Seven Planets around a Dwarf Star – Earth sized worlds with water

The European Southern Observatory (ESO) and NASA’s Spitzer Space Telescope have discovered a system of seven Earth-sized planets just 40 light-years away in the constellation of Aquarius. The planets were detected as they passed in front of the ultra-cool dwarf star known as TRAPPIST-1. The middle three of the planets lie in the habitable zone and most likely host oceans of water on their surfaces, increasing the possibility this exoplanetary system could host to life.  Water is key to all life as we know it on planet Earth, and is the first thing astronomers look for when discovering new planets around other stars.  This system has the largest number of Earth-sized planets and the largest number of worlds that could support liquid water on their surfaces to date.  Planets beyond our solar system, called exoplanets were first discovered in 1995 and as of February of this year there are just over 3500 confirmed ones.  A very small percentage (less than 100) are Earth sized, and even a smaller percentage are in the habitable zone where the planet is the right distance from its star creating temperatures where liquid water could exist on its surface.

TRAPPIST 1 Planetary System

TRAPPIST 1 Planetary System – image provided by European Southern Observatory

Using ESO’s TRAPPIST–South telescope, the Very Large Telescope (VLT) at Paranal,  and the NASA Spitzer Space Telescope along with other telescopes around the world, astronomers have now confirmed the existence of seven small planets orbiting the cool red dwarf star TRAPPIST-1 . These planets are similar in size to Earth are labelled TRAPPIST-1b, c, d, e, f, g and h in order of increasing distance from their parent star.  All of the planets were discovered via the transit method, watching for dips or decreases in the star’s brightness caused by each of the seven planets passing in front of it.  Transits allow astronomers to infer  a great deal of information about the planets including the sizes, compositions, and orbits. Based on these measurements, astronomers determined the inner six planets are comparable in both size and temperature to the Earth.

This system is different than our own solar system in numerous ways.  The parent start called TRAPPIST-1 is very small in size, just slightly larger than the planet Jupiter containing just 8% the mass of the Sun.  The orbits of the planets are about the size of Jupiter’s Galilean moon system being closer to their star than the orbit of Mercury is to our Sun. The range of orbital periods is from 1.51 days with b to approximately 20 days for h.  By comparison Mercury takes 88 days to orbit the Sun. Yet all the planets in the TRAPPIST-1 system are similar in size to Earth and Venus or slightly smaller. Density measurements suggest that at least the innermost six have rocky in compositions.

While all seven planets discovered in the TRAPPIST-1 system could potentially have water on their surfaces, the ones designated e, f, and g orbit in the star’s habitable zone and could host oceans of liquid water based on climate models.  The three innermost planets, b, c and d, are probably too hot for liquid water, and the distance of the system’s outermost planet h, is likely to be too distant and cold for water in its liquid form, but it might have ice.

This is a pivotal discovery for exoplanets and the possibility of life outside our Solar System.

Shawn Laatsch

About Shawn Laatsch

Shawn Laatsch is the director of the Emera Astronomy Center and Jordan Planetarium at the University of Maine. He started his astronomy education career in 1984 and has directed planetariums in university and science center facilities, taught undergraduate astronomy courses, and given numerous lectures around the globe. He serves as President (2017 & 2018) of the International Planetarium Society, Inc. the world’s largest organization of planetarium professionals. Shawn has a passion for sharing astronomy and stargazing with people of all ages.