Galaxy planets
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Galaxy Planets: Formation, Detection, and Characteristics
Cosmic Planet Formation Histories
The formation of planets within galaxies, including our Milky Way, has been a subject of extensive research. Studies combining galaxy formation histories with planet formation models suggest that the Milky Way hosts approximately 10^9 Earth-like planets and 10^10 giant planets. Across the entire Hubble volume, these numbers scale up to around 10^20 for both Earth-like and giant planets. Interestingly, metallicity thresholds, which were once thought to significantly influence planet formation, do not drastically alter these estimates. However, the formation of giant planets tends to occur later and in larger host galaxies compared to Earth-like planets1.
Planets in the Milky Way
Our galaxy, the Milky Way, is a gravitationally bound system comprising stars, stellar remnants, interstellar gas, dust, dark matter, and asteroids. It is estimated to contain at least a hundred billion planets, with many potentially being life-friendly. For a planet to be considered theoretically habitable, it must be relatively small, rocky, and located within the habitable zone of its star, where liquid water can exist. Future advancements in telescope technology will allow for more detailed assessments of these planets' environmental compositions and the activity levels of their parent stars2.
Circumbinary Planets
Most Sun-like stars in the Galaxy exist in binary systems, where two stars are gravitationally bound. The discovery of circumbinary planets, which orbit around both stars in a binary system, has been a significant milestone. Notable examples include Kepler-34 b and Kepler-35 b, which are gas-giant planets with orbits closely aligned with their parent stars. These discoveries suggest that more than 1% of close binary stars host giant planets in nearly coplanar orbits, indicating a Galactic population of several million such planets3. Additionally, the Kepler-47 system, which includes two planets orbiting a pair of stars, demonstrates that even in the chaotic environment of a close binary star, planetary systems can form and persist. The outer planet in this system resides within the habitable zone, where conditions could allow for liquid water4.
Detection of Planets in Nearby Galaxies
Detecting planets around stars in nearby galaxies, such as M31 (Andromeda), is challenging due to the vast distances involved. The primary method for such detections is microlensing, which can observe signals caused by planets around stars in these distant galaxies. This technique is particularly effective during high-magnification events, allowing for the detection of Jupiter-like planets. Although characterizing these planets is difficult, the absence of such signals can provide strong constraints on the abundance of Jupiter-like planets in these galaxies5.
Earth-Sized and Earth-Like Planets
Recent analyses of data from the Kepler spacecraft have revealed that planets with radii similar to Earth's are common throughout the Galaxy, orbiting at least 16.5% of Sun-like stars. One such planet, Kepler-78b, has a radius and density similar to Earth's, suggesting a composition of iron and rock. This finding supports the idea that Earth-sized planets with Earth-like densities are widespread6.
Planets in Multiple Star Systems
Planets are not only found around single stars but also in binary and higher-order multi-star systems. Over 100 planets have been discovered in such systems, both in circumbinary and circumstellar configurations. While the presence of a second star can disrupt planet formation, recent studies indicate that gas giants are as common around main sequence binaries as they are around single stars. However, there is a notable absence of circumbinary planets around the tightest main sequence binaries, suggesting a more disruptive formation history for these close stellar pairs7.
Kepler Mission Contributions
The Kepler Mission, launched by NASA in 2009, has been instrumental in advancing our understanding of planetary systems. By monitoring 170,000 stars, Kepler has detected over 4,600 planetary candidates, including several hundred Earth-sized planets. The mission's findings indicate that most stars have planets, many of which are similar in size to Earth, and that multi-planet systems are common. These discoveries have provided crucial data for estimating the frequency of planets in our galaxy8.
Conclusion
The study of planets within galaxies, particularly the Milky Way, has revealed a rich diversity of planetary systems. From Earth-like planets to gas giants and circumbinary planets, the Galaxy is teeming with a variety of worlds. Advances in detection methods, such as microlensing and data from missions like Kepler, continue to expand our knowledge, offering insights into the formation, distribution, and characteristics of these distant planets.
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On the history and future of cosmic planet formation
The universe will form over 10 times more planets than currently exist, implying a 92 percent chance that we are not the only civilization the Universe will ever have.
Planets in Milky Way
Small, rocky planets in the habitable zone of their star are most likely to support life.
Transiting circumbinary planets Kepler-34 b and Kepler-35 b
More than 1% of close binary stars have giant planets in nearly coplanar orbits, indicating a Galactic population of at least several million.
Highly CitedKepler-47: A Transiting Circumbinary Multiplanet System
Kepler-47, a transiting circumbinary multiplanet system, reveals that close binary stars can host complete planetary systems, with the outer planet residing within the habitable zone where liquid water could exist.
Highly CitedDetecting planets around stars in nearby galaxies
Photometric microlensing can detect planets around stars in nearby galaxies, such as M31, and the absence of such signals can constrain the abundance of jupiter-like planets.
An Earth-sized planet with an Earth-like density
Kepler-78b, a planet with a mass of 1.86 Earth masses, has a density similar to Earth's and is likely composed of iron and rock.
Highly CitedPopulations of planets in multiple star systems.
Over 100 planets have been found in multiple star systems, with most being circumstellar, suggesting a unique formation history for close stellar pairs.
KEPLER Mission: development and overview
The Kepler Mission has identified over 4600 planetary candidates, including over a thousand confirmed planets, and has discovered that most stars have planets similar to Earth, with many in the habitable zone.
Highly CitedDiscovery of a cool planet of 5.5 Earth masses through gravitational microlensing
This discovery suggests that cool, sub-Neptune-mass planets may be more common than gas giant planets, suggesting the core accretion theory may be correct.
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