October 06, 2016

Analysis of nearest Rocky Exoplanet Proxima B suggests it is an ocean planet with 120 miles of water covering the whole world

A rocky extrasolar planet with a mass similar to that of the Earth was recently detected around Proxima Centauri, the nearest star to our sun. This planet, called Proxima b, is in an orbit that would allow him to have liquid water on its surface, thus raising the question of its habitability. In a study to be published in The Astrophysical Journal Letters, an international team led by researchers at the Marseille Astrophysics Laboratory (CNRS / Aix-Marseille Université) has determined its dimensions and properties of its surface, which actually favor its habitability . She said the planet could be of the "ocean planet" with an ocean covering its entire surface, and a similar water than some icy moons around Jupiter or Saturn. The researchers also show that Proxima b composition might look like that of Mercury, with a metal core with two-thirds of the mass of the planet. These results provide the basis for future studies to determine the habitability of Proxima b.

Proxima Centauri, the star nearest the sun, has a planetary system consisting of at least one planet. By analyzing and supplementing earlier observations that such a discovery was recently made, marking the field of exoplanet research. These new measures have shown that this planet, named Proxima Centauri b - or simply Proxima b -, has a mass close to that of the Earth (1.3 times the latter) and orbits its star at a distance of 0, 05 astronomical units (one tenth of the Sun-Mercury distance). Contrary to what one might think, as little distance does not imply a high temperature on the surface of Proxima b. As Proxima Centauri is a red dwarf, its mass and radius correspond to only one-tenth of the Sun, and its brightness is a thousand times smaller than our star. At such a distance, b Proxima is therefore in the habitable zone of its star. It is likely to harbor liquid water at its surface and therefore to harbor life forms.


Diagram mass-radius comparing the positions of several exoplanets known to those in the solar system planets. The curves correspond to specific compositions used in the model of internal structure. The existing area of Proxima b is drawn in gray and takes into account the uncertainty of its mass and its different possible compositions

Arxiv - Possible Internal Structures and Compositions of Proxima Centauri b (5 pages)



However, very little is known about Proxima b, particularly its radius is unknown. It is therefore impossible to know what the planet looks like, or what it is made. The radius measurement of an exoplanet is normally done during transit, where it eclipses its star. But such an event is a low probability (1.5%), and more observations of the star does indeed show no signs of transit.

There is another way to estimate the radius of a planet if we know its mass, simulating the behavior of the constituent materials. This is the method used by a team of researchers Franco-American after the Marseille Astrophysics Laboratory (CNRS / Aix-Marseille University) and the Department of Astronomy at Cornell University. With the help of a model of internal structure, they explored the different compositions that could present Proxima b and deduced the corresponding values ​​of the radius of the planet. They restricted their study to the case of potentially habitable planets simulating dense and solid planets, formed of a metallic core and a rocky mantle as the terrestrial planets in the solar system, while allowing the incorporation of a large mass water in their composition.


Comparison of two extremes obtained for Proxima b with Earth. This diagram shows the internal structure of each planet. From left to right: Proxima b with the smallest attainable radius (65% metallic core surrounded by a rocky mantle separated into two phases), Earth (ditto with 32.5% core) and b with the Proxima maximum authorized range (50% of rocky mantle surrounded by a layer of water in solid and liquid form)


These assumptions allow a wide variety of compositions for Proxima b, the radius of the planet may vary between 0.94 and 1.40 times the radius of the Earth (6371 km). The study shows that Proxima b has a minimum radius of 5990 km, and the only way to get this value is to have a very dense planet, consisting of a metal core with a mass equal to 65% of the the planet, the rest being rocky mantle (formed silicate) until this surface. The boundary between these two materials is then located about 1500 km depth. With such a composition, Proxima b is very close to the planet Mercury, which also has a very solid metal core. This first case does not exclude the presence of water on the surface of the planet, as on Earth where the water body does not exceed 0.05% of the mass of the planet. In contrast, Proxima b can also have a radius of 8920 km, provided that it is composed of 50% of rocks surrounded by 50% water. In this case, Proxima b would be covered by a single liquid ocean 200 kilometers deep. Below, the pressure would be so strong that liquid water would turn high-pressure ice before reaching the boundary with the mantle to 3100 km depth. In these extreme cases, a thin gas atmosphere could cover the planet, as on Earth, making Proxima b potentially habitable.

Such findings provide important additional information to different training scenarios that have been proposed for Proxima b. Some involve a completely dry planet, while others permit the presence of a significant amount of water in its composition. The work of the research team used to have an estimate of the radius of the planet in each of these scenarios. Similarly, this will restrict the amount of water available Proxima b, which is prone to evaporation under the influence of ultraviolet rays and X of the host star, which are much more violent than those from the sun.

Future observations of Proxima Centauri will refine this study in the future. In particular, the measurement of stellar abundances of heavy elements (Mg, Fe, Si ...) will decrease the number of possible compositions for Proxima b, allowing determination more accurate radius Proxima b.

Abstract - Possible Internal Structures and Compositions of Proxima Centauri b

We explore the possible Proxima Centauri b's interiors assuming the planet belongs to the class of dense solid planets (rocky with possible addition of water) and derive the corresponding radii. To do so, we use an internal structure model that computes the radius of the planet along with the locations of the different layers of materials, assuming that its mass and bulk composition are known. Lacking detailed elementary abundances of the host star to constrain the planet's composition, we base our model on solar system values. We restrained the simulations to the case of solid planets without massive atmospheres. With these assumptions, the possible radius of Proxima Centauri b spans the 0.94--1.40 R⊕ range. The minimum value is obtained considering a 1.10 M⊕ Mercury-like planet with a 65% core mass fraction, whereas the highest radius is reached for 1.46 M⊕ with 50% water in mass, constituting an ocean planet. Although this range of radii still allows very different planet compositions, it helps characterizing many aspects of Proxima Centauri b, such as the formation conditions of the system or the current amount of water on the planet. This work can also help ruling out future measurements of the planet's radius that would be physically incompatible with a solid planetary body.

SOURCES- Arxiv, CRNS

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