1. There are three main ways in which the terrestrial planets could have acquired their volatiles.

2. Thus the four terrestrial planets are all affected in important but very different ways by comet and asteroid impacts.

3. If such planets exist, these missions—NASA's Terrestrial Planet Finder and ESA's Darwin—should be able to scan their atmospheres for the presence of gases that would indicate the existence of life.

4. The finding contradicts the prevailing notion that a terrestrial planet needs a large moon to stabilize the orientation of its axis and, hence, its climate.

5. Now the total number of extrasolar planets discovered up to more than 700, as defined in "terrestrial planet" standard is too narrow, most of the planet have been excluded.

6. "We have been calling Vesta the smallest terrestrial planet, " Chris Russell, Dawn's principal investigator, said at the news conference.

7. Those ideas about terrestrial planet formation and habitability of terrestrial planets will need to be re-evaluated from scratch.

8. And the Terrestrial Planet Finder, an even more ambitious project that might have been able to study alien atmospheres, has been postponed indefinitely.

9. The inner edge of the belt, the part closest to Mars and the other terrestrial planets, is compositionally complex.

10. The mean density of Mercury indicates that its interior is substantially different from the interiors of the other terrestrial planets.

11. Outline the evidence that some sort of late heavy bombardment occurred on all terrestrial planets. 5.

12. Finally, Mercury and the Moon may be much less thoroughly outgassed than the other terrestrial planets.

13. The cometary events responsible for these multiple episodes of fragmentation are clearly important to the terrestrial planets.

14. The most likely fate is to collide with one of the terrestrial planets.

15. Outline the arguments for and against the late heavy bombardment of a terrestrial planet occurring after its formation. 6.

16. This astonishing discovery of polar ice on Mercury makes it clear that impacts play a major role on all the terrestrial planets.

17. Information from the planetary probes indicates that all the terrestrial planets have undergone differentiation, but they have followed different evolutionary paths.

18. Through all these means small bodies can bring to the terrestrial planets volatiles in sufficient abundance to match the observations.

19. At three to four times the size of Earth, super-Earth Gliese 581 g is likely a terrestrial planet like ours, and like Earth[/terrestrial planet.html], it's large enough to hold an atmosphere [source: Lemonick].

20. The SIM, in turn, will pave the way for the Terrestrial Planet Finder, to be launched late in the next decade, which should identify even more earth-like planets.

21. She's also interested in determining what conditions other than a liquid ocean might help initiate life on a terrestrial planet.