Kepler and TESS unveiled that Sun-like stars frequently host exoplanets. These
exoplanets are subject to ionizing X-ray radiation and coronal mass ejections (CMEs)
from the host stars, that may cause changes in their atmospheric dynamics and
chemistry. Notably, young Sun-like stars often produce superflares with energy exceeding
10^33 erg, suggesting severe conditions in the early Sun-Earth environment. These
superflares are expected to be closely linked to large X-ray fluxes and gigantic CMEs,
but their evidence remains scarce. Recently, we have performed multi-wavelength
monitoring observations of nearby active young Sun-like stars (25-100 Myr-age ZAMS) with
optical photometry (TESS) and spectroscopy (3.8m Seimei telescope) and X-ray telescope
(NICER) during 2020-2023, and found >10 superflares. Our H-alpha spectroscopic
observations reveal for the first time that superflares are frequently associated with
gigantic and fast "prominence eruptions". The largest prominence eruption is found to be
associated with a possible post-flare X-ray dimming, an evacuation of coronal plasma
from the stellar surface. These consistently indicate that a gigantic CMEs occurs from
superflares and can influence the exoplanetary environment. Furthermore, we
observationally derived a unified empirical relation of energy partitions into optical
and X-ray superflares, which enables the evaluation of XUV environment around young
Suns. We have successfully obtained multi-wavelength results; however, the data and
evidence are limited. To conclude this presentation, I will outline the future direction
of our research and discuss potential collaboration opportunities with XRISM.