The first good evidence of a planet-like object beyond the solar system has emerged from detailed observations near a star about 50 light years from Earth. An object seen in the region may be the first known brown dwarf, a hypothetical body intermediate in size between a planet and a star.

If confirmed, the discovery would lend support to the idea of life in other star systems. As long as our solar system contained the only known planets, it was hard to estimate how common such objects might be. A second example so close by -- 50 light years is nearby on the galactic scale -- suggests planetary systems are fairly common.

The discovery may also help solve the so-called missing mass problem. The observed mass of the universe accounts for only about 10 percent of the gravitational force that appears to be operating in space. If it turns out there are many brown dwarfs, they could account for a share of the missing mass.

The discovery was reported in last week's Nature by Benjamin Zuckerman of the University of California at Los Angeles and Eric E. Becklin of the University of Hawaii.

Other planet-like objects have been reported in the past but all have faded from believability because repeated attempts to see them failed. Better established are signs of disks of material around stars that appeared to be early stages of forming planetary systems.

The difference between a star and a planet is chiefly one of mass. When interstellar gas and dust accrete into a ball, pressure builds up at the center. If enough mass builds up, the pressure and heat can become great enough to start a thermonuclear chain reaction, the same process that makes a hydrogen bomb blow up. This is what makes stars shine.

If the mass is too small to do this, the result can be anything from a cold lump of rock to a planet such as Jupiter that, unlike Earth, gives off more heat from its internal pressure than it receives from the sun. If Jupiter, the largest planet in our system, were about 75 times more massive, its internal pressures would be enough to ignite nuclear reactions and turn it into a star.

A brown dwarf would be a body many times larger than Jupiter but not big enough to turn into a star. It would send out heat resembling the infrared radiation that Zuckerman and Becklin detected as coming from a point in the sky near the white dwarf star Giclas 29-38. The mystery object appears to have a surface temperature of around 1,700 degrees Fahrenheit, much too cool to be a star but about 10 times hotter than Jupiter.