1I/'Oumuamua is a small body of interstellar origin discovered on October 19, 2017. As the first object identified from beyond the Solar System, and given its very unusual shape, it does not ceased to capture the attention of scientists. Several hypotheses on its origin have been put forward, some involving an artificial origin. The measurement of anomalous acceleration of the object revived the debate when in an article that made the "buzz", astrophysicists Schmuel Baily and Avi Loeb of Harvard University put forward the hypothesis of a solar sail.
For astronomers and astrophysicists, the law of gravity should always be the dominant factor when a large object moves through the Solar System. The Sun's gravitational pull has by far the greatest effect, pulling all mass towards it according to Newton's or Einstein's laws. According to Newton's law of gravitation, the only possible orbits should be a circle, ellipse, parabola or hyperbola.
For an object such as 'Oumuamua, the orbit must be hyperbolic. An object originating in our Solar System, from Neptune or beyond, could initially move at speeds of up to 1 km/s; 'Oumuamua began entering the Solar System at speeds above 20 km/s. Without passing close to the gas giants to benefit from a gravitational slingshot effect, the object is therefore necessarily of interstellar origin.
However, when it left our Solar System, it did not follow the predicted hyperbolic path. According to the scientists' best orbital reconstruction, 'Oumuamua accelerated as it headed toward the Sun, then slowed as it moved away from it. But this deceleration did not go as planned, as if there was a slight additional acceleration that pushed it further from the Sun than the simple effect of gravity.
In the case of an interstellar asteroid, several explanations can be advanced to explain a divergence of the orbit and the speed compared to the predictions:
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The outgassing explanation would make the most sense, since asteroids and comets in our solar system are full of volatile compounds on their surfaces.
A volatile, by definition, is an ice-like solid chemical that will heat up, transition to a gas phase, and evaporate when the body gets too close to the Sun.
Comets (and sometimes asteroids as well) develop outgassing tails, and thrust from the Sun can cause additional acceleration beyond what gravity predicts.
The only problem with this scenario is that very detailed observations of Oumuamua showed no outgassing phenomena or tail-like structures. The only remarkable property emerging from the observations, in addition to this unusual acceleration, is the smooth and elongated shape of the object.
The uneven heating mechanism is extremely interesting. When one side of the interstellar object faces the Sun, it heats up to temperatures that can be several hundred degrees higher than the opposite side, resulting in a large temperature gradient.
Every object gives off energy in proportion to its temperature. The hot side then radiates with higher energy than the cold side. The irregular nature of the asteroid implies uneven radiation emission, which could, in principle, lead to abnormal acceleration. Acceleration calculations carried out under this assumption still need to be compared to the measurements made.
The ultimate explanation is that of the pressure of solar radiation. This phenomenon is due to the transfer of the momentum of the photons on a given surface; in other words, the flow of photons impacting a surface causes the movement of the latter by applying a constant pressure to it. If 'Oumuamua has a large enough flat surface, coupled with a relatively low mass, then solar radiative pressure could be responsible for its excess acceleration.
This is the hypothesis developed by Baily and Loeb. By performing relatively simple calculations, the two astrophysicists showed that for an object with a particular mass/surface ratio, then the measured acceleration would correspond to that provided by the solar radiation pressure. The authors could have stopped their development here and thus provided a completely plausible scientific explanation.
Nevertheless, Baily and Loeb, specifying that such an object could be of natural origin, continued their explanations by also introducing the hypothesis of an artificial origin, then making direct reference to a spacecraft developed by an extraterrestrial civilization. This machine would thus use, to move, the principle of the solar sail, based on the phenomenon of radiative pressure described above.
Indeed, in their calculations, the authors show that to present such an acceleration according to the solar radiative pressure received, the structure should present a thickness between 0.3 and 0.9 mm.
They remind us that although extremely thin, such an object could survive interstellar travel on galactic scales, while resisting collisions with dust and gases, as well as rotational and tidal forces.
They conclude their hypothesis by specifying that if this structure is of natural origin, then it is necessarily composed of a material currently unknown with regard to the physical properties involved. If in science open-mindedness is a valued quality, we can blame the two astrophysicists for this somewhat unfortunate extrapolation.
There are several natural explanations for the observations and data collected that do not require the involvement of new physics, alien scenarios, or the intervention of extraterrestrial intelligence.
Especially since the spectral signature, color, reflectivity, and a whole host of other properties of the asteroid are entirely consistent with a natural origin. Baily and Loeb have therefore provided a hypothesis that is certainly attractive, but more science fiction as it is only supported by the speculations of its authors.
As cosmologist Carl Sagan said, extraordinary claims require extraordinary evidence. And, in this case, it is clear that the evidence collected is singularly ordinary.
When an extraordinary hypothesis is advanced, it is easy to overlook all the very ordinary explanations that already exist, and which usually involve known and well understood natural phenomena. And this is the case here, the natural hypotheses proposed do not in any way require the introduction of an artificial hypothesis.
Ockam's razor is a powerful tool of scientific methodology and, even if the two authors did not assert anything and were on the whole cautious, it would have been desirable for them to stick to it. Imagination is important, but it should not come at the expense of the scientific process. After all, 'Oumuamua is already extraordinary enough in himself, because if he is not the messenger of an extraterrestrial civilization, he remains the messenger of the stars.
