APPENDIX IV
Interplanetary dust, zodiacal light
Astronomers have detected remnant and new interplanetary dust - the product of the arrival of interstellar dust and comets - in our solar system. It is called zodiacal dust and generates a luminescence that can be seen in the plane of the planets, that is, the plane of the ecliptic, where the accretion disk was, in the beginning.
Interplanetary dust (IDPs) is composed of particles up to 100 mm, and from that size we would have Meteoroids and larger objects, so it is very small particles, interplanetary dust is a variant of cosmic dust, it is called interplanetary for it is comprised between the sun and the planets.
Several solar systems in formation have also been discovered in which one can appreciate the protoplanetary dust existing between the planets that are consolidating and the central star.
After 4,000 million years of life of the Sun and its solar wind, dust still floats in interplanetary space. The remaining dust of the original nebula to which the comets and the coming from the interstellar space are added permanently, that is, the dust that is beyond the solar system and that arrives to our system thanks to the wind currents of the stars, novae, gravitational waves and all the dynamics of the galaxy.
It is clear that the solar wind is not enough to clean interplanetary space.
Apparently there is a certain balance between the attractive force of the Sun and the solar wind, in addition to that the dust also has to describe orbits and interact with the planets.
This dust falls to the earth in the form of micrometeorites that, due to their size, reach the Earth’s surface almost without being altered.
This zodiacal dust is the remnant of that “sandstorm” that prevented the observer of genesis from seeing the sun and therefore understanding that it was the source of the light that he perceived.
Also a proof of the impossibility of seeing -from the Earth- the moment in which the star, the Sun, was ignited for the first time.
The Earth in its movement around the sun captures - even today - thousands of tons of this dust daily (about 2900tn a day), at that rate it is calculated that if this dust were not destroyed, on earth there would be a layer of one meter of height of dark colored powder, interplanetary dust.
Dynamic
Let’s see now the dynamics of interplanetary dust in the solar system.
Several forces act on this interplanetary micromatter:
- The radiation pressure; that appears as a force that acts on the dust pushing it and therefore slowing it down and trying to move it out of the solar system, it is a vector of poynting, that is, it is affected by the intensity of the electromagnetic wave coming from the sun, it is a very weak but very noticeable in the cometary tails when approaching the sun.
-The Poynting-Robertson effect, The interaction of dust with sunlight generates a braking force that is weak compared to that generated by radiation pressure but that dissipates energy and momentum causing the particle to fall very slowly in orbits of spiral towards the Sun. This effect is very important for very small particles, but when it is already bodies of mass near the meter is no longer appreciable.
-Other important effect is the existence of the interplanetary magnetic field which gives rise to a force that tends to increase the orbital inclination of the interplanetary dust.
The arrangement of the dust in the solar system is of a greater concentration between Mars and the Sun, in a crushed lenticular form, with its plane of main symmetry coinciding with the invariable plane of the solar system (or maximum plane of Aries or Laplace).
In the vicinity of the sun below 0.5 AU would be absent because high temperatures volatilize them.
Interplanetary dust can be visualized from Earth in a certain way. If the night is very dark and with great stability we can see what is called zodiacal light, it is called so we can observe a faint light in the plane of the ecliptic at dawn or dusk, This is the reflection of the light of the sun by interplanetary dust in the vicinity of the sun.
Beta Pictoris Accretion Disk
In 1983, the multinational satellite IRAS (Infrared Astronomical Satellite) discovered that some nearby stars emitted more infrared light than normal. Then, speculations began, and almost all of them pointed in the same direction: that excess of infrared radiation could be explained by the existence of huge (and hot) rings of matter around the stars. The following year, astronomers from the Las Campanas Observatory in northern Chile revealed something much more specific: one of the stars in question, called Beta Pictoris, had around him a colossal disk of matter, 30 times the diameter of the System Solar. It was very flat, and seemed to have a gap in the middle. And while no planets were detected inside, almost all astronomers interpreted what was seen around Beta Pictoris, was the embryo of a planetary system. Nothing less. And that the central hole was an area where planets were probably forming, growing as they incorporated all that scattering of cosmic debris. The emblematic case of Beta Pictoris was followed by many others to this day, including the “protoplanetary disks” observed by the Hubble Space Telescope in the bowels of the famous Orion Nebula. All these direct observations, added to new astrophysical models, and computer simulations, allowed us to understand how planetary systems are born. And how ours was born ...
The inconvenience of interplanetary dust in the search for exoplanets
That interplanetary dust (or interstellar if we talk about the one between the stars of the galaxy) is problematic, it affects how we see the exoplanets. Especially those who are in the habitable zone.
Imagine a zodiacal light that was a thousand times brighter than the one we see here. So bright that it darkens even the Milky Way. What impact would such a light have for astronomers? We can suspect that it would be serious, but how exactly? That’s what a group of researchers has tried to determine in a new study. It allows us to understand, for example, the difficulty of finding planets around certain stars.
In it, we try to determine how much interplanetary dust could prevent us from detecting planets around that star. It is something that can be useful for telescopes that are designed in the future. In this case, the 30 closest stars have been examined. The first results seem quite positive. In the observed stars, its zodiacal light is 15 times less than what we have in the habitable zone of the Solar System.
Problematic at large scales
But planets that are in areas with very intense zodiacal light can be problematic. Epsilon Eridani is an interesting system because of its proximity to the Sun. It is only 10.5 light-years away, very close on the astronomical scale. In addition, it is very similar to our star. So it is an objective that in theory would seem attractive. However, researchers have concluded that it has so much interplanetary dust that we could not identify a planet like the Earth around it.
Even so, Epsilon Eridani is interesting because it allows studying the process of planet formation, among other reasons. For years it has been claimed that he could have a planet around him. However, its detection is complicated and has been questioned. If there were, something that does not seem to have many supports, it would be a planet with a mass similar to Jupiter, between 60% and 155% of its size. With an orbit of 6.8 to 7.3 years.
The study, in any case, is a sample that is still beginning to study the distribution of interplanetary dust. Something that can allow us to deduce the possibility that there are planets in a star system. The standard model is that interplanetary dust forms during collisions between asteroids. That dust approaches the star and is scattered throughout the system.
The intriguing case of Vega
A striking result is that of Vega. Astronomers have known for some time that the star has a large cold interplanetary dust belt. It comes to be the equivalent of the Kuiper Belt in the case of Vega. It also has a hot dust disk near the star. But no tempered dust has been discovered, something that has been seen in the Solar System. That dust is what would be in the habitable zone.
So researchers are wondering what could be the mechanism that makes no interplanetary dust in that region. His absence, in his words, could be the sign that there is a very massive planet. Its severity could be responsible for keeping that region clean. There could also be several rocky planets with a mass similar to that of Earth. Other stars, on the other hand, have shown different results.
Instead of having those distant and nearby interplanetary dust belts, they have large amounts of interplanetary dust in the habitable zone. In those cases, its presence could be due to a massive belt of asteroids in which collisions are very frequent. Of those 30 stars analyzed, interplanetary dust has been detected in the habitable zone of four.
The difficulty to see planets in the habitable zone
Two of those stars, too, are stars in which until now no dust has been detected around them. Although this can be explained if we bear in mind that the instruments they are using are five to ten times more sensitive. In the next exoplanet searches, the researchers suggest extending the analysis to farther stars. They are clear that the better we know how much interplanetary dust there is in a system, the better.
The idea is, at least, interesting. However, we must not lose perspective. In fact, the study itself shows it. In most of the closest stars, the amount of dust between planets is lower than what we see here. So it does not seem that it could be a factor that will prevent us from discovering rocky planets in other systems. It does serve, on the other hand, to maintain the mystery in some cases.
That is, if planets have not been found around a star, but it is known to have a dense cloud of dust, it is possible that it is simply that cloud that prevents us from detecting it.
Be that as it may, the study is interesting to understand the first moments of the creation of the solar system and its central star the sun.
Ref .: Study of S. Ertel, D. Defrère, P. Hinz et al; “The HOSTS survey - Exozodiacal dust measurements for 30 stars”. Published in the Astrophysical Journal on April 17, 2018.
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