This week, Eternal Sunshine of the Positive Mindset.

If we boost positive memory by activating memory-carrying cells, we may be able to restore

more normal behaviour.

And our plant friends may be brainless, but are they intelligent?

Once you accept intelligence is a capacity for problem solving,

then plants solve an enormous number of problems.

Plus we explore a gaggle of little exoplanets.

This is the Nature Podcast for June the 18th, 2015.

I'm Kerry Smith.

And I'm Adam Levy.

Until the 1990th, 2015. I'm Kerry Smith. And I'm Adam Levy. Until the 1990s, astronomers could only fantasise about planets orbiting stars other than our sun.

Now we know about thousands of these exoplanets. But what can we learn about these distant worlds?

Mostly, not much, just their size.

In a few cases, though, it's been possible to calculate the size and mass of these exoplanets,

giving us clues to their composition.

Research led by Daniel John Toff Hutter of Pennsylvania State University

has been able to make these measurements for the smallest exoplanet

yet. The planet is one of a family of three going around a star named Kepler 138, about 200 light

years away. For comparison, our sun is just eight light minutes away. But how do we even spot these

planets in the first place? Here's Daniel. Planets orbit their stars in a flat plane,

and if we're lucky enough to be in that same plane as a planet as it orbits its star,

we can see it pass in front of the star every orbit.

...