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Gravity isn't just about mass, but about distance, too.

Our moon has a surface gravity of about 1/6th of Earth, because it is small and less dense than the Earth is. Surface gravity of a body is inversely proportional to the square of its radius (for constant mass). That means that if you compressed the moon such that it was $\frac{1}{\sqrt{6}}$th of its current radius, it would have the same surface gravity as the Earth even though its mass woudn't have changed at all.

It would have to have a density of about 50 tonnes per cubic metre though, and that's heavier than any normal material so this situation couldn't arise around Earth. You'd need to arrange for a very dense metallic moon to orbit a small or very low density planet... perhaps one mostly made of ice or water. It would be a little surprising to have this arrangement, but not actually impossible. Just unlikely.

As an example, you could imagine a planet a bit like Callisto, which has a surface gravity of about 1/8th of Earth despite its size due to being made largely of ice and rock. A spherical moon with a radius of 200km made of iridium would have a slightly higher surface gravity, but still weigh less than 1/150th of its parent planet. The barycenter of the system will still be comfortably within the radius of Callisto for a plausible orbital distance of the metal moon... for a 130000km orbit, the barycenter will be about 854km from the centre of Callisto, leaving the pair with less "wobble" than the Earth-Moon system. Seems fairly convincingly a planet-moon relationship, rather than a binary planet. At least to me, anyway.

Gravity isn't just about mass, but about distance, too.

Our moon has a surface gravity of about 1/6th of Earth, because it is small and less dense than the Earth is. Surface gravity of a body is inversely proportional to the square of its radius, holding mass constant. That means that if you compressed the moon such that it was $\frac{1}{\sqrt{6}}$th of its current radius, it would have the same surface gravity as the Earth even though its mass woudn't have changed at all.

It would have to have a density of about 50 tonnes per cubic metre though, and that's heavier than any normal material so this situation couldn't arise around Earth. You'd need to arrange for a very dense metallic moon to orbit a small or very low density planet... perhaps one mostly made of ice or water. It would be a little surprising to have this arrangement, but not actually impossible. Just unlikely.

As an example, you could imagine a planet a bit like Callisto, which has a surface gravity of about 1/8th of Earth despite its size due to being made largely of ice and rock. A spherical moon with a radius of 200km made of iridium would have a slightly higher surface gravity, but still weigh less than 1/150th of its parent planet. The barycenter of the system will still be comfortably within the radius of Callisto for a plausible orbital distance of the metal moon... for a 130000km orbit, the barycenter will be about 854km from the centre of Callisto, leaving the pair with less "wobble" than the Earth-Moon system. Seems fairly convincingly a planet-moon relationship, rather than a binary planet. At least to me, anyway.

Gravity isn't just about mass, but about distance, too.

Our moon has a surface gravity of about 1/6th of Earth, because it is small and less dense than the Earth is. Surface gravity of a body is inversely proportional to the square of its radius. That means that if you compressed the moon such that it was $\frac{1}{\sqrt{6}}$th of its current radius, it would have the same surface gravity as the Earth even though its mass woudn't have changed at all.

It would have to have a density of about 50 tonnes per cubic metre though, and that's heavier than any normal material so this situation couldn't arise around Earth. You'd need to arrange for a very dense metallic moon to orbit a small or very low density planet... perhaps one mostly made of ice or water. It would be a little surprising to have this arrangement, but not actually impossible. Just unlikely.

As an example, you could imagine a planet a bit like Callisto, which has a surface gravity of about 1/8th of Earth despite its size due to being made largely of ice and rock. A spherical moon with a radius of 200km made of iridium would have a slightly higher surface gravity, but still weigh less than 1/150th of its parent planet. The barycenter of the system will still be comfortably within the radius of Callisto for a plausible orbital distance of the metal moon... for a 130000km orbit, the barycenter will be about 854km from the centre of Callisto, leaving the pair with less "wobble" than the Earth-Moon system. Seems fairly convincingly a planet-moon relationship, rather than a binary planet. At least to me, anyway.

Gravity isn't just about mass, but about distance, too.

Our moon has a surface gravity of about 1/6th of Earth, because it is small and less dense than the Earth is. Surface gravity of a body is inversely proportional to the square of its radius (for constant mass). That means that if you compressed the moon such that it was $\frac{1}{\sqrt{6}}$th of its current radius, it would have the same surface gravity as the Earth even though its mass woudn't have changed at all.

It would have to have a density of about 50 tonnes per cubic metre though, and that's heavier than any normal material so this situation couldn't arise around Earth. You'd need to arrange for a very dense metallic moon to orbit a small or very low density planet... perhaps one mostly made of ice or water. It would be a little surprising to have this arrangement, but not actually impossible. Just unlikely.

As an example, you could imagine a planet a bit like Callisto, which has a surface gravity of about 1/8th of Earth despite its size due to being made largely of ice and rock. A spherical moon with a radius of 200km made of iridium would have a slightly higher surface gravity, but still weigh less than 1/150th of its parent planet. The barycenter of the system will still be comfortably within the radius of Callisto for a plausible orbital distance of the metal moon... for a 130000km orbit, the barycenter will be about 854km from the centre of Callisto, leaving the pair with less "wobble" than the Earth-Moon system. Seems fairly convincingly a planet-moon relationship, rather than a binary planet. At least to me, anyway.

Gravity isn't just about mass, but about distance, too.

Our moon has a surface gravity of about 1/6th of Earth, because it is small and less dense than the Earth is. Surface gravity of a body is inversely proportional to the square of its radius. That means that if you compressed the moon such that it was $\frac{1}{\sqrt{6}}$th of its current radius, it would have the same surface gravity as the Earth even though its mass woudn't have changed at all.

It would have to have a density of about 50 tonnes per cubic metre though, and that's heavier than any normal material so this situation couldn't arise around Earth. You'd need to arrange for a very dense metallic moon to orbit a small or very low density planet... perhaps one mostly made of ice or water. It would be a little surprising to have this arrangement, but not actually impossible. Just unlikely.

As an example, you could imagine a planet a bit like Callisto, which has a surface gravity of about 1/8th of Earth despite its size due to being made largely of ice and rock. A spherical moon with a radius of 200km made of iridium would have a slightly higher surface gravity, but still weigh less than 1/150th of its parent planet.

Gravity isn't just about mass, but about distance, too.

Our moon has a surface gravity of about 1/6th of Earth, because it is small and less dense than the Earth is. Surface gravity of a body is inversely proportional to the square of its radius. That means that if you compressed the moon such that it was $\frac{1}{\sqrt{6}}$th of its current radius, it would have the same surface gravity as the Earth even though its mass woudn't have changed at all.

It would have to have a density of about 50 tonnes per cubic metre though, and that's heavier than any normal material so this situation couldn't arise around Earth. You'd need to arrange for a very dense metallic moon to orbit a small or very low density planet... perhaps one mostly made of ice or water. It would be a little surprising to have this arrangement, but not actually impossible. Just unlikely.

As an example, you could imagine a planet a bit like Callisto, which has a surface gravity of about 1/8th of Earth despite its size due to being made largely of ice and rock. A spherical moon with a radius of 200km made of iridium would have a slightly higher surface gravity, but still weigh less than 1/150th of its parent planet. The barycenter of the system will still be comfortably within the radius of Callisto for a plausible orbital distance of the metal moon... for a 130000km orbit, the barycenter will be about 854km from the centre of Callisto, leaving the pair with less "wobble" than the Earth-Moon system. Seems fairly convincingly a planet-moon relationship, rather than a binary planet. At least to me, anyway.