Earth-like planet

Taylor, S. R. (1999). On the difficulties of making Earth-like planets. Meteoritics and Planet. Sci. 34, 317-329. 

One of the projects planned for the next decade is Darwin, to be organised by ESA. Darwin will be a flotilla of four or five spacecraft that will search for Earth-like planets around other stars and analyse their atmospheres for the chemical signature of life. Three of the spacecraft will carry 3 1 m space telescopes , which will form the Infrared Space Interferometer IRSI they will be stationed 1.5 million kilometres from Earth, in the opposite direction from the Sun, at the Lagrangian Point L2 (a libration point at which the gravitational forces of the Earth and the sun cancel out). 

In the galactic lifebelt , the amount of rock-forming elements is high enough for Earth-like planets to have been formed. However, two conditions must be fulfilled ... 

It is tempting to take a tour of the solar system, stopping off at each planet to look at the chemistry from the origin of the red colour in Jupiter s great red spot to the volcanic activity of Io, but this would be another book for each planet. Instead, we will generalise the study to the formation of Earth-like planets in order to focus on the possibilities for life. 

Calculate the effective surface temperature of an Earth-like planet with the same diameter, albedo and orbital radius around Aldeberan in Taurus, which has a surface temperature of 7200 K. Is the planet within the habitable zone of this star ... 

This is, of course, a very Terra-centric categorization, but so far we only have the unitary example of Earth from which to draw examples. Furfaro et al. (2007) have developed a fuzzy-logic-based scheme that goes more toward the fundamentals of coupled physical/biological planetary systems, but still it takes a restricted view oriented toward Mars- and Earth-like planets. It is entirely possible that Earth has a thin biospheric veneer compared to potentially more massive biospheres on worlds with deep ice-covered oceans such as Europa we just don t know. 

Covalent compounds make up the bulk of chemical compounds known to man, but they only a make-up a small percentage of the chemical compounds found on earth and earthly like planets, and virtually most solar systems. As previously stated, there are about 200,000 ionic compounds known to man, with a potential of another 100,000 left undiscovered throughout the universe however, covalent compounds number in the millions. For example, currently there are 16,000,000 covalent compounds known to man (as of 2003). The possible number of covalent compounds is practically endless, as the combination of these compounds is virtually infinite. 

Philosophically, the known as Principle of Mediocrity tell us about the possible fact that Earth is not a special place in the universe, and life on it is just the coincidence of some environmental conditions that could be found elsewhere in the universe [1]. Furthermore, from a scientific point of view, the recent development in Astrometry and Planetary Search has led to the discovery of Jupiter-like planets so far, but Earth-like planets are expected to be also detected. In the same way, recent studies have pointed to the fact that life can also evolve in different environments taking profit of some different energy sources, enhancing the probability of planets evolving life [2],... 

First omit the observed cosmic abundance of the biogenic elements that are favorable to life. Then omit the environments (earth-like planets or Europa-like satellites) that favor evolution and adaptive radiation. The consequence of omitting both factors is that life most likely would not arise. 

This question is obviously unanswerable in the present state of our knowledge. In any case, itis not for the biologist to answer. All that can be said is that the present direction of astronomical research favors multiplicity rather than uniqueness. It is already known that planetary systems are far from rare. It is true that only large planets orbiting close to their sun have been detected so far, but this is because of technical limitations that may well be overcome by the advances of tomorrow. The existence of earth-like planets is not excluded it is seen as likely by many experts. 

On the whole, sheer numbers would seem to argue against rarity. Considering the number of sunlike stars believed to exist in our galaxy (on the order of thirty billion) and the estimated number of galaxies in the universe (about one hundred billion), the odds seem to favor the existence of multiple earth-like planets capable of giving rise to life. Even some defenders of a rare earth share this opinion (Ward and Brownlee, 2000). 

Wolstencroft, R. D. and Raven, J. A. (2002). Photosynthesis likelihood of occurrence and possibility of detection on Earth-like planets. Icarus, 157, 535 8. 

The restricted three-body problem Two bodies of finite masses, called primaries, revolve around their common center of mass in circular orbits and a third body with negligible mass moves under their gravitational attraction, but does not affect the orbits of the two primaries. In most astronomical applications the second primary has a small mass compared to the first primary, and consequently the motion of the third, massless, body is a perturbed Keplerian orbit. This is a model for the study of an asteroid (Jupiter being the second primary), a trans-Neptunian object (Neptune being the second primary) or an Earth-like planet in an extrasolar planetary system. 

Figure 1. Chemical equilibrium composition of volatiles outgassedfrom average H-chondritic material as a function of temperature for the T-P profile of an Earth-like planet. Modifiedfrom (7).

Spacecraft missions are being planned by both NASA (the Terrestrial Planet Finder) mid ESA (Darwin) to search for Earth-like planets around other stars and to take spectra of their atmospheres and surfaces 44, 45). These missions are designed to detect planets located in the habitable zones (HZ) of other stars. The... 

HZ is defined as the orbital area around a star where a planet can sustain liquid water at the surface. Several studies have attempted to define the HZ as a function of stellar type (e.g. F, G, K, M dwarf stars) and time (see e.g. 46, 47, and references therein). The HZ is of interest because it is widely believed that liquid water is necessary for the genesis of (recognizable) life. The particular emphasis of the planned space missions is to search for signs of life on extrasolar Earth-like planets via spectroscopy. Atmospheric compounds such as O2, O3, N2O, CH4, and CH3CI are considered biomarkers, and their spectroscopic detection in a terrestrial-type atmosphere, particularly O2 or O3 found together with a reduced gas such as CH4, would suggest life (48, 49). Detection of CO2 would indicate that the planet is indeed a terrestrial-type planet... 

By the end of the episode, after discovering how much the crew needs her, she has made her peace with working in space. She drops the slug off on an Earth-like planet, where it will survive just fine. 

As telescopes have searched the sky for other planets, we have developed a reasonably big sample. We guess that about 10% of sun-like stars have an Earth-like planet around them, and find that in general, planets only up to 1.6 times the mass of Earth can turn out rocky and Earth-like, so that gas giants are much more common. 

About 10% of sun-like stars have an Earth-like planet... E. A. Petiguraa et al. Prevalence of Earth-size planets orbiting Sun-like stars. 2013. Proc Natl Acad Sci. 110(48), p. 19273. DOl 10.1073/pnas.l319909110. 

---