Artwork depicting Ross 128 and its planet. Credit: ESO
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Artwork depicting Ross 128 and its planet. Credit: ESO

A nearby Earth-sized planet has been found… and it *may* be a nice place to live

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Nov 16, 2017

Oh, this is very interesting exoplanet news indeed: Astronomers have found a planet that is likely around the same size as Earth, and it's orbiting a star that's only 11 light years away! Better yet, depending on the planet's characteristics, it could have temperate conditions on the surface. In other words, it may be — may be — Earth-like.

The planet orbits Ross 128, a cool red dwarf star. The star is something of a dim bulb; it has a sixth the mass of the Sun, and shines only about 0.004 times as brightly. Even though it's close (it's the 14th closest known stellar system to ours, including brown dwarfs) it's so faint you need a telescope to see it. At a magnitude of 11.5, the dimmest star you can see with your naked eye is still 100 times brighter than Ross 128!

This makes it a tough star to study. Looking for planets around it takes a big telescope and sensitive equipment. The planet, called Ross 128 b, was found using what's called reflexive motion. As it orbits the star, the gravity of the planet pulls back on the star. While the planet makes a big circle, the star makes a smaller one. As that happens, sometimes the star is approaching Earth, and sometimes moving away. That causes the starlight from it to shift in color very slightly (via the Doppler shift).

Two objects of different masses orbit each other; the more massive one makes a big circle and the lower mass one a smaller circle. Credit: NASA/Spaceplace

Two objects of different masses orbit each other; the more massive one makes a little circle and the lower mass one a bigger circle. Credit: NASA/Spaceplace

This is a very small effect, but it's measurable. Astronomers used the High Accuracy Radial velocity Planet Searcher to observe the star 157 times over nearly 12 years. They then checked the signal to see if there was a periodic shift in the star's light… and there was!

The period of the shift was about 9.9 days, which is therefore the orbital period of the planet: Its year. That means it huddles close to the star, about 7.4 million kilometers out. The Earth's orbit is 150 million km from the Sun, but again remember Ross 128 is a pretty feeble thing! When you account for the orbital size and stellar output, the planet gets about 1.4 times as much sunlight as Earth does.

The temperature of the planet, though, isn't possible to know accurately. If it's dark, it absorbs more light from the star and is warmer. If it's shinier it reflects more light, and will be colder. If it has an Earth-like reflection (absorbing about 60% of the Sun's light and reflecting 40%) then it will have a temperature of roughly 0° Celsius. That's cold, but remember, that's average. The Earth's average temperature is only 16°C, but the variations are pretty big.

And the planet could be darker than Earth, making it warmer. Of course, it could be more reflective, in which case it's an iceball. We just don't know.

Artwork depicting Ross 128 and its planet. Credit: ESO/ M. Kornmesser

Artwork depicting Ross 128 and its planet. Credit: ESO/ M. Kornmesser

We have a better understanding of its mass. That comes from how hard it pulls on its parent star, and that comes out to be about 1.4 times Earth's mass. Unfortunately, that's a lower limit. It could be higher, though how much isn't known.

Still, that's not a deal breaker for a planet. Earth is pretty dense. A planet less dense but bigger than Earth would have more mass, yet conditions on the surface (meaning gravity) would be roughly Earth-like.

Again, we don't know. But this is fairly promising!

Even better news is that the star looks to be stable. Some red dwarfs have very strong magnetic fields, and can have pretty intense stellar activity like flares. The nearest star to ours, Proxima Centauri, has an Earth-sized planet as well, but Proxima is an active star. It has flares so intense they could damage the potential ecosystem of the planet.

That won't be the case with Ross 128. It's a slow rotator (it spins once every 121 days), and magnetic field strength is generally tied to rapid spinning (the magnetic field is generated deep inside the star by its spin). The observations showed only weak activity from the star, so that's nice.

Slow rotation is also usually an indicator of age, and a different study (in that link it's listed in Table 7 under Gl 447, another name for the star) shows Ross 128 to be about two billion years old. That's half the age of our solar system, but long enough for a planet to develop life (by that age, Earth was covered in goo).

All in all, this is a very interesting planet. We don't know if it's Earth-like, but given what we do know I don't see any show stoppers to preclude it. And it's close by! One of the reasons these observations were done was to look for Earth-sized planets around nearby red dwarfs, so that future large telescopes can look for signs of atmospheres around them. Ross 128 b is a solid candidate for that.

… and if you're patient, it gets easier. Ross 128 is what's called a high proper motion star; it's apparently moving across the sky relatively rapidly due to it being close to us. When you project its motion into the future it turns out that in about 70,000 years it'll be the closest star to Earth! That means Ross 128 b will be the closest exoplanet to us as well.

That'll make telescopic observations of it easier, and cut down any travel time to it by a factor of two or so. I don't think that last bit will help much though.

But that underscores an important aspect of this: Red dwarf stars are by far the most common kind of star in the galaxy. And two of the nearest have Earth-sized planets! Extrapolating from a small sample size is tricky, but that seems to me a strong indicator that there are a lot of planets like this in the Milky Way. Billions. Maybe tens of billions. More.

If Earth is rare, say one in a million, then this alone implies there are thousands of Earth-like planets in the galaxy. We don't know for sure yet, but we're getting close to having a handle on this number.

That's an amazing thing to know, and a hopeful one.