A、They are rocky planets. B、They are gaseous like Jupiter. C、They are covered by water. D、They are covered by sea ice. A

Astronomers have found at least seven Earth-sized planets orbiting the same star 40 light-years away, according to a study published Wednesday in the journal Nature. The findings were also announced at a news conference at NASA headquarters in Washington.    (16) This discovery outside of our solar system is rare because the planets have the winning combination of being similar in size to Earth and being all temperate, meaning they could have water on their surface and potentially support life.
    "This is the first time that so many planets of this kind are found around the same star," said Michael Gillon, lead study author and astronomer at the University of Liege in Belgium.
    The seven exoplanets were all found in tight formation around an ultra-cool dwarf star called TRAPPIST-1. (17) Estimates of their mass also indicate that they are rocky planets, rather than being gaseous like Jupiter. Three planets are in the habitable zone of the star, known as TRAPPIST-1 e, f and g, and may even have oceans on the surface.
    (18) The researchers believe that TRAPPIST-lf in particular is the best candidate for supporting life. It’s a bit cooler than Earth, but could be suitable with the right atmosphere and enough greenhouse gases.
    "I think we’ve made a crucial step towards finding if there is life out there," said Amaury Triaud, one of the study authors and an astronomer at the University of Cambridge. "I don’t think any time before we had the right planets to discover and find out if there was life. Here, if life managed to thrive and releases gases similar to what we have on Earth, we will know."
Questions 16 to 18 are based on the passage you have just heard.
16. Why is the discovery outside of our solar system rare?
17. What do estimates of the mass of the seven exoplanets indicate?
18. Why do researchers think that TRAPPIST-1f is the most suitable for life?