Forest planet, sometimes called jungle planet or swamp planet, is a theoretical class of terrestrial planet with most or nearly all of the surface covered with forests, jungles, or swamps. Forest planets are seen in science fiction, such as Dagobah in Star Wars.
Forest planets provide habitats very suitable for complex life over much of the surface, resulting in life more diverse than there are on Earth. Around Sun-like stars, forest planets appear mostly green from space. Such planets are rare in the universe, comprising less than 0.1% of all planets. At present, the prospects of finding evidence of forests on exoplanets are slim.
The surface of forest planets are mostly covered with trees or tree-like structures, as well as shrubs, grasses, and possibly fungi, including mushrooms, along with densely-populated wildlife roaming through forests. Forest planets have more intricated ecosystems than on Earth. Forests provide great habitats such as shelter from the elements and abundant food supply.
Forest planets tend to have more diverse forms of plant and animal life than there are on Earth. Kelps, which are underwater trees, as well as corals, are more diverse than there are on Earth due to mostly shallow bodies of water. Some organisms can also fly, especially birds or bird-like creatures. Another form of flying creature in theory is in the shape of a balloon, skyfish, flying whales, or even wingless flying creatures. Aquatic creatures on such planets are less diverse than there are on Earth because of fewer niches in water. Such creatures use kelp as habitats just like trees used as habitats for land organisms.
Gravitational effects on organisms
The average height of trees, shrubs and other organisms around the planet depend on the strength of planet's gravity. Gravity affects how organisms grow and develop in certain ways. Because gravity acts as breaking mechanism for tree growth, shorter trees tend to be grown in higher gravity environments, while taller trees tend to be grown in lower gravity enviroments. The same mechanism also applies to animals; in addition, they're stronger on higher gravity planets than in counterparts on Earth, and vice versa.
Gravity can also affect evolution of organisms, especially the likelihood of flying creatures. On lower gravity planets, flying creatures would be more diverse, but the ability diminish on higher gravity planets. Though less likely in higher gravity, large floating creatures in the form of gas-filled balloons would actually be more likely, especially in atmospheres denser than Earth's.
Swamps may be abundant over much of the planet as well as bodies of water in the form of rivers, lakes, seas, and streams. If swamps cover most of the surface of forest planet, then the term "swamp planet" is often preferred. In addition to forests and bodies of water, mountain ranges, valleys, and volcanoes where they also provide habitats. Deserts and polar ice caps are often found in small areas where there is little vegetation.
Atmosphere and climate
Forest planets tend to have atmospheres thicker than Earth's but with similar proportions of gases, like nitrogen, oxygen, carbon dioxide, and methane. The combination between thicker atmosphere and greater amounts of greenhouse gases would mean heating be distributed more evenly around the planet. As a result, climate would be similar worldwide, allowing forests and wildlife to thrive over much of the planet's surface. If it is tropically warm over much of the world, the term "jungle planet" would often be used instead. Because of the mentioned atmospheric tendencies, three quarters of all forest planets are believed to be jungle planets.
Atmospheric carbon dioxide level on forest planets usually range from about 500 parts per million to about 2000 parts per million, compared with 400 parts per million for Earth. Dense population of plants covering much of the planet's surface would deplete atmospheric CO2 quickly because it uses this gas for photosynthesis, but it is dedicately balanced by dense population of animals roaming much of the surface which releases CO2 into the atmosphere through respiration. Rain, volcanic eruptions and other natural factors also play a role in the CO2 balance.
Few places on forest planets can have climates drastically different than in forests, like deserts where they are hot and get little rain. Snow would mostly occur only near the poles and near the summits. Hurricanes would almost never occur on forest planets since they don't have large and deep oceans, although thunderstorms can still form in some places.
Orbit and tilt
Since forest planets are the class of habitable planets, it must orbit in the star's habitable zone or so-called Goldilock's zone where temperatures is not too hot nor too cold for life to form and evolve as we know it on Earth. It must also have circular orbit with eccentricity of less than 0.03 and should have rotational axes of less than 8° in order for seasonal variations to be minimal, resulting in similar climate over much of the surface throughout its year. If the axial tilt is greater than 8°, seasons would result which can cause snow to fall on forest lands during its winter thus causing trees to be dormant and forcing some animals to hibernate. If orbital distances and eccentricities change over the long timescales on forest planets, often cause by the gravitational forces of other planets in the system, climate would change in certain places, which can damage complex habitats. Orbital changes and altering of rotational axes can even trigger ice ages, causing widespread destruction of habitats.
Chlorophyll is a pigment causing plants to appear green on Earth. However, plants on other worlds don't have to be green, depending on the color of parent stars. Planets orbiting around F, G, K, and M-type stars would be suitable for vegetation, but stars brighter than F would emit too much light for plants to absorb energy and thus would not survive. Around late K and M-type stars, plants would need to absorb nearly all of visible light wavelengths, resulting in gray to black plants, and corresponding forest planets would appear dark from space with geometric albedo less than 0.1. Around early to mid F-type stars, vegetations would need to absorb less energy and reflect more light, thus making plants light in color, and corresponding forest planets would appear bright from space with geometric albedo greater than 0.7. Around solar-type stars (from late F to mid G-type star), plants would be green like they are on Earth, but on planets orbiting late G to early K-type stars, vegetations would be brown.
Other factors affecting color of plants on planets is their atmospheric makeup, orbit, rotation, and evolution.
The detection of forests on extrasolar planets would be extremely challenging since it requires direct imaging of small planets orbiting near the glare of their parent stars. Determining color of planet orbiting around different color of star can provide evidence of forest planet. To confirm the evidence, structures and terrain must be detected on exoplanets. Astronomers were simulating how to detect features on exoplanets for years using techniques like spectroscopy, and eventually came up with an idea of reflectance, which is the study of multiple reflecting light at various phase angles and the study of shadows. Satellites performed reflectance tests on Earth and saw that different terrain and structures, including trees and buildings, produce different reflected light at different angles. This test would prove to be a very useful tool for detecting surface features and clouds on exoplanets, including the signatures of trees or tree-like structures.
Bidirectional Reflectance Distribution Function (BRDF) would be used to study shadow variations or changes of planetary albedos throughout the planet's orbit, thus determining what's on the planet's surface. Shadows seen on the planet's surface would most often be caused by structures such as mountains, hills, boulders, cliffs or other geologic features. Unique variations of shadows cast on the surface would be used to determine what structure it is casting shadows and would be used to detect signatures of trees or tree-like structures. By studying geography throughout the planet, it would be used to determine if it is a forest planet.
According to the Rare Earth Hypothesis, complex life in our galaxy would likely be very rare; so since forest planets require dense vegetation to colonize much of the surface, it would correspondingly be very rare. In the Milky Way Galaxy, there are an estimated 620 million forest planets out of 820 billion total planets (including giant planets) and 524 billion terrestrial planets, meaning there is one such planet per 1,323 total planets and 845 terrestrial planets. By comparison, forest planets are just 47% more abundant than ferrosilicate giants, which are among the hottest class of giant planets, and 38 times more abundant than city planets.
Forest planets are only concepted in science fiction as science never defined this term. The most famous fictional forest planet is Dagobah, which is a Star Wars planet depicted in Revenge of the Sith, The Empire Strikes Back, and Return of the Jedi where Yoda lives. This planet is covered in forests as well as 88% of surface covered in water in the form of swamps. Another forest world in Star Wars is Endor, a moon orbiting the gas giant, which is the homeworld of Ewoks. The second Death Star is constructed in orbit around Endor and sequently destroyed by Rebels.
- City planet
- Ocean planet
- Extraterrestrial life
- Planetary habitability
- Habitable planet
- Extrasolar planet