Why is Jupiter a Planet if the Star EBLM J0555-57Ab is Smaller in Size?

The classification of celestial bodies as planets or stars is a complex matter, based on specific criteria primarily related to their mass, size, and ability to undergo nuclear fusion. This article explores Jupiter's characteristics and the unique properties of the star EBLM J0555-57Ab to understand why Jupiter is classified as a planet while EBLM J0555-57Ab is a brown dwarf or star.

Definition of a Planet

According to the International Astronomical Union (IAU), a planet must meet the following criteria:

1. Orbit a Star

A planet must orbit a star, and in our case, this star is the Sun.

2. Be Massive Enough for Hydrostatic Equilibrium

A planet must be massive enough for its gravity to shape it into a nearly round object, a state known as hydrostatic equilibrium.

3. Clear its Orbit of Other Debris

A planet must have cleared its orbit of other debris, allowing for its own orbital path to be dominated by its own gravity.

Jupiter's Characteristics

Jupiter, the largest planet in our solar system, is a prime example of a planet that meets these criteria. Here are some key characteristics:

1. Size

With a diameter of about 86,881 miles (139,822 kilometers), Jupiter is significantly larger than most other planets.

2. Mass

Jupiter has a mass of approximately 1.9 x 1027 kg, which is about 318 times that of Earth. This size and mass allow Jupiter to maintain its planetary status despite its large size.

3. No Nuclear Fusion

Jupiter, like other gas giants, does not have enough mass to ignite nuclear fusion in its core. This characteristic distinguishes planets from stars.

EBLM J0555-57Ab: A Unique Object

EBLM J0555-57Ab, a unique object known as a brown dwarf, challenges the traditional boundaries between planets and stars. Here are some key points about EBLM J0555-57Ab:

1. Size and Mass

EBLM J0555-57Ab is one of the smallest known stars or brown dwarfs with a radius smaller than that of Jupiter. However, it has a mass that can exceed the mass of Jupiter, typically in the range of 85 to 100 times that of Jupiter, allowing it to be classified as a star or brown dwarf.

2. Composition

Brown dwarfs and low-mass stars like EBLM J0555-57Ab are primarily composed of hydrogen and helium, similar to stars. They may also contain heavier elements and compounds, but hydrogen and helium are the predominant constituents. If EBLM J0555-57Ab has more mass than Jupiter, it is likely undergoing some degree of fusion, specifically deuterium fusion, but not sustained hydrogen fusion like true stars.

Summary

In summary, Jupiter is classified as a planet because it meets the criteria for planetary status, even if it has a larger size than EBLM J0555-57Ab. The latter, while potentially smaller in size, has more mass and consists mainly of hydrogen and helium, placing it in the category of brown dwarfs or low-mass stars. This example highlights the dynamic nature of our understanding of celestial objects and the ongoing exploration of their properties and classifications.