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figures/venus_eros.jpg
Venus as seen by Magellan, and the Eros asteroid observed by NEAR.
Copyright : NASA

Exoplanets resemble the Earth :

  • Exoplanets are spherical, like the Earth.

    In general, planets are spherical due to the gravitational force.

    If a planet had a very high mountain, it would be too heavy and would collapse under its own weight.

    This is the reason why on Earth, there are no mountains higher than Mount Everest (8 km).

    Mars is 10 times less massive than the Earth. Due to the smaller gravitational force, the weight of the Martian mountains is weaker. The highest mountain on Mars, Olympus Mons, is three times higher than Mount Everest. On Venus, the highest point (Mount Maxwell) is at 12  km. The mass of Venus is 80 percent of the mass of the Earth.

    Small planets, satellites, and asteroids are also spherical, if they are large enough (about 100 km). If they are small, like Eros for instance (about 20 km), they keep the shape they had when they formed.

  • Like the Earth, exoplanets do not shine.

    The planets of the Solar System are visible in the sky because they are illuminated by the Sun. From the Earth, we only see their illuminated parts. Even if the planets (and the moon) are spherical, we only see their phases from the Earth. Similarly, exoplanets are visible because they reflect the light from their host stars.