The Line Between a Planet and a Star is Much Finer than you might Think

The Line Between a Planet and a Star is Much Finer than you might Think

Scientists may have to reconsider the mass that a celestial object needs to have to initiate nuclear fusion.

The term ‘Stellar Evolution’ refers to the process by which a star changes over the course of time. The lifetime of a star is inversely proportional to its mass. Consequently, massive stars live for a few million years while the smallest ones can have a lifespan of trillions of years. The collapsing clouds of dust and gas called molecular clouds give birth to all the stars. These protostars have a long, struggling journey (millions of years) before reaching the equilibrium state which gives them the status of a main-sequence star.

The gravitational collapse of a giant molecular cloud breaks it into smaller fragments. The temperature and pressure of each of these pieces increases due to the heat energy released by the collapsing gas. This creates an ideal environment for these fragments to condense into each other and they form a rotating sphere of superhot gas called a protostar. The accretion of dust and gas keeps it growing until it reaches its final mass. Once the temperature of a star reaches 10,000 Kelvin, the fusion of Hydrogen starts. These proton-proton chain reactions fuse Hydrogen to Deuterium and then Helium. Similarly, the stars that are slightly bigger than our Sun are also heated by the carbon-nitrogen-oxygen (CNO cycle) fusion reactions.

On the other hand, some of the protostars never experience the nuclear fusion of Hydrogen as they lack the necessary mass. The stars having masses less than 1.6 x 1029 kg never achieve the temperature needed for initiating a fusion reaction. They are classified as ‘Brown Dwarfs’. The formal definition of a brown dwarf suggests that it is more massive than a planet but not big enough to be called a star. Having said that, a recent discovery of two humungous brown dwarfs might change our understanding of the stellar evolution.

According to an estimate of astronomers, Epsilon Indi B and C are more than 70 times the mass of Jupiter. This fact takes it them incredibly close to being stars. However, the luminosity of these brown dwarfs indicates that they haven’t achieved the status of stars, at least for now. Despite all that, this discovery has urged the scientists to reconsider the mass that a celestial object must possess to initiate a fusion reaction because the current upper limit for a brown dwarf is 70 Jupiter masses.

The Epsilon Indi System was discovered in 2003 at a distance of 12 light-years from Earth. At that time, astronomers found a pair of brown dwarfs and a small main sequence star in this system. Recently, a Jupiter-sized object (orbiting the star) was also found there. It is regarded as the nearest known exoplanet to our solar system. Prior to its discovery, scientists calculated the masses of the other three celestial bodies of the Epsilon Indi System by making use of the Infrared Imaging and Low-resolution Spectroscopy. That experiment predicted the masses of the brown dwarfs to be 47 and 28 Jupiter masses with a probable error of 25%.

The researchers of the Carnegie Institution for Science in Washington choose a different approach for their analysis. They decided to have a look at their wiggling in order to determine the approximate masses of the two brown dwarfs. They mapped the smallest movements of these objects against their cosmic background and got a more precise measurement for the masses of both, Epsilon Indi B and C. The latter of the two has around 70.1 times the mass of Jupiter while the former is 75 times heavier than Jupiter. Serge Dieterich, the First Author of the study, explained their findings in the following words:

Taken together, our results mean that the existing models need to be revised. We showed that the heaviest brown dwarfs and the lightest stars may only have slight differences in mass. But despite this, they are destined for different lives – one racing to dim and cool, the other shining for billions of years.”

Brown dwarfs are quite hard to locate but the commonality of dark solar systems suggests that the possibility of life in the cosmos is certainly there. Alycia Weinberger, a member of the researching team, referred to that by saying,

We are interested in whether stars and brown dwarfs always exist in the same proportion to each other in star-forming regions, which could help us understand the overall habitability of our galaxy.”

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