as well as additional observations from other projects designed to follow up on MOA and OGLE alerts.
From this information, the researchers determined the frequency of
planets compared to the mass ratio of the planet and star as well as the
distances between them. For a typical planet-hosting star with about 60
percent the sun's mass, the typical microlensing planet is a world
between 10 and 40 times Earth's mass. For comparison, Neptune in our own
solar system has the equivalent mass of 17 Earths.
The results imply that cold Neptune-mass worlds are likely to be the
most common types of planets beyond the so-called snow line, the point
where water remained frozen during planetary formation. In the solar
system, the snow line is thought to have been located at about 2.7 times
Earth's mean distance from the sun, placing it in the middle of the
main asteroid belt today.
A paper detailing the findings was published in The Astrophysical Journal on Dec. 13.
"Beyond the snow line, materials that were gaseous closer to the star condense into solid bodies, increasing the amount of material available to start the planet-building process," said Suzuki. "This is where we think planetary formation was most efficient, and it's also the region where microlensing is most sensitive."
NASA's Wide Field Infrared Survey Telescope (WFIRST), slated to launch in the mid-2020s, will conduct an extensive microlensing survey. Astronomers expect it will deliver mass and distance determinations of thousands of planets, completing the work begun by Kepler and providing the first galactic census of planetary properties.
NASA's Ames Research Center manages the Kepler and K2 missions for NASA's Science Mission Directorate. The Jet Propulsion Laboratory (JPL) in Pasadena, California, managed Kepler mission development. Ball Aerospace & Technologies Corporation operates the flight system with support from the Laboratory for Atmospheric and Space Physics at the University of Colorado in Boulder.
WFIRST is managed at Goddard, with participation by JPL, the Space Telescope Science Institute in Baltimore, the Infrared Processing and Analysis Center, also in Pasadena, and a science team comprising members from U.S. research institutions across the country.
For more information on how NASA’s Kepler is working with ground-based efforts, including the MOA and OGLE groups, to search for planets using microlensing, please visit: https://www.nasa.gov/feature/ames/kepler/searching-for-far-out-and-wandering-worlds/
By Francis Reddy
NASA's Goddard Space Flight Center in Greenbelt, Maryland
Editor: Karl Hille
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Neptune-mass exoplanets like the one shown in this
artist's rendering may be the most common in the icy regions of
planetary systems. Beyond a certain distance from a young star, water
and other substances remain frozen, leading to an abundant population of
icy objects that can collide and form the cores of new planets. In the
foreground, an icy body left over from this period drifts past the
planet. Credits: NASA/Goddard/Francis Reddy. Hi-res image
A paper detailing the findings was published in The Astrophysical Journal on Dec. 13.
"Beyond the snow line, materials that were gaseous closer to the star condense into solid bodies, increasing the amount of material available to start the planet-building process," said Suzuki. "This is where we think planetary formation was most efficient, and it's also the region where microlensing is most sensitive."
NASA's Wide Field Infrared Survey Telescope (WFIRST), slated to launch in the mid-2020s, will conduct an extensive microlensing survey. Astronomers expect it will deliver mass and distance determinations of thousands of planets, completing the work begun by Kepler and providing the first galactic census of planetary properties.
NASA's Ames Research Center manages the Kepler and K2 missions for NASA's Science Mission Directorate. The Jet Propulsion Laboratory (JPL) in Pasadena, California, managed Kepler mission development. Ball Aerospace & Technologies Corporation operates the flight system with support from the Laboratory for Atmospheric and Space Physics at the University of Colorado in Boulder.
WFIRST is managed at Goddard, with participation by JPL, the Space Telescope Science Institute in Baltimore, the Infrared Processing and Analysis Center, also in Pasadena, and a science team comprising members from U.S. research institutions across the country.
For more information on how NASA’s Kepler is working with ground-based efforts, including the MOA and OGLE groups, to search for planets using microlensing, please visit: https://www.nasa.gov/feature/ames/kepler/searching-for-far-out-and-wandering-worlds/
By Francis Reddy
NASA's Goddard Space Flight Center in Greenbelt, Maryland
Editor: Karl Hille
Source: NASA/Exoplanets
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