**4. Space probes**

*Solar System Planets and Exoplanets*

contains seven potential Earth-like exoplanets. Trappist-1 is an ultra-cool red dwarf star with a radius that is somewhat larger than Jupiter's, but has a mass of about 84 times Jupiter's [13, 16]. It has a surface temperature of about 2600 K [16], which partially explains the nature of the habitable semi-major axes and rotation periods

*Radial velocity profile for a transiting exoplanet. Positive (negative) radial velocities result in a light profile* 

ρ ρ

Trappist-1b 1.02 1.12 0.73 0.81 0.012 1.51 Trappist-1c 1.16 1.10 0.88 0.97 0.016 2.42 Trappist-1d 0.30 0.78 0.62 0.48 0.022 4.05 Trappist-1e 0.77 0.91 1.02 0.93 0.029 6.10 Trappist-1f 0.93 1.05 0.82 0.85 0.039 9.21 Trappist-1 g 1.15 1.15 0.76 0.87 0.047 12.4 Trappist-1 h 0.33 0.77 0.72 0.56 0.062 ~20

( <sup>⊕</sup> ) **Surface** 

**gravity (g)**

**Semi-major axis (AU)**

**Orbital Periodb (d)**

In 2017, NASA announced the observation that seven rocky exoplanets similar

in size to Earth were discovered orbiting the host star Trappist-1 [16]. These planets resided in the habitable zone and had the potential for the existence of liquid water on their surfaces. Trappist-1 is about 40 light years from Earth, and this proximity creates the possibility that its planetary systems could be imaged with future generations of telescopes [16]. Further investigation could reveal the existence of atmospheric constituents that would indicate the possible presence of

**Table 7** summarizes selected details of the Trappist-1 exoplanetary system [16, 17]. These exoplanets have nearly circular orbits, and orbit in proximity to their host star. The Trappist-1 exoplanets are in the range of 0.3–1.2 Earth masses with

**20**

life forms.

noted in **Table 7**.

*Derived from Ref. [17].*

*Derived from Ref. [16].*

**Figure 3.**

*a*

*b*

**Table 7.**

*that is blue (red) shifted.*

**Exoplaneta** m M( <sup>⊕</sup> ) *r R*( <sup>⊕</sup> )

*Selected characteristics of Trappist-1 exoplanets.a,b*

A variety of probes have investigated Solar System planets as well as exoplanets. These devices are growing in capability, and future probes could have the capability to reveal significant details regarding the exoplanetary structure and atmospheric composition of the increasing number of observed exoplanetary systems.
