See a large planet orbiting a small star

Astronomers have discovered a planet the size of Saturn in close proximity to a small cool star 35 light-years from Earth. This is the first discovery of an extrasolar planet with a radio telescope using a technique that requires extremely precise measurements of the position of a star in the sky, and only the second discovery of a planet for this technique and for radio telescopes. The results were obtained using VLBA (Very Long Baseline Array) – an antenna array with a very long bases. It is a radio interferometer owned by the US National Radio Astronomy Observatory. The VLBA consists of ten radio telescopes monitored remotely from a command center located in Socorro, New Mexico. The results of the discovery are published by the Astronomical Journal.

One of the space discovery techniques involves tracking the actual motion of a star in space, and then detecting a slight “wobble” in that motion, caused by the planet’s gravitational effect. The star and planet revolve around a place that represents the center of mass for both of them. A planet opens up indirectly if the location, the barycenter, is far enough from the center of the star to cause the wobble to be detected by the telescope.



This astrometric method is especially good for detecting planets like Jupiter in orbits far from the star. After all, when a massive planet orbits a star, the wobble created in the star increases with the distance between the planet and the star. And at a given distance from the star, the more massive the planet, the greater the wobble.

Since June 2018, astronomers have tracked TVLM 513-46546, a cold dwarf star with a mass of less than one-tenth. In addition, they used data from nine previous observations of the VLBA star between March 2010 and August 2011.

Extensive analysis of data from these time periods revealed a clear wobble in the star’s motion, indicating the presence of a planet comparable in mass to Saturn, orbiting the star once every 221 days. By the way, this planet is closer to the star than Mercury is to the Sun.

Cool dwarfs such as TVLM 513-46546 are the most abundant type of stars in our Milky Way galaxy. Many of them have planets, but smaller than Earth and Mars.

Giant planets like Jupiter and Saturn are not expected by scientists to see small stars like this around. In addition, astrometric techniques are best at finding planets like Jupiter in wide orbits. Therefore, scientists were extremely surprised to find a planet like Saturn in a relatively compact orbit.

We expected to find a more massive planet like Jupiter in a wider orbit.

Salvador Curiel of the National Autonomous University of Mexico.



More than 4,200 planets orbiting stars other than the Sun have been discovered, but the planet around TVLM 513-46546 is only the second to be found using astrometric techniques. Another very successful method, the radial velocity technique, is also based on the gravitational effect of a planet on a star. This technique shows the slight acceleration of a star towards or away from Earth caused by the motion of the star around the barycenter.

The third method, the transit method, is also very successful, detecting a slight darkening of the star’s light as a planet passes in front of it, as seen from Earth.

The astrometric method was successful in detecting nearby binary stellar systems, and as early as the 19th century was recognized as a potential tool for detecting extrasolar planets. Over the years, a number of such discoveries have been announced, which have then failed to pass further verification. The difficulty was that the stellar wobble created by the planet, when viewed from Earth, is so small that it requires extraordinary precision in positional measurements.

The VLBA with antennas 5,000 miles apart provided us with the high resolution and extremely high precision required for this discovery. In addition, the improvements that have been made to VLBA sensitivity have given us the data quality that allows us to do this job now.

Amy Miodushevski of the National Radio Astronomy Observatory