Stars shine until their supply are exhausted by the gravitational force produced by their very core and therefore, as regards their mass, will eventually lose their ability to shine with a big explosion or running out of nuclear fuel at the end. But, what if a star does not have adequate fuel to burn hydrogen properly, at its very beginning?
Stars cores are merged at the center of the cloud of dust and gas, mostly, consisting hydrogen because of the gravitational force produced by the mass of the cloud of dust and gas. The reason is as follows. When the cloud mass is forced and squeezed by the gravitational force to become smaller and smaller throughout the space, if its fuel supply is sufficient to maintain hydrogen fusion, it will ignite and shine before exhausting its fuel supply. The result is a star. However, if the cloud of dust and gas is insufficient to produce the gravitational force to get fusion started, the result is a brown dwarf. It never shines as a star does, but glitters because of the heat generated at its core. In other words, brown dwarfs are like aborted stars, in a sense, but self-luminous, generating heat from their slow gravitational contraction(1). The more massive brown dwarfs enjoy a brief period of thermonuclear activity, burning deuterium(or lithium), but they quickly exhaust their limited supply of fuel(1), never reach a stable state, and quietly go extinct throughout the Universe. In that regard, we can say that, at the creation of celestial bodies, brown dwarfs are intermediate between stars and massive Jovian planets, named after Jupiter. As to what would happen if one of them had ignited in our solar system, it is getting much more interesting.
The mass of brown dwarfs is too low to maintain hydrogen fusion but not as small as expected(more than 10 times that of Jupiter but less than 7% the mass of the Sun). If one of them had ignited in the solar system, it would become a star, and therefore our solar system would be a double-star – mostly known binary star system – system. ”Binary star systems run the gamut from two stars nearly touching each other, pulled tidally into football shapes and whirling crazily around their common center of gravity every few hours, to twins so far apart that millions of years must elapse before they complete a single mutual orbit(2).” In that regard, the answer is that the life on Earth would not continue to evolve because the Sun could not entrench itself as a stable source of energy as it could without a brown dwarf.
(1) Bely, Pierre-Yves, Christian, Carol, and Roy, Jean-René. "What is a brown dwarf?." A Question and Answer Guide to Astronomy. Cambridge: Cambridge University Press, 2010. 49. Print.
(2) Berman, Bob. "Genesis." The Sun's Heartbeat: And Other Stories from the Life of the Star That Powers Our Planet. New York: Little, Brown and Company, 2011. 18. Print.
Figure - 26.1 https://3c1703fe8d.site.internapcdn.net/newman/gfx/news/hires/2018/universe.jpg
Figure - 26.2 https://spaceplace.nasa.gov/review/spitzer-slyder/text-version.html
Figure - 26.3 http://large.stanford.edu/courses/2011/ph241/olson1/
Figure - 26.4 https://www.universetoday.com/wp-content/uploads/2009/06/jovian-planets.jpg
Figure - 26.5 https://phys.org/news/2010-03-wise-nemesis.html
Figure - 26.6 https://phys.org/news/2017-08-scientists-brown-dwarf-weather.html