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What is the giant mirror that they have found in the cosmos and why it should not exist

PorSam Miller

Jul 25, 2023

The European Space Agency (ESA) has managed to locate the brightest body known to date that does not emit its own light. Its about ultrahot exoplanet LTT9779blocated about 260 light years from us and orbiting its host star in just 19 hours.

About the size of the planet Neptune, this scorching world—where temperatures reach up to 2,000⁰C on its day side—is blanketed in highly reflective metallic titanium clouds. As a consequence, LTT9779b reflects up to 80% of the light it receives from its parent star, making it the brightest exoplanet ever found. It is literally a giant mirror in the middle of the cosmos.

This is the result of research carried out by the Satellite for the Characterization of Exoplanets (CHEOPS) from ESA. With only dimensions of 1.5 x 1.4 x 1.5 meters and a weight of about 300 kg with fuel, CHEOPS is capable of measuring the size of exoplanets from the minute variations in luminosity of their host stars.

Indeed, because LTT9779b reflects light from its parent star directly back at us, the satellite’s sensors will get a decrease in the amount of reflected light as the exoplanet passes behind its star. This is precisely what the CHEOPS satellite instruments found, yielding a reflectance of 80% for our particular “mirror planet”. And it is not a term far from reality, since the exoplanet LTT9779b constitutes the best mirror (and the largest) found in the Universe to date.

About metallic mirrors in Optics

But what do we really understand by a mirror and what is its operation? The definition is relatively simple: a mirror is an optical component capable of reflecting the light that falls on it.

In particular, only those devices in which the reflection is of the specular type and satisfy the law of reflection (where the angle of incidence of light is equal to the reflected angle).

Among all the characteristics of a good mirror, we can highlight two fundamental ones:

  1. The reflectivity (or reflectance), which is the percentage of light intensity that a mirror reflects. Generally, it depends on the wavelength of the light and the angle of incidence.

  2. The reflection bandwidth, which corresponds to the range of light wavelengths in which the reflectance is high. Thus, for example, while some mirrors are designed to reflect in the visible range, others have a bandwidth in the infrared (such as the primary mirror of the James Webb Space Telescope).

On the other hand, the common mirrors that we use in homes basically consist of a glass plate with a silver coating on the back. They are the so-called second surface mirrors, which present minor reflections in the glass layer (primary reflection), increasing considerably in the metal layer (secondary reflection). As a counterpart, its reflectivity is below 100%, since there are absorption losses for visible light in the metallic layer.

For other applications, first surface mirrors are used, where light falls directly on the metal coating (or an enhancement coating) and does not reach the mirror support.

Precisely, the titanium present in the clouds of LTT9779b would act as a gigantic first-surface mirror (although without a substrate), reflecting 80% of the light received by its parent star.

Brighter than Venus

In the night sky we can identify the Moon and the planet Venus as the brightest objects (discounting our Sun, whose light is reflected by the previous ones).

In the case of our satellite, the beautiful selene it reflects only 8% of the total energy it receives from the Sun (ie, it has an albedo of 0.08 on a scale from 0 to 1). However, we enjoy its nocturnal splendor due to its proximity to our planet.

On the other hand, the planet Venus was (to date) the brightest star that did not emit its own light, reflecting up to 75% of the light it received. The fact that Venus is known as The star gives us an idea of ​​the magnitude of its brightness, due to its dense layer of clouds composed basically of CO2.

On the other hand, Earth (as seen from space) has an albedo of 0.37, while the gas giant Jupiter reflects up to 52% of sunlight.

Consequently, the exoplanet LTT9779b (with an albedo of 0.80) is even brighter than Venus although, due to its remoteness, we cannot see it with the naked eye.

An exoplanet that shouldn’t exist

To date, all exoplanets discovered that orbit their host star in less than a day are so-called «hot Jupiters» (ie gas giants with a radius 10 times greater than Earth) or rocky planets smaller than our planet.

According to the researcher and co-author of the study Vivien Parmentier, from the Observatoire de la Côte d’Azur in France, «is an exoplanet that should not exist», since its atmosphere would be expected to have been dragged by its parent star, leaving behind the core of the exoplanet.

Furthermore, due to its proximity to its star, the temperature of LTT9779b on its day side reaches 2000 ⁰ C, too high a value for clouds to form in its atmosphere. However, this peculiar exoplanet has an atmosphere made up of metallic clouds.

How did these characteristic clouds form on our mirror planet?

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In the words of this group of researchers, «you have to think in the same way as the condensation that forms in a bathroom after a hot shower.» In the case of LTT9779b, the metallic clouds originated from a supersaturation of the exoplanet’s atmosphere, when the excess of silicates and titanium exceeded their retention capacity and forced the appearance of these clouds.

In future observations, the collaboration of the Hubble and James Webb space telescopes is expected to gather more information on the composition and structure of this exoplanet, the best (and largest) mirror found to date in the cosmos.

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