Low probability of expulsion of planets from a Solar System due to gravitational tug

Exoplanets is one of the most exciting research fields in planetary science and astrophysics. Seeded in 1995 with the discovery of the first planets outside the Solar System, the field of exoplanets research have expanded rapidly with the use of more sensitive instruments capable of detecting the small drop in luminosity of the parent star, i.e., the transit approach, or slight changes in radial velocity.


Methods for detecting planets include radial velocity, gravitational microlensing and transit. Specifically, the first exoplanets were detected by measuring small changes in radial velocity. Due to measurement difficulty, first detection of exoplanets typically find gas giants of size similar or larger than Jupiter. With improvement in detection technologies and higher measurement fidelity, exoplanets of size smaller than gas giants (such as rocky Earth sized planets) can be detected, but with difficulty.


With the launch of Kepler in 2009, exoplanets could finally be detected in a reliable manner by the transit method. In more detail, there would always be small dips in light intensity from the parent star of the exoplanets as the planet move past the star; hence, detecting this change in luminosity would help define the relative size of the astronomical object in front of the star. But, is detecting the small dip in starlight sufficient condition for defining a planet in a star’s solar system? The answer is no, because for an astronomical object to qualify as a planet, it must have a regular orbit of defined periodicity. Hence, if Kepler space telescope is able to repeatedly observed the transit of the same planet across the face of the star at a regular periodicity (which allows the orbit of the planet to be calculated), there is a higher chance and probability of the detected entity being a planet.


Kepler’s first phase of observation (Kepler K1) ended in 2013 after the failure of two out of four reaction wheels. Nevertheless, Kepler K1 led to the detection of about 3000 unconfirmed planets. Confirmation of planets require multiple transits of the candidate planets across the star, but due to the lengthy observation period involved, many of the candidate planets could not be reliably observed transiting their parent star. In what is the handicap of the transit method, spacecraft capable of long duration observation of a specific area of the sky must be designed with sufficient fuel and backup reaction wheels, for handling the inevitable damage resulting from exposure to cosmic radiation at a stable orbit possibly removed from the protective effects of Earth’s magnetosphere.

But, there exists a hypothesis in the astronomical literature on the possible removal of specific planets from a planetary solar system due to unexpected gravitation tug by other transiting planets or solar system. First and foremost, to be defined as a solar system, the cloud of dust and debris around a new born star must coalesce into definable astronomical objects with regular, stable orbits around the star. To have a stable orbit around the star meant that the planets around the star have achieved optimal separation between themselves and with the host star. Hence, it is highly inconceivable that objects such as planets outside of the solar system could exert a significant gravitational tug on the planets within the solar system to result in the expulsion of the planet out of its home solar system.


Overall, it is an untenable idea that planets outside of a solar system could exert a gravitational tug significant enough to move a planet out of its solar system, which results in an orphan planet. The concept, while possible for a solar system in the process of planet formation, is not possible for a mature solar system with definable planets, asteroids, comets and a host star. In the case of a nascent solar system, significant gravitational perturbation within newly formed planets in the system could possibly result in the expulsion of small rocky planets out of the planetary system. However, once out of the solar system, lack of dust and gas limits the further growth of the planet; thus, resulting in a poorly defined astronomical object not definable as a planet (or sometimes manifesting as a dwarf planet). Hence, pictures in magazines of lone planets kicked out of their home solar system due to gravitational perturbation by transiting planets or other large gravitational bodies is false. There is very likely no lone orphan planet without a host star in the universe. Similarly, with the universe expanding, there is relatively little chance of one solar system coming into near contact with another. In fact, distances between solar systems is lengthening due to possible effects from dark energy overwhelming the cohesive nature of dark matter.


Category: space exploration,

Tags: exoplanets, planetary system, lone planet, dwarf planets, orphan planets, orphan planets, gravitational tug, transit, Kepler, radial velocity,


Acknowledgement: Ng Wenfa thank Seah Kwi Shan for co-authoring this blog post.




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