Astronomers Measure Mass and Distance of Rogue Planet Using Dual Observations

Astronomers have directly determined both the mass and distance of a free-floating planet by observing a microlensing event from two locations: Earth and the Gaia space telescope. This approach marks the first time such measurements have been achieved for a rogue planet using simultaneous ground- and space-based data.

The research, led by Subo Dong and colleagues, was published in the journal Science. The team combined survey data collected from Earth with observations from the Gaia telescope to enable precise measurement of microlensing parallax, which is essential for determining the properties of distant objects.

The planet identified in this study is designated KMT-2024-BLG-0792, also known as OGLE-2024-BLG-0516. It is located toward the galactic bulge, approximately 9,950 light-years from Earth and about 3,000 parsecs from the center of the Milky Way.

Analysis of the microlensing event revealed that the planet has a mass close to 22 percent of Jupiter’s mass, which is similar to Saturn. The mass estimate corresponds to roughly 70 times the mass of Earth, confirming its classification as a Saturn-class object.

What the numbers show

• The planet's mass is about 22 percent of Jupiter's mass
• The object is nearly 9,950 light-years from Earth
• The planet lies about 3,000 parsecs from the Milky Way's center
• Microlensing parallax was measured using a 1.5-million-kilometer baseline

The dual observations from Earth and Gaia, separated by approximately 1.5 million kilometers, allowed astronomers to resolve the mass-distance ambiguity that often complicates microlensing studies. This technique provided the necessary data to measure both the mass and distance of the planet directly.

According to the published findings, this is the first instance where a rogue planet’s mass and distance have been directly established using this dual-observation method. The results support the view that low-mass rogue planets likely originated in planetary systems and were later ejected, rather than forming independently like stars.

A commentary by Gavin Coleman stated that this dual-perspective approach could inform future space missions, such as those planned for the Nancy Grace Roman Space Telescope. The method demonstrated in this research may be applied to additional discoveries of free-floating planets in the future.

The study demonstrates how combining ground-based and space-based observations can advance the understanding of objects that do not orbit stars, providing new information about their origins and characteristics.

* This article is based on publicly available information at the time of writing.