Researchers from Osaka University, using gravitational microlensing, have determined that there are cold planets throughout our galaxy. Cold planets would have average temperatures below the freezing point.

The results show that the planetary distribution is not strongly dependent on the distance from the Galactic center. Instead, cold planets orbiting far from their stars seem to exist universally in the Milky Way. This includes the Galactic bulge, which has a very different environment to the solar neighborhood, and where the presence of planets has long been uncertain.

“Stars in the bulge region are older and are located much closer to each other than stars in the solar neighborhood,” explains lead author of the study Naoki Koshimoto. “Our finding that planets reside in both these stellar environments could lead to an improved understanding of how planets form and the history of planet formation in the Milky Way.”

Osaka University, “Cold planets exist throughout our galaxy, even in the galactic bulge, research suggests” at ScienceDaily (August 30, 2021) The paper requires a subscription.

Gravitational microlensing “is an observational effect that was predicted in 1936 by Einstein using his General Theory of Relativity. When one star in the sky appears to pass nearly in front of another, the light rays of the background source star become bent due to the gravitational “attraction” of the foreground star. This star is then a virtual magnifying glass, amplifying the brightness of the background source star, so we refer to the foreground star as the lens star. If the lens star harbors a planetary system, then those planets can also act as lenses, each one producing a short deviation in the brightness of the source. Thus we discover the presence of each exoplanet, and measure its mass and separation from its star. This technique will tell us how common Earth- like planets are, and will guide the design of future exoplanet imaging missions.” – NASA

Knowing more about where different types of planets exist and where they don’t will, of course, help target our searches for life. The researchers recommend combining their results with measurements from microlens parallax or lens brightness.

Galactic bulge? “Galactic bulge is a tightly packed group of stars within a larger star formation. The term almost exclusively refers to the central group of stars found in most spiral galaxies. Bulges were historically thought to be elliptical galaxies that happened to have a disk of stars around them, but high-resolution images using the Hubble Space Telescope have revealed that many bulges lie at the heart of a spiral galaxy. It is now thought that there are at least two types of bulges: bulges that are like ellipticals and bulges that are like spiral galaxies.” – Worldwide Scientific Discussion Community

Here are average temperatures for planets in our solar system.

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