  The existence of multiple regions of space beyond the observable Universe
(within the so-called "multiverse") where the vacuum energy density takes
different values, has been postulated as an explanation for the low non-zero
value observed for it in our Universe. It is often argued that our existence
pre-selects regions where the cosmological constant is sufficiently small to
allow galaxies like the Milky Way to form and intelligent life to emerge. Here
we propose a simple empirical test for this anthropic argument within the
boundaries of the observable Universe. We make use of the fact that dwarf
galaxies formed in our Universe at redshifts as high as z~10 when the mean
matter density was larger by a factor of ~10^3 than today. Existing technology
enables to check whether planets form in nearby dwarf galaxies and globular
clusters by searching for microlensing or transit events of background stars.
The oldest of these nearby systems may have formed at z~10. If planets are as
common per stellar mass in these descendents as they are in the Milky Way
galaxy, then the anthropic argument would be weakened considerably since
planets could have formed in our Universe even if the cosmological constant was
three orders of magnitude larger than observed. For a flat probability
distribution, this would imply that the probability for us to reside in a
region where the cosmological constant obtains its observed value is lower than
\~10^{-3}. A precise version of the anthropic argument could then be ruled-out
at a confidence level of ~99.9%, which constitutes a satisfactory measure of a
good experimental test.
